Stereoselective syntheses of five naturally occurring, pharmacologically active medium and large ring lactones are described. Key synthetic methods used were, depending on the cases, olefin metatheses, asymmetric allylations and C-glycosidations.
For some recent reviews on various types of lactones, see: (a) RossiR., BellinaF. (2001) Recent advances in the regio- and stereocontrolled synthesis of natural and unnatural stereodefined 5-ylidene-2(5H)-furanones. Targets in Heterocyclic Systems, 5, 169–198;(b) Barrero AF, Quilez Del Moral JF, Herrador MM. (2003) Podolactones: a group of biologically active norditerpenoids. In Studies in Natural Products Chemistry (Vol. 28, Bioactive Natural Products, Part I), 453-516; (c) van Beek TA. (2005) Ginkgolides and bilobalide: their physical, chromatographic and spectroscopic properties. Bioorganic & Medicinal Chemistry, 13, 5001-5012; (d) Konaklieva MI, Plotkin BJ. (2005) Lactones: generic inhibitors of enzymes?. Mini-Reviews in Medicinal Chemistry, 5, 73-95. (e) De Fatima A, Modolo LV, Conegero LS, Pilli RA, Ferreira CV, Kohn LK, de Carvalho JE. (2006) Styryl lactones and their derivatives: biological activities, mechanisms of action and potential leads for drug design. Current Medicinal Chemistry, 13, 3371-3384; (f) Mirjalili MH, Moyano E, Bonfill M, Cusido RM, Palazón J. (2009) Steroidal lactones from Withania somnifera, an ancient plant for novel medicine. Molecules, 14, 2373-2393.
2.
For some recent reviews on chemical, and pharmacological aspects of several types of macrolides, see: (a) NorcrossR.D., PatersonI. (1995) Total synthesis of bioactive marine macrolides. Chemical Reviews, 95, 2041–2114;(b) Yeung KS, Paterson I. (2005) Advances in the Total Synthesis of Biologically Important Marine Macrolides. Chemical Reviews, 105, 4237-4313; (c) Paterson I, Findlay AD. (2008) Total synthesis of cytotoxic marine macrolides: callipeltoside A, aurisides A and B, and dolastatin 19. Pure and Applied Chemistry, 80, 1773-1782; (d) Lespine A, Dupuy J, Alvinerie M, Comera C, Nagy T, Krajcsi P, Orlowski S. (2009) Interaction of macrocyclic lactones with the multidrug transporters: the bases of the pharmacokinetics of lipid-like drugs. Current Drug Metabolism, 10, 272-288; (e) Murphy DM, Forrest IA, Curran D, Ward C. (2010) Macrolide antibiotics and the airway: antibiotic or non-antibiotic effects? Expert Opinion on Investigational Drugs, 19, 401-414.
3.
(a) ZotchevS.B. (2003) Polyene macrolide antibiotics and their applications in human therapy. Current Medicinal Chemistry, 10, 211–223;(b) Caffrey P, Aparicio JF, Malpartida F, Zotchev SB. (2008) Biosynthetic engineering of polyene macrolides towards generation of improved antifungal and antiparasitic agents. Current Topics in Medicinal Chemistry, 8, 639-653.
4.
(a) GaynorM., MankinA.S. (2003) Macrolide antibiotics: binding site, mechanism of action, resistance. Current Topics in Medicinal Chemistry, 3, 949–960;(b) Zeitlinger M, Wagner CC, Heinisch B. (2009) Ketolides - the modern relatives of macrolides: the pharmacokinetic perspective. Clinical Pharmacokinetics, 48, 23-38.
5.
(a) MulzerJ., ÖhlerE. (2003) Microtubule-stabilizing marine metabolite laulimalide and its derivatives: synthetic approaches and antitumor activity. Chemical Reviews, 103, 3753–3786. (b) Altmann KH, Pfeiffer B, Arseniyadis S, Pratt BA, Nicolaou KC. (2007) The chemistry and biology of epothilones-the wheel keeps turning. ChemMedChem, 2, 396-423.
6.
(a) CardaM., GonzálezF., CastilloE., RodríguezS., MarcoJ.A. (2002) Stereoselective synthesis of the naturally occurring lactones (–)-osmundalactone and (–)-muricatacine using ring-closing metathesis. European Journal of Organic Chemistry, 2649–2655. (b) Ruiz P, Murga J, Carda M, Marco JA. (2005) Stereoselective synthesis of the naturally occurring styryllactones (+)-goniofufurone and (+)-cardiobutanolide. Journal of Organic Chemistry, 70, 713-716.
7.
Saturated δ-lactones: (a) CardaM., CastilloE., RodríguezS., MarcoJ.A. (2000) A stereoselective synthesis of (+)-malyngolide via a ring-closing olefin metathesis. Tetrahedron Letters, 41, 5511–5513;(b) Carda M, Rodríguez S, Castillo E, Bellido A, Díaz-Oltra, S, Marco JA. (2003) Stereoselective synthesis of (+)-malyngolide, (–)- malyngolide and (+)-tanikolide using ring-closing metathesis. Tetrahedron, 59, 857-864.
8.
5,6-Dihydro-2-pyran-2-ones: (a) MarcoJ.A., CardaM., MurgaJ., FalomirE. (2007) Stereoselective syntheses of naturally occurring 5,6-dihydropyran-2-ones. Tetrahedron, 63, 2929–2958, and pertinent references therein; (b) Álvarez-Bercedo P, Falomir E, Murga J, Carda M, Marco JA. (2008) Stereoselective synthesis of the naturally occurring 2-pyranone dodoneine. European Journal of Organic Chemistry, 4015-4018.
9.
Álvarez-BercedoP., MurgaJ., CardaM., MarcoJ.A. (2006) Stereoselective synthesis of the published structure of feigrisolide A. Structural revision of feigrisolides A and B. Journal of Organic Chemistry, 71, 5766–5769.
10.
(a) MurgaJ., FalomirE., García-FortanetJ., CardaM., MarcoJ.A. (2002) Stereoselective synthesis of microcarpalide. Organic Letters, 4, 3447–3449;(b) García-Fortanet J, Murga J, Falomir E, Carda M, Marco JA. (2005) Stereoselective total synthesis and absolute configuration of the natural decanolides (–)-microcarpalide and (+)-lethaloxin. The identity of (+)-lethaloxin and (+)-pinolidoxin. Journal of Organic Chemistry, 70, 9822-9827; (c) García-Fortanet J, Carda M, Marco JA. (2007) Stereoselective synthesis of the bacterial DNA primase inhibitor Sch 642305 and its C-4 epimer. Tetrahedron, 63, 12131-12137.
11.
Aspergillides (14-membered): (a) Díaz-OltraS., Angulo-PachónC.A., KneetemanM.N., MurgaJ., CardaM., MarcoJ.A. (2009) Stereoselective synthesis of the cytotoxic macrolide aspergillide B. Tetrahedron Letters, 50, 3783–3785;(b) Díaz-Oltra S, Angulo-Pachón CA, Murga J, Carda M, Marco JA. (2010) Stereoselective synthesis of the cytotoxic 14-membered macrolide aspergillide A. Journal of Organic Chemistry, 75, 1775-1778; (c) Díaz-Oltra S, Angulo-Pachón CA, Murga J, Falomir E, Carda M, Marco JA. (2011) Synthesis and biological properties of the cytotoxic 14-membered macrolides aspergillide A and aspergillide B. Chemistry-A European Journal, 17, 675-688.
12.
Macrolide FD-891 (16-membered): (a) MurgaJ., García-FortanetJ., CardaM., MarcoJ.A. (2004) Stereoselective synthesis of the C14-C26 fragment of the cytotoxic macrolide FD-891. Tetrahedron Letters, 45, 7499–7501;(b) García-Fortanet J, Murga J, Carda M, Marco JA. (2004) Stereoselective synthesis of the C1-C12 fragment of the cytotoxic macrolide FD-891. Synlett, 2830-2832; (c) García-Fortanet J, Murga J, Carda M, Marco JA. (2006) Stereoselective synthesis of the cytotoxic macrolide FD-891. Organic Letters, 8, 2695-2698; (d) García-Fortanet J, Murga J, Carda M, Marco JA, Matesanz R, Díaz JF, Barasoain I. (2007) The total synthesis and biological properties of the cytotoxic macrolides FD-891 and its non-natural (Z)-C12 isomer. Chemistry-A European Journal, 13, 5060-5074.
13.
For reviews on various synthetic, biosynthetic or pharmacological aspects of medium-ring (8-11 membered) lactones, see: (a) DrägerG., KirschningA., ThierickeR., ZerlinM. (1996) Decanolides, 10-membered lactones of natural origin. Natural Product Reports, 13, 365–375;(b) Shiina I. (2007) Total synthesis of natural 8- and 9-membered lactones: recent advancements in medium-sized ring formation. Chemical Reviews, 107, 239-273; (c) Ferraz HMC, Bombonato FI, Longo LS Jr. (2007) Synthetic approaches to naturally occurring ten-membered-ring lactones. Synthesis, 3261-3285; (d) Ferraz HMC, Bombonato FI, Sano MK., Longo LS Jr. (2008) Natural occurrence, biological activities and synthesis of eight-, nine-, and eleven-membered-ring lactones. Quimica Nova, 31, 885-900; (e) Ishigami K. (2009) Synthetic studies of natural 10-membered lactones, mueggelone, microcarpalide, and Sch642305, which have interesting bioactivities. Bioscience, Biotechnology and Biochemistry, 73, 971-979.
14.
RatnayakeA.S., YoshidaW.Y., MooberryS.L., HemscheidtT. (2001) The structure of microcarpalide, a microfilament disrupting agent from an endophytic fungus. Organic Letters, 3, 3479–3481.
15.
ArnoneA., AssanteG., MontorsiM., NasiniG., RaggE. (1993) Secondary mold metabolites. XLIII. Isolation and structure determination of lethaloxin, a fungal macrolide from Mycosphaerella lethalis. Gazzetta Chimica Italiana, 123, 71–73. The structure of lethaloxin depicted in this paper corresponds to the enantiomer of 2.
16.
ChuM., MierzwaR., XuL., HeL., TerraccianoJ., PatelM., GulloV., BlackT., ZhaoW., ChanT.M., McPhailA.T. (2003) Isolation and structure elucidation of Sch 642305, a novel bacterial DNA primase inhibitor produced by Penicillium verrucosum. Journal of Natural Products, 66, 1527–1530.
17.
Bacterial DNA primases are DNA-dependent RNA polymerases required for the replication of chromosomal DNA, and constitute therefore attractive targets for drug discovery: AraiK.I., KornbergA. (1979) A general priming system employing only dnaB protein and primase for DNA replication. Proceedings of the National Academy of Sciences USA, 76, 4308–4312.
18.
JayasuriyaH., ZinkD.L., PolishookJ.D., BillsG.F., DombrowskiA.W., GenilloudO., PeláezF.F., HerranzL., QuaminaD., LinghamR.B., DanzeizenR., GrahamP.L., TomassiniJ.E., SinghS.B. (2005) Identification of diverse microbial metabolites as potent inhibitors of HIV-1 Tat transactivation. Chemistry & Biodiversity, 2, 112–122.
19.
KitoK., OokuraR., YoshidaS., NamikoshiM., OoiT., KusumiT. (2008) New cytotoxic 14-membered macrolides from marine-derived fungus Aspergillus ostianus. Organic Letters, 10, 225–228.
20.
For reviews on cross metathesis, see: (a) Connon SJ, Blechert S. (2003) Recent developments in olefin cross-metathesis. Angewandte Chemie International Edition, 42, 1900–1923;(b) Chatterjee AK, Choi TL, Sanders DP, Grubbs RH. (2003) A general model for selectivity in olefin cross metathesis. Journal of the American Chemical Society, 125, 11360-11370; (c) Vernall AJ, Abell AD. (2003) Cross metathesis of nitrogen-containing systems. Aldrichimica Acta, 36, 93-105; (d) Schrodi Y, Pederson RL. (2007) Evolution and applications of second-generation ruthenium olefin metathesis catalysts. Aldrichimica Acta, 40, 45-52.
21.
For reviews on ring-closing metathesis, see: (a) Fürstner A. (2000) Olefin metathesis and beyond. Angewandte Chemie International Edition, 39, 3012–3043;(b) Jafarpour L, Nolan SP. (2001) Transition-metal systems bearing a nucleophilic carbene ancillary ligand: from thermochemistry to catalysis. Advances in Organometallic Chemistry, 46, 181-222; (c) Trnka TM, Grubbs RH. (2001) The development of L2X2Ru=CHR olefin metathesis catalysts: an organometallic success story. Accounts of Chemical Research, 34, 18-29; (d) Love JA. (2003) Olefin metathesis strategies in the synthesis of biologically relevant molecules. In Handbook of Metathesis. Vol. 2, Grubbs, RH (Ed). Wiley-VCH, Weinheim, Germany, 296-322; (e) Grubbs RH. (2004) Olefin metathesis. Tetrahedron, 60, 7117-7140; (f) Astruc D. (2005) The metathesis reactions: from a historical perspective to recent developments. New Journal of Chemistry, 29, 42-56; (g) Hoveyda AH, Zhugralin AR. (2007) The remarkable metal-catalysed olefin metathesis reaction. Nature, 450, 243-251.
22.
For previous synthesis of microcarpalide, see ref. [13e].
23.
GuivisdalskyP.N., BittmanR. (1989) Regiospecific opening of glycidyl derivatives mediated by boron trifluoride. Asymmetric synthesis of ether-linked phospholipids. Journal of Organic Chemistry, 54, 4637–4642.
24.
(a) LipshutzB.H., SenguptaS. (1992) Organocopper reagents: substitution, conjugate addition, carbo/metallocupration, and other reactions. Organic Reactions, 41, 135–631;(b) Krause N. (Ed) (2004) Modern Organocopper Chemistry, Wiley-VCH, Weinheim, Germany.
25.
WutsP.G.M., GreeneT.W. (2007) Greene's Protective Groups in Organic Synthesis (4. Ed). John Wiley and Sons, New York, 30–38.
26.
(a) MancusoA.J., SwernD. (1981) Activated dimethyl sulfoxide: useful reagents for synthesis. Synthesis, 165–185; (b) Tidwell TT. (1990) Oxidation of alcohols to carbonyl compounds via alkoxysulfonium ylides: the Moffatt, Swern, and related oxidations. Organic Reactions, 39, 297-572. Racemization of aldehyde (S)-10 (ee ≥ 98%) was minimized when N,N-diisopropyl ethylamine (DIPEA) was used as the base: Dondoni A, Perrone D. (1997) Synthesis of N-(tert-butoxycarbonyl)-N,O-isopropylidene serinal from serine methyl ester by a reduction-oxidation sequence. Synthesis, 527-529.
27.
For reviews on reactions with allyl tin reagents, see: (a) NishigaichiY., TakuwaA., NarutaY., MaruyamaK. (1993) Versatile roles of Lewis acids in the reactions of allylic tin compounds. Tetrahedron, 49, 7395–7426;(b) Yamamoto Y, Shida N. (1994) Stereochemistry and mechanism of allylic tin-aldehyde condensation reactions. In Advances in Detailed Reaction Mechanisms, Vol. 3, JAI Press, Inc., 1-44.
28.
Acid 12 was prepared from (S,S)-tartaric acid through modification of a described procedure: BattyD., CrichD. (1992) Acyl radical cyclizations in synthesis. Part 4. Tandem processes: the 7-endo/5-exo serial cyclization approach to enantiomerically pure bicyclo [5.3.0]decan-2-ones. Journal of the Chemical Society, Perkin Transactions I, 3193–3204.
29.
(a) MikołtajczykM., KiełtbasińskiP. (1981) Recent developments in the carbodiimide chemistry. Tetrahedron, 37, 233–284;(b) Mulzer J. (1991) Synthesis of esters, activated esters and lactones. In Comprehensive Organic Synthesis. Trost BM, Fleming I, Winterfeldt E (Eds). Vol. 6, Pergamon Press, Oxford, 323-380.
30.
For reviews on the uses of RCM for the synthesis of macrocycles, see: (a) PrunetJ. (2003) Recent methods for the synthesis of (E)-alkene units in macrocyclic natural products. Angewandte Chemie International Edition, 42, 2826–2830;(b) Gradillas A, Pérez-Castells J. (2006) Macrocyclization by ring-closing metathesis in the total synthesis of natural products: reaction conditions and limitations. Angewandte Chemie International Edition, 45, 6086-6101; (c) Majumdar KC, Rahaman H, Roy B. (2007) Synthesis of macrocyclic compounds by ring-closing metathesis. Current Organic Chemistry, 11, 1339-1365.
31.
(a) FürstnerA., RadkowskiK., WirtzC., GoddardR., LehmannC.W., MynottR. (2002) Total syntheses of the phytotoxic lactones herbarumin I and II and a synthesis-based solution of the pinolidoxin puzzle. Journal of the American Chemical Society, 124, 7061–7069;(b) Liu D, Kozmin SA. (2002) Synthesis of (–)-pinolidoxin: divergent synthetic strategy exploiting a common silacyclic precursor. Organic Letters, 4, 3005-3007.
32.
For reviews on non-metathetic reactions catalyzed by ruthenium complexes, see: (a) TrostB.M., TosteF.D., PinkertonA.B. (2001) Non-metathesis ruthenium-catalyzed C–C bond formation. Chemical Reviews, 101, 2067–2096;(b) Schmidt B. (2003) Ruthenium-catalyzed cyclizations: more than just olefin metathesis! Angewandte Chemie International Edition, 42, 4996-4999; (c) Alcaide B, Almendros P. (2003) Non-metathetic behavior patterns of Grubbs’ carbene. Chemistry-A European Journal, 9, 1258-1262; (d) Schmidt B. (2004) Catalysis at the interface of ruthenium carbene and ruthenium hydride chemistry: organometallic aspects and applications to organic synthesis. European Journal of Organic Chemistry, 1865-1880; (e) Arisawa M, Terada Y, Takahashi K, Nakagawa M, Nishida A. (2007) Non-metathesis reactions of ruthenium carbene catalysts and their application to the synthesis of nitrogen-containing heterocycles. The Chemical Record, 7, 238-253.
33.
Theoretical calculations were first performed at the semiempirical level (AM1) and gave a difference in energy contents of 2 kcal/mol between both stereoisomers. When the calculations were made with ab initio methods (HF/3-21G), the difference turned out to be 1.9 kcal/mol.
34.
SinhaS.C., KeinanE. (1997) Total synthesis of (+)-aspicilin. The naked carbon skeleton strategy vs the bioorganic approach. Journal of Organic Chemistry, 62, 377–386.
35.
MoonH.R., ChoiW.J., KimH.O., JeongL.S. (2002) Improved and alternative synthesis of D- and L-cyclopentenone derivatives, the versatile intermediates for the synthesis of carbocyclic nucleosides. Tetrahedron: Asymmetry, 13, 1189–1193.
36.
(a) MitsunobuO. (1981) The use of diethyl azodicarboxylate and triphenylphosphine in synthesis and transformation of natural products. Synthesis, 1–28; (b) Hughes DL. (1992) The Mitsunobu reaction. Organic Reactions, 42, 335-656. (c) Valentine DH, Jr., Hillhouse JH. (2003) Alkyl phosphines as reagents and catalysts in organic synthesis. Synthesis, 317-334.
37.
DixonD.J., LeyS.V., ReynoldsD.J. (2002) The total synthesis of the Annonaceous acetogenin, muricatetrocin C. Chemistry-A European Journal, 8, 1621–1636.
38.
InanagaJ., HirataK., SaekiH., KatsukiT., YamaguchiM. (1979) A rapid esterification by means of mixed anhydride and its application to large-ring lactonization. Bulletin of the Chemical Society of Japan, 52, 1989–1993.
39.
As in the case of microcarpalide, catalyst Ru-II provided only the undesired Z-23 in 72% yield. Again, the preferential formation of the more stable Z-isomer is due to thermodynamic control of the RCM process by catalyst Ru-II.
40.
After selective cleavage of the MOM group in olefin Z-23, crystalline hydroxy lactone Z-24 was formed in 70% yield. Its structure and absolute configuration were confirmed by means of an X-ray diffraction analysis.
41.
NaitoH., KawaharaE., MarutaK., MaedaM., SasakiS. (1995) The first total synthesis of (+)-bullatacin, a potent antitumor annonaceous acetogenin, and (+)-(15,24)-bisepi-bullatacin. Journal of Organic Chemistry, 60, 4419–4427.
42.
EvidenteA., LanzettaR., CapassoR., VurroM., BottalicoA. (1993) Pinolidoxin, a phytotoxic nonenolide from Ascochyta pinodes. Phytochemistry, 34, 999–1003.
43.
For a discussion of previous incorrect structural representations of lethaloxin, see ref. [10b].
44.
For previous syntheses of 3, see: (a) Mehta G, Shinde HM. (2005) Enantioselective total synthesis of bioactive natural product (+)-Sch642305: a structurally novel inhibitor of bacterial DNA primase and HIV-1 Tat transactivation. Chemical Communications, 3703–3705; (b) Snider BB, Zhou J. (2006) Synthesis of (+)-Sch 642305 by a biomimetic transannular Michael reaction. Organic Letters, 8, 1283-1286; (c) Ishigami K, Katsuta R, Watanabe H. (2006) Stereoselective synthesis of Sch 642305, an inhibitor of bacterial DNA primase. Tetrahedron, 62, 2224-2230; (d) Wilson EM, Trauner D. (2007) Concise synthesis of the bacterial DNA primase inhibitor (+)-Sch 642305. Organic Letters, 9, 1327-1329 (this synthesis, which resembles ours in the last steps, appeared when we were in the last stage of our own work).
45.
For a review on this methodology, see: ChapdelaineM.J., HulceM. (1990) Tandem vicinal difunctionalization: β-Addition to α,β-unsaturated carbonyl substrates followed by α-functionalization. Organic Reactions, 38, 225–653.
46.
DanishefskyS.J., SimoneauB. (1989) Total syntheses of ML-236A and compactin by combining the lactonic (silyl) enolate rearrangement and aldehyde-diene cyclocondensation technologies. Journal of the American Chemical Society, 111, 2599–2604.
47.
Such reactions take place via intermediate formation of very reactive “naked” tris(dialkylamino)sulfonium (TAS) enolates, followed by in situ alkylation of the latter: NoyoriR., NishidaI., SakataJ. (1983) Tris(dialky1amino)sulfonium enolates. Synthesis, structure, and reactions. Journal of the American Chemical Society, 105, 1598–1608.
48.
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49.
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50.
HandeS.M., UenishiJ. (2009) Total synthesis of aspergillide B and structural discrepancy of aspergillide A. Tetrahedron Letters, 50, 189–192.
51.
DixonD.J., LeyS.V., TateE.W. (2000) Diastereoselective oxygen to carbon rearrangements of anomerically linked enol ethers and the total synthesis of (+)-(S,S)-(cis-6-methyltetrahydropyran-2-yl)acetic acid, a component of civet. Journal of the Chemical Society, Perkin Transactions I, 2385–2394.
52.
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53.
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54.
(a) MislowK., O'BrienR.E., SchaeferH. (1962) Stereochemistry of (+)-(S)-2-propanol-l-d3. Partial asymmetric reduction of 4′,1″-dimethyl-l,2,3,4-dibenzcyclohepta-l,3-diene-6-one. Journal of the American Chemical Society, 84, 1940–1944;(b) Takai K, Heathcock CH. (1985) Acyclic stereoselection. 32. Synthesis and characterization of the diastereomeric (4S)-pentane-1,2,3,4-tetrols. Journal of Organic Chemistry, 50, 3247-3251; (c) Marshall JA, Seletsky BM, Luke GP. (1994) Synthesis of protected carbohydrate derivatives through homologation of threose and erythrose derivatives with chiral γ-alkoxy allylic stannanes. Journal of Organic Chemistry, 59, 3413-3420; (d) Yokokawa F, Inaizumi A, Shioiri T. (2005) Synthetic studies of the cyclic depsipeptides bearing the 3-amino-6-hydroxy-2-piperidone (Ahp) unit. Total synthesis of the proposed structure of micropeptin T-20. Tetrahedron, 61, 1459-1480.
55.
(a) RamachandranP.V., ChenG.M., BrownH.C. (1997) Efficient synthesis of enantiomerically pure C2-symmetric diols via the allylboration of appropriate dialdehydes. Tetrahedron Letters, 38, 2417–2420;(b) Ramachandran PV. (2002) Pinane-based versatile allyl” boranes. Aldrichimica Acta, 35, 23-35.
56.
(a) HuY.J., DominiqueR., DasS.K., RoyR. (2000) A facile new procedure for the deprotection of allyl ethers under mild conditions. Canadian Journal of Chemistry, 78, 838–845;(b) Wipf P, Rector SR, Takahashi H. (2002) Total synthesis of (–)-tuberostemonine. Journal of the American Chemical Society, 124, 14848-14849.
57.
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58.
Compound 46 was obtained a ca. 2:1 mixture of anomers, each of them being a ca. 9:1 mixture of E/Z isomers.
59.
SimchenG., WestW. (1977) Synthese von Keten-O-trimethylsilyl-S-alkyl-monothioacetalen. Synthesis, 247–248.
60.
(a) LewisM.D., ChaJ.K., KishiY. (1982) Highly stereoselective approaches to α- and β-C-glycopyranosides. Journal of the American Chemical Society, 104, 4976–4978. (b) Paterson I, Luckhurst CA. (2003) Toward the total synthesis of phorboxazole A: synthesis of an advanced C4–C32 subunit using the Jacobsen hetero-Diels–Alder reaction. Tetrahedron Letters, 44, 3749-3754.
61.
(a) IkemotoN., SchreiberS.L. (1992) Total synthesis of (–)-hikizimycin employing the strategy of two-directional chain synthesis. Journal of the American Chemical Society, 114, 2524–2536;(b) Crimmins MT, Emmitte KA. (1999) Total synthesis of (+)-laurencin: an asymmetric alkylation-ring-closing metathesis approach to medium ring ethers. Organic Letters, 1, 2029-2032.
62.
The energy contents and relative stabilities of several of the synthetic lactones have been evaluated with the aid of semiempirical methods (PM3). Energies were then refined by means of HF/6-31G* calculations. For the results of these, see the Supporting Information in ref. [11c].
63.
OokuraR., KitoK., SaitoY., KusumiT., OoiT. (2009) Structure revision of aspergillides A and B, cytotoxic 14-membered macrolides from Aspergillus ostianus, by X-ray crystallography. Chemistry Letters, 38, 384.
64.
(a) BodenE.P., KeckG.E. (1985) Proton-transfer steps in Steglich esterification: a very practical new method for macrolactonization. Journal of Organic Chemistry, 50, 2394–2395;(b) Trost BM, Chisholm JD. (2002) An acid-catalyzed macrolactonization protocol. Organic Letters, 4, 3743-3745; (c) Shiina I, Kubota M, Oshiumi H, Hashizume M. (2004) An effective use of benzoic anhydride and its derivatives for the synthesis of carboxylic esters and lactones: a powerful and convenient mixed anhydride method promoted by basic catalysts. Journal of Organic Chemistry, 69, 1822-1830.
65.
The aromatic solvent toluene acts here as a sensitizer: InoueY., YamasakiN., TaiA., DainoY., YamadaT., HakushiT. (1990) The temperature- and viscosity-dependent photostationary E/Z ratio in triplet-sensitized photoisomerization of cyclooctene. Journal of the Chemical Society, Perkin Transactions II, 1389–1394.
66.
WutsP.G.M., GreeneT.W. (2007) Greene's Protective Groups in Organic Synthesis (4. Ed.). John Wiley and Sons, New York, pp. 106–120.
