Naturally Occurring Homoisoflavonoids: Phytochemistry,Biological Activities,and Synthesis (Part II)

Abstract

This review documents all the new homoisoflavonoids (HIFs) that have been reported since 2007, whose total number has grown from 159 in 2007 to 295 at the present time. This review contains their structures, biological sources, plant parts they are obtained from, and, if reported, their optical rotations and melting points. The same classification is followed as an earlier review to ease reference to both reviews. This review takes note of the recent revision of plant families that were known to contain HIFs that have now been merged into one big family, Asparagaceae. Homoisoflavonoids also occur in Fabaceae and others. Two taxa, Ophiopogoan japonicus (Asparagaceae) and Caesalpinia sappan (Fabaceae), have been the source of many HIFs. These are briefly summarized. The biological properties of HIFs are also reviewed under the topics such as antioxidant, anti-inflammatory, antimicrobial, antidiabetic, and cytotoxic. The review also surveys the total synthesis of natural HIFs. All new compounds are classified and tabulated following the same style as the previous review.

Dedicated to Professor Andrew Paul Krapcho on the occasion of his 87th Birthday.

Homoisoflavonoids represent a subclass of the larger family of flavonoids (1) that are uniquely characterized by having one more carbon (C-9) than the regular flavonoids in their 16-carbon skeleton (2) (Figure 1). Labeling studies^1,2 have provided evidence to support the biogenetic hypothesis that they are biosynthesized from a 15-carbon flavonoid precursor.

Figure 1

General scaffolds of a flavonoid and an HIF.

This review is a follow-up of an earlier one in 2007 ³ listing the 159 compounds which were known then. The total number of naturally occurring HIFs has now reached 295 and this review documents the new metabolites belonging to this class that have been discovered since then. This review is based on a survey of publications that have appeared in Scopus and the Science Citation Index during the period 2007 to 2018.

Five important review publications relevant to the field of HIFs have appeared during this review period. The first is a review of the chemical structures, plant origins, ethnobotany, and biological activities of homoisoflavanones, ⁴ listing 129 compounds. This number is less than the 159 recorded in the earlier review, because the authors focused only on metabolites having a selected number of structural types under this class. The second is a review of HIFs and their pharmacological activities. ⁵ This lists 240 naturally occurring HIFs and contains a comprehensive listing of all natural HIFs known at the time of the review (2014). The coverage is limited to recording all the metabolites and discusses pharmacological properties. The third review is devoted to one homoisoflavonoid, namely, brazilin. ⁶ Brazilin (3) (Figure 2) is the major and most biologically active of the 3 HIFs found in the heartwood of Caesalpinia sappan. The review documents the scientific evidence supporting the folkloric uses of brazilin as an antioxidant and antibacterial, with anti-inflammatory, antiphotoaging, hypoglycemic, vasorelaxant, hepatoprotective, and antiacne activity (more on this compound, later). The fourth is a phytochemical, ethnomedicinal, and pharmacological review of the taxon Ophiopogon japonicus, a functional food and commercial medicinal plant, whose biological activities are due to 3 major classes of metabolites: saponins, HIFs, and polysaccharides. ⁷ The published evidence summarized in this review points to the HIFs contributing to the anti-inflammatory and antioxidant properties of the medicinal plant. The fifth review is the first comprehensive report in recent times devoted to what is often referred to as Dragon’s blood. This review sheds much needed light to clarify the confusion that prevailed on the source and composition of Dragon’s blood which refers to red saps and resins derived from a few disparate taxa. ⁸ Although this multipurpose material was originally from Dracaena species, the review reveals that there are 4 distinct plant genera, namely Croton (Euphorbiaceae), Dracaena (Dracaenaceae, now: Asparagaceae), Daemonorops (Palmaceae), and Pterocarpus (Fabaceae) that are described in the literature as sources of Dragon’s blood. Of the 4 genera, only Dracaena cinnabari ⁸ and Dracaena cochinchinensis^9-11 contain HIFs.

Figure 2

Examples of HIFs.

Distribution of HIFs

Many of the plant families that were known to produce HIFs, such as Anthericaceae, Liliaceae, Convallariaceae, Dracaenaceae, and Hyacinthaceae, have now been grouped under one big family, Asparagaceae (Accessed September 13, 2018 [http://www.plantsoftheworldonline.org/taxon/urn:lsid:ipni.org:names:481889-1]). So, all references to these families are recorded as Asparagaceae in this review. There are now 7 other families besides Asparagaceae that are known to produce HIFs: Amarillidaceae, Araliaceae, Fabaceae, Orchidaceae, Polygonaceae, Portolucaceae, Rosaceae, Meliaceae, Polypodiaceae, and Similacaceae. By far the majority of HIFs is derived from the family Asparagaceae, a much lesser number from Fabaceae, and 1 or 2 genera belonging to the other families. Many names of species have also changed. For example, C. sappan L. is considered as the synonym of Biancaea sappan (L.) Tod. and taxonomic information for this taxon from Kew’s http://www.plantsoftheworldonline.org/ (Accessed September 13, 2018 [http://www.plantsoftheworldonline.org/taxon/urn:lsid:ipni.org:names:481889-1]) is given using the latter. To avoid confusion, we have retained the earlier names at the taxon level.

Advances in Extraction and Separation Methods

One of the major challenges in the study of secondary metabolites has always been their purification and isolation from complex plant extracts.

The usual methods involve silica gel chromatography, which inevitably results in the irreversible adsorption of some of the components and in some cases even a change in the integrity of the metabolites. In this regard, there have been some advances reported during the period under review of gentler methods of isolation of HIFs. Xu et al have reported the successful separation of 4 HIFs from C. sappan by high-speed counter-current chromatography (HSCCC). ¹² The crude extract of C. sappan was fractionated by HSCCC using a 2-phase solvent system consisting of chloroform-methanol-water (4:3:2, v/v/v). The separation conditions were: flow rate, 1.0 mL/min; revolution speed, 900 rpm; detection wavelength, 280 nm; separation temperature, 25°C; sample size, 120 mg crude sample dissolved in a mixture of the upper and lower phases (10 mL each). The retention of the stationary phase was 83%. This method delivered 5 mg of 3′-deoxysappanol (4), 8 mg of 3-deoxysappanone (5) B, 20 mg of 4-O-methylsappanol (8), and 18 mg of brazilin (3) in one step from 120 mg of an ethyl acetate extracted fraction of C. sappan (Figure 2).

In another instance Ma et al ¹³ reported a method that combines supercritical fluid extraction and HSCCC to extract and purify HIFs from O. japonicus. Thus, in a single operation, 140 mg crude extracts were separated and yielded 15.3 mg of methylophiopogonanone A (9) (96.9% purity), 4.1 mg of 6-formyl-isoophiopogonone A (10) (98.3% purity), and 13.5 mg of 6-formyl-isoophiopogonanone A (11) (97.3% purity) (Figure 2).

Uddin et al have reported a centrifugal partition chromatography (CPC)-based one-step isolation of sappanol (7) and brazilin (3) from C. sappan. ¹⁴ Using the solvent system, ethyl acetate:acetonitrile:water, 1:1:2, v/v, the ethyl acetate-soluble material (350 mg) was subjected to CPC to yield the 2 HIFs.

Theoretical Studies on HIFs

As this class of compounds are more explored in terms of their structural diversity and biological properties, it becomes important to obtain a deeper insight into their properties through theoretical studies. We have observed such studies during the last decade that address the electronic distribution of HIFs. Discrete Fourier Transform (DFT) calculations of nuclear magnetic shielding for optimized geometries of HIFs show good correlations with experimentally measured values. ¹⁵

More effective chiroptical methods are now available^16,17 to solve absolute configuration issues. These involve combinations of electronic and vibrational chiroptical spectroscopic methods and interpretations of measured spectra with the aid of density functional theory calculations.

These methods ¹⁸ were employed in determining the absolute configuration of 5,7-dihydroxy-6-methoxy-3-(9-hydroxy-phenylmethyl)-chroman-4-one (12). This substance had been isolated from the dry leaves of Polygonum ferrugineum, ¹⁹ now renamed as Persicaria ferruginea (Wedd.) Soják (Polygonaceae), and from Polygonum senegalense ²⁰ (Persicaria senegalense (Meisn.) Soják), with widely differing optical rotation values. The former group reported [α]_D −8.67 in dichloromethane, while the latter group reported a value of +41 in methanol. Interestingly, a third group ²¹ had isolated the same substance from Polygonum libatum with [α]_D of zero and presumed the isolate to be a racemic mixture. Batista et al ¹⁸ investigated and first made the unexpected discovery that the differences were a result of the solvents used for the measurements and not due to different ratios of enantiomers. The specific rotation of 12 in chloroform was +44, which is much closer to the value obtained in methanol than to that measured in dichloromethane. These authors used this observation to caution on the risks of comparing optical rotation values in different conditions, even when apparently similar solvents are used. The absolute configuration of 12 was unambiguously assigned as 3R,9R by means of a combination of chiroptical methods and theoretical calculations.

Ophiopogon japonicus and C. sappan

Ophiopogon japonicus (Thunb.) Ker Gawl. and C. sappan (now called Biancaea sappan (L.) Tod. have been the subject of intense investigations during the last 10 years. Ophiopogon japonicus is an important medicinal plant which is native to Central and South China to Vietnam, temperate East Asia to the Philippines and has been introduced to some areas of central South America (Argentina, Uruguay, and Paraguay). Besides its use in the treatment of a variety of ailments, it is considered as a functional food in China and other East Asian countries. The main active ingredients are believed to be steroidal compounds and HIFs. ⁷ The Chen review ⁷ of this plant records 36 HIFs reported from O. japonicus. Further metabolites have been reported since then. Thus, 18 new HIF metabolites have been reported from this taxon since our last review, homoisopogon B-D (14, 18, 19), (3R)-2,3-dihydro-7-hydroxy-5-methoxy-3-(4′-methoxybenzyl)-6,8-dimethyl-4H- chromen-4-one (39), 8-formylophiopogonanone B (42), ophiopogonanone G (61), homoisopogon A (62), (3R)-3-(1,3-benzodioxol-5-ylmethyl)-2,3-dihydro-7-hydroxy-5-methoxy-6- methyl-4H-chromen-4-one (63), (3R)-3-(1,3-benzodioxol-5-ylmethyl)-2,3-dihydro-7-hydroxy-5-methoxy-6,8-dimethyl-4H- chromen-4-one (70), ophiopogoside A (73), ophiopogoside B (75), 4′-O-demethylophiopogonanone E (81), 5,7,2′,3′-tetrahydroxy-6-methyl-8-methoxy-3-(4′-methoxybenzyl)chroman-4-one, (84), ophiopogonanone H (98), 6-formylophiopogonone B (128) 8-formyl-7-hydroxy-5,4′-dimethoxy-6-methylhomoisoflavone (129), ophiopogonone D (130), and ophiopogonone E (131).^22-28

For C. sappan, the native range is the Indian subcontinent to Indo-China. It has been introduced to Nigeria, the Congo, Mozambique, Tanzania, and Uganda in Africa (Accessed September 13, 2018 [http://www.plantsoftheworldonline.org/taxon/urn:lsid:ipni.org:names:481889-1]). The heart wood of C. sappan is the most used part and studies have shown that the main homoisoflavonoid ingredient of the heartwood of this plant is brazilin (3), which is responsible for several of the observed biological activities, which include cytotoxic, antitumor, antimicrobial, antiviral, immunostimulant, and other properties. This plant has been the subject of several studies which have resulted in the identification of many new HIFs including the first report of a Δ^3,4-unsaturated homoisoflavan from a natural source (20), 3′-deoxy-4-O-methylepisappanol (28), (3R,4S)-3,7-dihydroxy-3-(3ʹ-methoxy-4ʹ-hydroxybenzyl)-4-ethoxychroman (caesalpiniaphenol F, 29), (3S,4R)-3,7,2ʹ,3ʹ-tetrahydroxy-3,4-dihydro-9H-indeno[6,5 c]chromene (caesalpiniaphenol E) (142), ²⁹ caesalpinophenols G (84) and H (146), (3R)-3,7-dihydroxy-3-(4′-hydroxy-3′-methoxybenzyl)-chroman-4-one (85), ³⁰ 7,10,11-trihydroxydracaenone (143), epicaesalpin J (144), ³¹ caesappin A (145), and caesappin B (150). ³²

Novel Structures Containing the HIF Skeleton

Novel structures that have been reported during this period will be summarized hereunder. In the earlier review ³ a few HIFs with attachment of sugar units were reported. A 5-O-glucoside, 5-O-rutinoside, and a 5-O-neohesperoside were listed, which were linked to 3-hydroxy-3-benzylchroman-4-ones. These were isolated from Ornithogalum caudatum. During the current review period a few more glycosides have been reported. Simple 7-O-glucosides (74 and 75) were reported from O. japonicus. ²⁶ Two disaccharide glycosides of benzylchroman-4-ones (44 and 45) were also reported from Ledebouria floribunda. ³³ A slightly more complex glycoside, (3R)-5,7-dihydroxy-8-(2ʺ-O-veratroyl-β-d-glucopyranosyl)-3-(4ʹ- hydroxyphenyl)-6-methylchroman-4-one (43) (Figure 3) has been reported from the Araliaceae plant Acanthopanax brachypus. ³⁴

Figure 3

Glycosylated HIFs.

Furthermore, a biogenetically intriguing set of 3 homoisoflavonoid glycosides (133, 134, and 135) (Figure 3) has been reported from Prunus domestica belonging to the Rosaceae family. ³⁵ What is distinct about these 3 compounds is the unique carbon framework which has a methyl substituent at C-2, a Δ^2,3-unsaturation, and glucosylation of a sterically crowded C-4ʹ-hydroxyl group of ring β-propenoic acid, whose β-carbon is attached to C-6, is seen to extend further the homoisoflavonoid carbon framework. Further elaboration takes place where esterification with propyl alcohol gives 134 and with hydroquinone 135. It is to be noted that these unusual glycosides are found in the 2 families, Araliaceae and Rosaceae, where HIFs are unknown or extremely rare. The compounds 133 to 135 showed potent inhibitory activity against α-glucosidase and this is further discussed in a later section on antidiabetic properties of HIFs.

Although prenylated and geranylated flavonoids are very common in certain families of plants, compounds 38, 40, and 41 (Figure 4) are the first examples of prenylated HIFs. These unusual compounds were reported from the bulbs of L. floribunda. ³⁶

Figure 4

Examples of unusual HIFs.

Zhao et al ³⁷ reported the isolation of the 3-benzylchromen 20 (Figure 4) from C. sappan and observed that this may have been the first natural product possessing a double bond located at C-3 and C-4 of a homoisoflavan system. In 2016 came a further report on the isolation of 3 more similar Δ^3,4-unsaturated compounds, but with additional hydroxy and methoxy substituents^38,39 from Caesalpina spinosa and Herreria montevidensis, respectively. Interestingly, He et al also reported 2 more compounds 23 and 24 from the same plant, which contain a carbonyl group at C-2. This compound was identified as a 3-benzyl coumarin, although it can also be characterized as an oxidized homoisoflavonoid. The natural occurrence of 23 and 24 (Figure 4) has a special biogenetic significance and may indicate a new biosynthetic route to coumarins that have not been realized before.

In 2014 Wang and coworkers isolated caesappin A (145) together with caesappin B (146), where the latter is most probably the precursor of 146 (Wang et al., 2014). These compounds are unusual in that there has been a rearrangement of the carbon framework (cf. epihematoxylol B 147 (Lin et al., 2014a)) for which these workers propose the mechanism depicted in Scheme 5 showing how they may be biosynthesized in the plant. Scheme 5. Biogenetic scheme to caesappin A (145).

Classification of HIFs

This report is prepared by following the classification of HIFs proposed in the 2007 review ³ and carefully building on it to update the total number of HIFs that is known from natural sources to date. This will enable readers to consult the 2 reviews and get a full and broader coverage of the subject. Therefore, the HIFs are classified into 5 major groups A to E as restated below and depicted in Figure 5.

Figure 5

The five groups (A-E) of homoisoflavoids.

The first group, A (Table 1), is based on the 3-benzylflavan skeleton, with a new subclass containing Δ^3,4-unsaturation and 3,4-dioxygenation.

Table 1

Group A: 3-Benzylflavan (20-22 and 26-30), Δ^3,4- (13-19 and 23-26) and 3,4-dihydroxy (31) derivatives

No	5	6	7	8	2′	3′	4′	5′		Source [Family] a , part of plant	References
13			OMe				OMe		+30 (3R)	H. montevidensis [A], root	39
14			OMe		OH		OMe		+23	O. japonicus [A], tuber	22
15		OH		OMe			OH		+41.5. 3R	Soymida febrifuga [M], bark	40
16	OMe	Me	OMe				OH		+50.7 (3R)	H. montevidensis [A], root	39
17	OMe	Me	OH				OH		+53.8 (3R)	H. montevidensis [A], root	39
18		Me	OMe		OH		OMe		+18	O. japonicus [A], tuber	22
19		Me	OH			OCH2O			+12.5	O. japonicus [A], tuber	22
20			OH				OH		-	C. spinosa [F], twigs and leaves	38
21			OH			OH	OH		-	C. sappan [F], heartwood	37
22			OH	OMe			OH		-	H. montevidensis [A], root	39
23			OH					OH	-	Anemarrhena asphodeloides [A], root	41
24			OMe					OH	-	C. spinosa [F], twigs and leaves	38
25		OMe	OH				OH		-	S. febrifuga [M], bark	40
26		OH	OH			OH	OH		-	O. japonicus [PP], aerial	42
No	3	4	7	8	2′	3′	4′	5′	Mp, [α]D, R/S	Source [Family] a , part of plant	References
31									-	C. sappan [A], heartwood	45

^aA, Araliaceae; F, Fabaceae; Po, Polygonaceae; Pt, Portulacaceae; M, Meliaceae; PP, Polypodiaceae.

The second group, B (Table 2), consists of the 3-benzyl-chroman-4-ones. The majority of the recently discovered HIFs belongs to this class. Also included in this section are the 3-hydroxy derivatives (Table 3), which are fewer, with just 2 compounds 102 and 103 having 9-hydroxy substituents.

Table 2

Group B-1: 3-Benzylchroman-4-Ones: Compounds 32 to 84.

No	Dioxygenated
	5	6	7	8	2′	3′	4′	5′	Mp, R/S, [α]D	Source [Family] a , part of plant	References
32	-	-	OMe				OH			3R, +30	H. montevidensis [A], root	39
	Trioxygenated
33	OH		OH				OMe		3S, +6	Urginea depressa [A], whole plant	46
34	OH		OMe		OH		OMe		3R, −8	Polygonatum sibircum [A], rhizome	47
35	OMe		OMe		OH				160°C; +0.7	Portulaca oleracea [Pt], [fresh aerial]	48
36	OH	Me	OMe				OH		3R, −42.3	Liriope platyphylla [A], root	49
37	OH		OH	Me			OH		3R, −22.9	Polygonatum odoratum [A], rhizome	50
38	OH	Dimethylchromene					OMe		3R, −1.3	L. floribunda [A], bulb	36
39	OMe	Me	OH	Me			OMe		3R.+60.5	O. japonicus [A], root	23
40	OH	OH	Geranyl				OMe		3R, −23.7	L. floribunda [A], bulb	36
41	OH	OH	Geranyl	Prenyl			OMe		3R, −9.5	L. floribunda [A], bulb	36
42	OH	Me	OH	CHO			OMe		3R, −23	O. japonicus [A], root	25
43	OH	Me	OH	Gly b			OH		3R, −43.5	A. brachypus [Ar], root	34
44	OH		Rha-Glu c				OH		−22.7	L. floribunda [A], bulb	33
45	OH		Rha-Glu c				OMe		−23.7	L. floribunda [A], bulb	33
Tetraoxygenated
46			OH	OMe		-O-CH2O-			3R, −12.3	Sansevieria trifaciata [A], aerial part	51
47		OH			OH	OH		OH	3S, +35	Caesalpinia bonduc [F], bark	52
48	OH		OH			OH	OMe		3R, +67.5	Massonia bifolia [A], bulb	53
49	OH		OMe			OH	OH		3R, −27	Bellevalia flexuosa [A], bulb	54
50	OH		OH			OH	OMe		3S, +5.2	Rhodocodon campanulatus [A], bulb	55
51	OH		OH		OH		OMe	3R	3R, −36.1	P. odoratum [A] rhizome	50
52	OH	OH	OH				OMe		3R, −36	B. flexuosa [A], bulb	54
53			OMe		OH	OH		OH	3S, +33	B. flexuosa [A], bulb	52
54	OMe	OH	OMe				OH		-	Scilla nervosa [A], bulb	56
55	OH	OMe	OMe		OH				+0.5	P. oleracea [Pt], aerial part	48
56	OH		OMe		OH		OMe		3R, −8.0	P. sibircum [A], rhizome	47
57	OMe		OH	OMe			OMe		3R, +41.5	D. cochinchinensis [A], resin	11
58	OMe	OMe	OMe		OH				+0.3	P. oleracea [Pt], aerial part	48
59	OH	OMe	OMe				OMe		-	S. nervosa [A], bulb	56
60	OMe	OMe	OMe				OMe		3S, +9	U. depressa [A], whole plant	46
61	OH	Me	OH		OH		OH		3R, −15.3	O. japonicus [A], root	24
62	OH	Me	OMe		OH		OMe		3R, −25	O. japonicus [A], tuber	22
63	OMe	Me	OH			-O-CH2O-			3R, +42.3	O. japonicus [A], root	23
64	OH	Me	OH			OH	OH		3R, −4.24	Bessera elegans [A], bulb	57
65	OH		OH	Me	OH		OH		+2.8	Gan luo Xin d	58
66	OH	Me	OMe		OH		OH		3R, −14.6	L. platyphylla [A], root	49
67	OH		OH	Me	OH		OH		3R, −24.7	P. odoratum [A], rhizome	50
68	OH		OH	Me	OH		OMe		3R, −8.2	Polygonatum cyrtonema [A], rhizome	59
69	OH	Me	OH	Me	OH		OH		−2.1	Gan luo Xin d	58
70	OMe	Me	OMe			OCH2O			3R, +67.1	O. japonicus [A], root	23
71	OH	Me	OH	Me		OH	OMe		229°C to 230°C; ( e ) −44.9	P. odoratum [A], root	60
72	OH	Me	OH	Me	OH		OMe		168°C to 169°C, (±)	P. odoratum [A], root	60
73	OH	Me	OH		OH		OMe		3R, −16.4	P. cyrtonema [A], rhizome	61
74	OH	Me	OGlc	CHO		OCH2O			−81.4	O. japonicus [A], root	26
75	OMe	Me	OGlc	CHO		OCH2O			−127.7	O. japonicus [A], root	26
	Pentaoxygenated
76	OH	OMe	OMe			OH	OH		3S, +17.4	Scilla scilloides [A], bulb	62
77	OH	OH	OMe	OMe			OH		3R, −14	Bellevalia eigii [A], bulb	63
78	OH	OH	OMe			OH	OMe		3S, +29.4	R. campanulatus [A] bulb	55
79	OH		OMe	OMe		OH	OH		3R, −86	B. eigii [A], bulb	63
80	OMe		OMe	OH		OH	OH		102°C to 104°C	Scilla persica [A], bulb	64
81	OH	Me	OH	OMe	OH		OMe		-	O. japonicus [A], rhizome	27
81b	OH	Me	OH	OMe	OH		OMe		−58.6	P. odoratum [A], rhizome	65
82	OH		OMe	OMe		OH	OMe		3R, −66	B. eigii [A], bulb	63
83	OMe	OMe	OMe			OCH2O			3S, +12	U. depressa [A], whole plant	46
	Hexaoxygenated
84	OH	Me	OH	OMe	OH	OH	OMe		3R, −14.4	O. japonicus [A], root	26

^aA, Asparagaceae; Ar, Araliaceae; F, Fabaceae; Po, Polygonaceae; Pt, Portulacaceae.

^bSubstitutent at C-8 is a 2″-veratroyl-substituted-C-glucoside.

^cO-[α-rhamnopyranosyl-(1→6)-β-glucopyranosyl.

^dGan Luo Xin is a traditional Chinese medicine for the treatment of hepatitis B formulated using 20 herbs including Polygonatum sibiricum.

^eThis compound was isolated as 84.9% 3R and 15.1%3S.

Table 3

Group B-2: 3-Hydroxy-3-Benzylchroman-4-Ones and 9-Hydroxy-3-Benzylchroman-4-Ones: Compounds 85 to 102; and Compounds 103 to 104.

	Tri-oxygenated								Mp (°C), 3 R/S [α]D	Source [Family] a , part of plant; 3α/β	References
	5	6	7	8	2′	3′	4′	5′
85			OH			OMe	OH		183°C; 3S, +49.2	C. sappan [F], heartwood, β	66
86	OH		OMe				OH		3S, −63	B. eigii [A], bulb, α	63
87			OH	OMe			OH		3R,	D. cochinchinensis [A], leaves	65
88	OH		OH				OMe		3S, −85	D. cochinchinensis [A], α	11
89	OH		OMe				OMe		3S, −96	B. flexuosa [A], bulb, α	54
90	OH	Me	OH				OMe		3R, −24.0	Dracaena cambodiana [A], stem, β	67
91	OH	Me	OMe				OH		81°C to 83°C; 3R, +178.7	Smilax glabra [S], rhizome, β	68
	Tetraoxygenated
92	OH		OH			OH	OMe		−76.92	Pseudoprospero firmifolium [A], bulb	69
93	OH		OMe	OMe			OH		−170	B. flexuosa [A], bulb, α	54
94	OMe	OMe	OMe				OH		+18	U. depressa [A], whole plant, β	46
95	OH		OMe			OMe	OMe		3S, −86.11	P. firmifolium [A], bulb	69
96			OH	OMe		OCH2O			3S, −28.3	Sansevieria trifasciata [A], aerial β	51
97	OMe	OMe	OMe				OMe		3S, +26	U. depressa [A], whole plant, β	46
98	OH	Me	OH	Me		OCH2O			3R, −97.6	Ophiopogon japonicas [A], root, β	28
99	OH		OMe	OMe		OMe		OMe	−131.6	P. firmifolium [A],	69
100	OH		OMe	OMe		OH	OMe		−75.0	P. firmifolium [A], bulb	69
101	OH	OH	OMe			OH	OMe		−38.46	P. firmifolium [A], bulb	69
102	OMe	OMe	OMe			OCH2O			3R, +23	U. depressa [A], whole plant, β	46
103	OH	OMe	OH				OH		240°C to 242°C	Lespedeza juncea [F], aerial	70
104	OH	OMe	OH	Me			OH		190°C to 192°C	L. juncea [F], aerial	70

^aA, Asparagaceae; F, Fabaceae; R, Rosaceae; S, Smilacaceae.

Group C compounds have the Δ^3,9 with either E or Z stereochemistry at the Δ^3,9-double bond and derivatives (Table 4). Many HIFs in this subgroup belong to the E series. The observation made in the previous review where the Δ^2,33 unsaturated HIFs were largely restricted to the genus Ophiopogon is repeated once again.^24-26,28,35

Table 4

Group C: Δ^3,9-Z (105-106), E (107-127), and Δ^2,3-Unsaturated (133-135) -3-Benzylchroman-4-Ones.

105			OH	OH			OMe		-	Caesalpinia diigyna [F], root	71
106			OH	OMe			OMe		-	Caesalpinia millettii [F], stem	72
107			OMe				OMe		130°C to 132°C	Caesalpinia pulcherima [F], whole	73
108	OH		OMe		OH				146°C	P. oleracea [Po], aerial	48
109			OH			O-CH2O			-	C. pulcherima [F], aerial	74
110			OMe			O-CH2O			-	C. pulcherima [F], aerial	74
111			OH			OH	OMe		-	C. pulcherima [F], aerial	74
112			OMe			OMe	OMe		-	C. pulcherima [F], aerial	74
113	OH		OGlc				OH		−67.4	P. odoratum [A], root	60
114	OH	Me	OH	Me			OH		217°C to 218°C	P. odoratum [A], root	60
115	OH		OH			OH	OH		-	Ledebouria ovatifolia [A], bulb	75
116	OH		OMe			OH	OH		-	Massonia biolia [A], bulb	53
117	OH		OMe	OMe			OH		-	B. eigii [A], bulb	63
118			OH			OMe	OMe	OMe	202°C to 204°C	C. pulcherima [F], whole	73
119		OMe	OMe			OH	OMe		168°C to 170°C	C. pulcherima [F], aerial	74
120	OH	Me	OH	Me		OH	OH		233°C to 234°C	P. odoratum [A], rhizome	76
121	OH	OH	OH	Me			OH		-	Eucomis pallidiflora subsp pole-evansii [A], bulb	77
122			OH		OH	OH	OH	OH	-	C. sappan [F], heartwood	30
123	OH		OMe	OMe		OH	OH		-	B. eigii [A], bulb	63
124	OH	OH	OH			OH	OMe		-	R. campanulatus [A], bulb	55
125	OH	Me	OH				OH		-	A. asphodeloides [A], rhizome	41
126	OMe	Me	OMe	OH		OH	OH		-	S. persica [A], bulb	78
127	OH	Me	OH	OMe		OH	OH		250°C to 251°C	P. odoratum [A], rhizome	73
128	OH	CHO	OH	Me			OMe		-	O. japonicus [A], root	25
129	OMe	Me	OH	CHO			OMe		-	O. japonicas [A], root	25
130	OH	Me	OH		OH		OH		-	O. japonicus [A], root	24
131	OH	Me	OH	OMe	OH		OH		-	O. japonicus [A], root	28
132			OH		OH	OH	OH	OH	-	C. sappan [F], heartwood	30
133	See structure R = H								+14.5	P. domestica [R] shoot	35
134	See Structure R = Propyl								+16.0	P. domestica [R] shoot	35
135	See structure R = p-hydroxyphenyl								+24.0	P. domestica [R] shoot	35

^aA, Asparagaceae; F, Fabaceae; Po, Polygonaceae; R, Rosaceae.

Group D is the scillascillins (Table 5). Only 6 HIFs in this class (136-141) are reported in this period.

Table 5

Group D: Scillascillins: Compounds 136 to 141.

No	5	6	7	8	2′	3′	4′	5′	Mp; [α]D	Source [Family], a part of plant	References
136	OH	Me	OH			OH	OMe		+6.96	B. elegans [A], bulb	57
137	OH		OH			OH	OMe	OMe	+65.6	S. scilloides [A], bulb	62
138	OH		OMe			OH	OMe	OH	+32	Drmiopsis barteri [A], bulb	79
139	OH		OMe			OH	OMe	OMe	+40	Ledebouria socialis [A] bulb	75
140	OH		OH			OMe	OMe	OMe	197°C to 199°C;	D. barteri [A], bulb	79
141	OH		OMe			OMe	OMe	OMe	+55.0	D. barteri [A], bulb	79

^aA = Asparagaceae.

Group E contains the rearranged polycyclic HIFs (Figure 6). Ten HIFs (130-138) with 5 carbon skeletons have been reported from 2 species, C. sappan and Haematoxylon campechianumi.

Figure 6

Group E: Rearranged HIFs: Compounds 142 to 150.

Biological Activities of HIFs

Pharmacological Properties of Brazilin

A recent review ⁶ has dealt with the pharmacological activities of the homoisoflavonoid brazilin. This review states: Brazilin is the safe natural compound having potential to develop as a medicinal compound with application in food, beverage, cosmetics and pharmaceutical industries to screen its clinical use in modern medicine but cautions that more studies are needed to evaluate the potential application of brazilin as preservative and coloring agent in food processing industries. Brazilin is reported to have a high antioxidant property (IC₅₀8.8 µM) comparable with or better than (+)-catechin (10.2 µM). It has also been found to be effective against drug-resistant Gram-positive bacteria including MRSA, vancomycin-resistant enterococci, and multi-drug-resistant Burkholderia cepacian. Brazilin showed antibacterial activity with a Minimum inhibitory concentration of 0.50 mg/mL. The 50% inhibitory concentration (IC₅₀) for lipase inhibition was lowest for brazilin (6 µM), which showed strong inhibition compared with chloramphenicol (677 µM, positive control). ⁸⁰ Experimental evidence is outlined in the Nirmal review supporting the beneficial hypoglycemic role of brazilin by enhancing glucose metabolism in adipose tissue, by inhibiting protein kinase C and insulin receptors serine kinase, and by inducing glucose transport in isolated rat epididymal adipocytes, including details on its mechanism of action. ⁶ The vasorelaxing property of brazilin has also been documented. Brazilin relaxed phenylephirine-induced vasoconstriction and increased cGMP in isolated rat aorta. More recently, brazilin is reported to increase the life span of the nematode worm Caenorhabditis elegans. The mean life of 12.3 ± 0.4 days was increased to 14.5 ± 0.4 days when the worms were fed 100 µM brazilin. ⁸¹

Antioxidant and Anti-Inflammatory Properties

The antioxidant and anti-inflammatory properties of several HIFs have been reported using the simple 2,2-diphenyl-1-picrylhydrazyl (DPPH) method, the inhibition of nitric oxide (NO) or the suppression of inducible nitric oxide (iNOS) synthase, inhibiting prostaglandin (PGE₂), interleukin (IL)−1b production, etc. Ledebouria floribunda has been identified as a source of HIFs, some with prenyl and geranyl groups and others with glycosidic linkages.^33,36 The 3 prenylated and geranylated derivatives (38, 40, and 41) are the first reports of HIFs with triple (acetate, shikimate, and mevalonate) biosynthetic pathways. These were assessed for their DPPH scavenging properties (Table 6) and were found to be either stronger or equal to the usual reference antioxidants like Butylated Hydroxytoluene (BHT) and Butylated hydroxyanisole (BHA). The expected activity due to the ortho dihydroxy groups is probably enhanced by other factors introduced by the prenyl and geranyl groups.

Table 6

Antioxidant Data of HIFs Using DPPH Radical Scavenging (System A), and B-Carotene/Linoleic Acid (System B) Assay Systems.

HIF	System AIC50 (μg/mL)	System B	References
2	3.11	57.0	33
3	8.8	-	82
6	14.5	-	82
38	112	-	36
40	23.2	-	36
41	28.7	-	36
44	212.4	26.0	36
45	273.1	11.4	36
151	84.3	41.9	36
152	39.4	64.5	36
154	80.2	42.0	36
BHT	103		33
BHA	98.7		33

Calvo et al ³⁶ also reported HIFs with sugar groups attached to the C-7 phenolic group of ring A. Their antioxidant properties have been assessed using both the DPPH scavenging as well as the β-carotene/linoleic acid system. The results showing strong antioxidant properties are shown in Table 6.

Hung et al ²⁶ investigated the effect of HIFs on the release of the inflammatory chemokine eotaxin, stimulated by IL-4 and in combination with TNF-α in BEAS-2B cells, which mimics the in vivo conditions in bronchial allergic asthma. They investigated the ability of HIF ophiopogoside A (62), ophiopogoside B (63), and ophiopogonanone G (72) to regulate cytokine-induced eotaxin expression in the human bronchial epithelial cell line BEAS-2B. IL-4 stimulation (20 ng/mL) of BEAS-2B cells for 48 hours increased eotaxin production from 10.7 to 60.5 pg/mL. The HIFs at 1.0, 5.0, 10.0, and 25.0 µM significantly downregulated IL-4-induced eotaxin production in a dose-dependent manner. At a concentration of 25 µM, ophiopogoside A (62), ophiopogoside B (63), and ophiopogonanone G (69) reduced eotaxin production to 30.8, 28.5, and 25.5 pg/mL, respectively. It was also shown that these compounds could inhibit eotaxin production in the combination of IL-4 and TNF-α stimulation; the cells were pre-incubated with compounds 62 and 63 for 2 hours and then activated with IL-4 (10 ng/mL) and TNF-α (10 ng/mL). The BEAS-2B cells produced 28.1 pg/mL eotaxin in the resting stage, but, after stimulation, eotaxin production increased to 456.2 pg/mL

The anti-inflammatory properties of sappanone A (159) were assessed by measuring its ability to inhibit the production of NO, PGE₂, and IL-6, as well as the expression of iNOS, cyclooxygenase-2 (COX-2), and IL-6 in lipopolysaccharide (LPS)-stimulated RAW 264.7 cells. ⁸³ These authors present evidence that leads to the conclusion that the anti-inflammatory activity of sappanone A occurs by activating the Nrf2/HO-1 pathway and by suppressing LPS-induced NF-κB activation. Their investigations have provided mechanistic details underlying the anti-inflammatory activities of sappanone A and suggest that this natural product may be useful for either the prevention or treatment of Nrf2- and/or NF-κB-dependent pathological conditions, such as inflammatory diseases.

Antibacterial, Antiviral, and Antifungal Activities

An interesting study by Jeong et al ⁴⁵ presents the antiviral activities of closely related HIFs assessed against neuraminidases (NAs) on the surface of influenza viruses (A/PR/8/34 [H1N1], A/Hong Kong/8/68 [H3N2], and A/Chicken/Korea/MS96/96 [H9N2]) and confirmed by the positive control with oseltamivir (IC₅₀ = 5.8 nM [H1N1], 5.6 nM [H3N2], and 1.2 nM [H9N2]). The HIFs included in these studies were brazilin (3), sappanone B (6), 3-deoxysappanone B (5), sappanol, episappanol, 4-O-methylsappanol (8), 3-deoxy-4-O-methylsappanol, 4-O-methylepisappanol, 4-(7-hydroxy-2,2-dimethyl-9βH-1,3,5-trioxa-cyclopenta[α]naphtalen-3α-methyl)-benzen-1,2-diol (17), 3,4,7-trihydroxy-3-benzyl-2H-chromene, and brazilin (3). Brazilin and sappanone A (159) were the most active with IC₅₀ (µM) values of 0.2 and 0.7 against [H1N1], 0.3 and 1.1 against [H3N2], and 0.4 and 1.0 for [H9N2]. These 2 were in a class of their own and were singled out as lead compounds with high potency against 3 viral NAs. The study also suggested that the α, β-unsaturated group of sappanone A (159) (Figure 7) was critical for activity as the others without this functional group showed higher IC₅₀ values in the range 63 to 204 µM. A further observation was also that the 4R HIF 4-O-methylepisappanol was more active with IC₅₀ values of 63.2 µM [H1N1, H3N2] and 42.8 µM for [H9N2] than the 4S isomers 4-O-methylsappanol (8) with IC₅₀s of 94.5 µM [H1N1], 134.7 [H3N2], and 99.6 µM for [H9N2]. Similar values were obtained for sappanol and episappanol. ²⁹ Cruz et al ⁸⁴ tested various compounds, including brazilin (3) and hematoxylin (161), isolated from the methanolic extract of Haematoxylon brasiletto against 12 bacteria and the fungus Candida albicans, but found these 2 compounds only weakly active against the tested organisms.

Figure 7

Example of antiviral HIF.

Antihyperglycemic Properties

The hypoglycemic properties of HIFs have been assessed in terms of their ability to modulate the insulin receptor, lower blood glucose, and regulate the levels of various kinases (protein kinase C, phosphotidylinositol 3-kinase, pyruvate kinase, and 6-phosphofructo-2 kinase), which are involved in the gluconeogenesis and glycolytic pathways and glucose transport. ⁶⁰

The 2 protein components of the glucose absorption system of the intestine are the sodium-dependent glucose transporter-1 (SGLT1), and glucose transporter (GLUT). Several HIFs have been evaluated for their ability to either activate adenosine monophosphate-activated protein kinase (AMPK) or to inhibit the activity of the GLUT transmembrane carriers.^85,86 Adenosine monophosphate protein kinase activators are potential therapeutic candidates for the treatment of diabetes as they can increase the uptake of glucose via membrane translocation of GLUT 4. Six previously known HIFs from the rhizomes of P. odoratum were evaluated for their ability to activate AMPK, and 3 of them (153, 154, and 157) exhibited significant activation effects. 153, 154, and 157 (Figure 8) also emerged as the most effective of 19 other HIFs as novel potent GLUT2 inhibitors. In addition, they also demonstrated synergistic inhibition of glucose transport when combined with previously identified SGLT1 specific antagonists. ⁸⁵

Figure 8

Examples of antihyperglycemic HIFs.

In another earlier study ⁸⁷ 2 of the HIFs, 155 and 157, were identified as the active constituents of P. odoratum acting as peroxisome proliferator-activator receptor agonists (PPARAs) by binding to PPARc ligand-binding pocket. The authors caution the herbal use of the plant against side effects resulting from PPARC activation.

The 3 esters, prunuside A to C, with a rather unusual substitution and carbon framework, were obtained from the shoots of the Rosaceae plant, P. domestica. The source of these prunusides is a tree whose fruits are edible and used to lower blood sugar in India and Pakistan. The HIFs (133-134) were assayed for α-glucosidase inhibitory activity. ³⁵ It is known that α-glucosidase inhibitors, such as acarbose, miglitol, and voglibose, can reduce the rate of cleavage of glucose from dietary carbohydrates and hence suppress postprandial hyperglycemia. Accordingly, Kosar et al ³⁵ assessed the α-glucosidase inhibitory activity of 133 to 134 and found the IC₅₀ ± SEM (μM) to be: 216.6 ± 0.027, 268.4 ± 0.047, and 203.6 ± 1.700, respectively. The value obtained for the positive control, deoxynojirimycin, used in the study was 281.3 ± 2.8. All the esters were more active than the control, particularly 134, which is probably due to the pharmacophoric contribution of the hydroquinone moiety.

Adenosine monophosphate protein kinase activation is known to affect glucose and lipid metabolism, gene expression, and protein synthesis. Two out of 6 HIFs isolated from the rhizomes of P. odoratum, (3R)-5,7-dihydroxyl-6-methyl-8-methoxyl-3-(4ʹ-hydroxybenzyl)-chroman-4-one and (3R)-5,7-dihydroxyl-6,8-dimethyl-3-(4ʹ-hydroxybenzyl)-chroman-4-one were found to have significant activation effects. ⁸⁶

Cytotoxic Activities

The cytotoxicity of HIFs on various cell lines, including tumor cells has been documented.

Uesawa et al ⁸⁸ undertook a SUGITA3 quantitative structure-activity relationships of 16 HIFs (3-benzylidinechromanones) and concluded that those 3-benzylidenechromanone derivatives that have a methoxy group at position C-7 of the chromanone ring and either hydroxyl or methoxy group at C-4ʹ of the B ring showed relatively higher tumor-specificity values, exceeding those of doxorubicin and 5-fluorouracil. The paper concluded by pointing out that molecular shape, size and polarization are useful indicators for the evaluation of tumor specificity of this class of compounds. Compound 132 (caesalpiniaphenol G) isolated from the heartwood of Vietnamese C. sappan contains a hydroxyl group at C-7 and 4 such groups in ring B. ³⁰ It was isolated together with the highly modified caesalpiniaphenol and structurally rearranged compound caesalpiniaphenol H (148). Both compounds showed potent inhibitory activity against HL-60 cancer cell lines with respective IC₅₀ values of 16.7 and 22.5 µg/mL. Treating HL-60 cells with 132 resulted in inhibition of growth and introduction of apoptosis.

The anticancer potential of intricatinol (160) (Figure 9) in combination with the known anticancer drug cisplatin in nonsmall-cell lung carcinoma (A549 cells) was demonstrated by the observed apoptosis showing DNA fragmentation and Annexin V positive cells. It was shown that treatment with the HIFs alone reduced the viability of the A549 cells in a dose-dependent manner, as reported by Singh et al ⁸⁹

Figure 9

Cytotoxic HIFs.

In studying the roles of protein tyrosine kinase (PTK) inhibitors for cancer chemoprevention, Lin et al^90,91,108 found that hematoxylin (161) was a “most remarkable” c-Src inhibitor in an orthogonal compound-mixing library of 32 200 compounds, and that the inhibition was associated with PTK phosphorylation and subsequent downstream signaling pathways. Subsequently, ⁹² brazilein, an oxidized form of hematoxylin, was found to inhibit adipocyte differentiation, inducing apoptosis through capsase-3 activity.

In a recent study Yang et al ⁹³ investigated the antitumor properties of protosappanin B (134). Both in vitro, by trypan blue exclusion and MTT assays, and in vivo, on H22 mouse liver cancer cells invasion and the survival of tumor-bearing mice were investigated. In both cases, they found that protosappanin B (134) exerts marked antitumor effects.

Antiproliferative activity guided investigation of the South African plant U. depressa led to the isolation of 6 new HIFs 33, 60, 83, 94, 97, 102 and all except 33 were shown to possess good antiproliferative activity against the A2780 ovarian cancer, A2058 melanoma, and H522-T1 human nonsmall-cell lung cancer cells, and urgineanin A (97) had submicromolar activity against all 3 cell lines. ⁴⁶

The finding that 5,7-dihydroxy-3-(4ʹ-hydroxy-3ʹ-methoxybenzyl)-6-methoxyhomoisoflavanone (cremastrone) possesses antiangiogenic activity both in vitro and in vivo and was a potent inhibitor of the proliferation of human umbilical vein endothelial cells, ⁹⁴ prompted further work to identify other natural HIFs and their derivatives which may be developed as small molecule therapeutic agents.^95,96 Among natural HIFs methylophiopogonanone A (9) and B have been identified as potentially significant hits. The former (9) has been shown to protect against cerebral ischemia/reperfusion injury and attenuates blood-brain barrier disruption in vitro. ⁹⁷

Synthesis of Naturally Occurring HIFs

The commonly employed methodologies for the construction of the homoisoflavonoid scaffolds can be categorized into 2 general groups:

Aldol condensation of pre-prepared chroman-4-ones with aryl aldehydes (Scheme 1a) provides HIFs (165) with an exoΔ^3,9 double bond. Furthermore, the homoisoflavonoid isomer 166 with the endoΔ^2,3 double bond can be prepared via migration of the exo double bond by treatment with a base or rhodium catalysts;

Acid catalyzed cyclization of dihydrochalcones 167, in the presence of one-carbon extension reagents, such as CH₃SO₂Cl//DMF, HCO₂Et/Na, 2,4,6-trichloro-1,3,5-triazine/DMF, and HC(OEt)₃/HClO₄, yields HIFs 166 with the endoΔ^2,3 double bond (Scheme 1b).

Scheme 1

General protocols for the synthesis of HIFs.

Since our last review, few novel synthetic methodologies have been developed for the construction of the homoisoflavone scaffolds. The cyclobenzylation reaction of (E)-3-(dimethylamino)-1-(2-hydroxy-4,5-methylenedioxy-phenyl)prop-2-en-1-one 168 and benzyl bromide at 80°C in the presence of NaI is a recent methodology for the synthesis of homoisoflavonoid derivatives 172 (Scheme 2). ⁹⁸ The reaction is proposed to proceed as shown in Scheme 2 and involves intramolecular cyclization followed by benzylation of the resulting enolate 170 and subsequent elimination of the dimethyl amine group to provide the homoisoflavonoid 172.

Scheme 2

BnBr, NaI, acetone, 80°C, 5 hours, 80%.

In a serendipitous discovery, Lee and co-workers developed an interesting ruthenium-catalyzed synthesis of 2-arylhomo-isoflavonoid 179 scaffolds by reacting salicylaldehyde and alkynoic acids via C-H activation, as depicted in Scheme 6. ⁹⁹ The reaction is proposed to proceed via initial formation of chalcone 175 that undergoes intramolecular cyclization to provide the flavonoid intermediate 176.

Aldol condensation between the flavonoid 176 and an additional salicylaldehyde provides homoisoflavonoid 178 with an exoΔ^3,9 double bond that rearranges to the endoΔ^2,3 isomer 179. However, the rearrangement reaction from the exoΔ^3,9 double bond to the endoΔ^2,3 double bond was uncontrollable to prepare selectively either the homoisoflavonoid 178 or 179 as desired. Bhagavathy and co-workers reported an interesting tandem thermal [1,3 - 1,3]-para and [1,3]-ortho- rearrangement of chromone-3-ylmethyl aryl ethers for the synthesis of homoisoflavonoids 181 and 182 as shown in Scheme 7. ¹⁰⁰ Despite its simplicity and generality, the protocol has not been applied to the synthesis of natural homoisoflavonoids.

The major advance made in the synthesis of HIFs post 2007 is the focus on asymmetric synthesis. Yu et al, for instance, reported the asymmetric synthesis of several sappanin-type natural HIFs (190 to 194) ¹⁰¹ using Evans’ chiral auxiliaries 183 and 195. ¹⁰² The use of Evans’ chiral auxiliary 183 led to the synthesis of homoisoflavan 190 in >99% ee via aldol, intramolecular cyclization and deoxygenation reactions as key steps, as shown in Scheme 3. By analogy, using Evans’ chiral auxiliary 195, the synthesis of enantiomer 194 was achieved in >099% ee (Scheme 3). Oxidation of 190 using DDQ gave 191, while demethylation of 190 afforded 192 and upon treatment with DDQ provided homoisoflavanone 193 (Scheme 3) (192 and 193 have been reported from commercial Dragon’s blood Dracaena draco and D. cambodiana. ¹⁰³ )

Scheme 3

(i) TiCl₄, (+)-sparteine, DCM, 0°C, 2 hours, 83%; (ii) Pd/C, EtOAc, rt, 5 hours, 95%; (iii) TiCl₄, TMEDA, DCM, −20°C, 2 hours, 62%; (iv) NaBH₄, THF/H₂O, 5 hours, 86%; (v) p-TsCl, n-Bu₂SnO, DMAP, Et₃N, MeCN, rt, 3 hours, 84%; (vi) n-Bu₄NF, THF, rt, 30 minutes, 92%; (vii) Ac₂O, Et₃N, DMAP, DCM, rt, 30 minutes, 94%; (viii) Et₃SiH, CF₃CO₂H, DCM, 0°C, 30 minutes, 88%; (ix) K₂CO₃, MeOH, rt, 30 minutes, 92%; (x) 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), dioxane, H₂O, rt, 30 minutes, 74%; (xi) BBr₃, CH₂Cl₂, rt, 1 hour, 89%; (xii) DDQ, dioxane, H₂O, rt, 30 minutes, 57%.

By exploiting the versatility of Sharpless dihydroxylation and epoxidation reactions, a number of sappanone-type natural HIFs have been prepared. This is demonstrated by Kim and co-workers who successfully synthesized (+)-urgineanin A-D and (−)-urgineanin A (196a-d) (196a-d were isolated from the South African plant U. depressa. ⁴⁶ ), as depicted in Scheme 8. ¹⁰⁴ Palladium-catalyzed allylic arylation of 197 with arylboronic acids provided coupled product 198. Asymmetric dihydroxylation using (DHQ)₂PHAL, followed by a regioselective oxidation led to the preparation of (+)-urgineanin A-D (196a-c) while the use of (DHQD)₂PHAL as a dihydroxylating agent afforded (−)-urgineanin A (196d).

Similarly, by changing the substituents at the phosphorus atom of enol phosphates 201, synthesis of enantiopure (R)-(+)-eucomol 202 and (S)-(−)-eucomol 203 (202/203 was first isolated from the bulbs of Eucomis bicolor Bak. ¹⁰⁵ ) was achieved via asymmetric dihydroxylation as shown in Scheme 9. ¹⁰³

In addition to asymmetric dihydroxylation strategies for the synthesis of enantiopure sappanones, an enantioselective cyclization protocol has also been reported. Takikawa and Suzuki designed and synthesized a modified Rovis catalyst 204, a chiral triazolium salt that possesses a strongly electron-withdrawing group and applied it in the enantioselective benzoin cyclization of suitably prepared keto-aldehydes 205 for the preparation of (+)-sappanone B 6 (Scheme 4). ¹⁰⁷ The presence of the strongly electron-withdrawing substituent (–CF₃) on the aromatic ring of the triazolium salt suppresses enolization that leads to formation of side products via aldol reactions. This is due to the ease with which the hydrogen can be abstracted to generate a carbene catalyst by the weaker base, Et₃N, which is not capable of enolizing the substrate. Furthermore, the generated carbene is less basic to enolize the keto-aldehyde 205.

Scheme 4

(i) cat. (15 mol %), Et₃N, toluene, rt, 12 hours, 92%, 95% ee; (ii) NaSC₁₂H₂₅, DMF, 80°C, 5 hours, 92%; (iii) BBr₃, DCM, 0°C, 0.5 hour, 85%.

Scheme 5

Biogenetic scheme to caesappin A (145).

The epoxidation and dihydroxylation reactions have been also instrumental in the total synthesis of (±)-brazilein, (±)-brazilin, and (±)-brazilane.^108,109 Zhang and co-workers utilized asymmetric Sharpless dihydroxylation reaction for the enantioselective synthesis of (+)-brazilin (3), (−)-brazilein (162), and (+)-brazilide A (214) from a common starting material, as shown in Scheme 10. ¹¹⁰ Accordingly, coupling of indene 207 and phenol 208, followed by asymmetric dihydroxylation led to the formation of diol 210 in 81% ee. Acid catalyzed intramolecular cyclization and deprotection of the benzoate protecting group afforded the key intermediate tetracyclic 211. Demethylation of 211 provided (+)-brazilin (3). DIB-mediated oxidation of (+)-brazilin (3) gave (−)-brazilein (162). Further manipulation of 211 provided 212 that underwent diastereoselective epoxidation to form epoxide 213. Lactonization of epoxide 213 afforded (+)-brazilide A (214) in 82% yield.

In conclusion, HIFs are now recognized as an important subclass of flavonoids. They display broad structural diversity and biological properties. They occur principally in 2 plant families, Asparagaceae and Fabaceae, and several genera including Ophiopogon, Caesalpinia, Polygonatum, Ledebouria, Belevalia, Dracaena, Hematoxylon, Liriope, Portulaca, Scilla, Pseudoprospera, and Urginea. Many species belonging to these genera are important ingredients in many Chinese, Indian, Japanese, and other countries’ traditional medicine. HIFs are the major bioactive constituents of the popular medicinal plants O. japonicus, ⁷ C. sappan, ⁶ and of Dragon’s blood (Dracanea cinnabari). ⁸ Ophiopogon japonicus alone has yielded more than 38 HIFs, ⁷ more than any other species, followed by Caesalpinia, Polygonatum, and Ledebouria species. Homoisoflavonoids have been shown to possess a wide range of pharmacological properties including antimicrobial, antimutagenic, antioxidant, immunomodulatory, cytotoxic, antiangiogenic, anti-inflammatory, antiphotoaging, hypoglycemic, vasorelaxant, hepatoprotective, and antiacne activities. Brazilin (3) is the major and most biologically active of the 4 HIFs found in the heartwood of C. sappan. ⁶ The folkloric uses of brazilin include antioxidant, antibacterial, anti-inflammatory, antiphotoaging, hypoglycemic, vasorelaxant, hepatoprotective, and antiacne applications. The scientific evidence generated for brazilin strongly suggests its potential to be developed as a medicinal compound with application in the food, beverage, cosmetics, and pharmaceutical industries. Brazilin may also have applications as a preservative and coloring agent in the food processing industries. Some of the significant pharmacological data generated during the current review period include those of brazilin (3), sappanone B (6), ⁸⁰ ledebourin B (40), and ledebourin C (41) ³⁶ as antioxidants; sappanone A (159), ⁸³ ophiopogoside A (62), ophiopogoside B (63), and ophiopogonanone G (69) ²⁶ for their anti-inflammatory properties, and brazilin (3) and sappanone A (159) for their exceptional antiviral potencies ²⁹ ; HIFs 155 and 157, ⁸⁷ and 133 to 134 ³⁵ for their potential application as hypoglycemic agents; and methylophiopogonanone A (9), ⁸⁶ urgineanin A (97), ⁴⁶ protosappanin B (134), ⁹³ intricatinol (160), ⁸⁹ and hematoxylin (161) ⁹⁰ for their cytotoxic and tumor-inhibiting applications. The increasing importance of HIFs has also raised the interest of synthetic organic and medicinal chemists. Both the earlier ³ and the current reviews include various synthetic methodologies for the total synthesis of natural HIFs, as well as their derivatives. The development of asymmetric synthesis, transition metal catalyzed coupling reactions as well as H-activation synthetic methodologies is less explored for the synthesis of HIF.

Scheme 6

[Ru(p-cymene)Cl₂]₂ (2.5 mol %), CsOAc, DMSO, 120°C, 12 hours, 64% to 82%.

Scheme 7

Ph₂O, Cs₂CO₃, reflux, 65% to 84%.

Scheme 8

(i) ArB(OH)₂, Pd(PPh₃)₄, K₃PO₄, THF, 150°C; (ii) OsO₄, (DHQ)₂PHAL, K₃Fe(CN)₆, K₂CO₃, MeSO₂NH₂, 0°C; (iii) IBX; (iv) OsO₄, (DHQD)₂PHAL, K₃Fe(CN)₆, K₂CO₃, MeSO₂NH₂, 0°C.

Scheme 9

(DHQD)₂PHAL, OsO₄, CH₃SO₂NH₂, K₃Fe(CN)₆, K₂CO₃, ^t BuOH/H₂O, 0°C.

Scheme 10

(i) DEAD, Ph₃P, THF, −78°C, 60%; (ii) AD-mix-b, OsO₄, MeSO₂NH₂, ^t -BuOH/H₂O/DCM, 3 days, 85%, 81% ee; (iii) PPTS, toluene, 90°C, 80%; (iv) LiOH, THF/MeOH, 67%, 99.8% ee; (v) BBr₃, DCM, 81%; (vi) PhI(OAc)₂, THF, 0°C, 76%; (vii) m-CPBA, 32°C, 3 days, DCM, 22%; (viii) a. BF₃·Et₂O, DCM; b. 65% H₂SO₄, AcOH, 110°C, 82%.