The addition of dialkyl acetylenedicarboxylates to isatin, a strong NH acid, in the presence of triphenylphosphine leads to stable crystalline phosphorus ylids in excellent yields and one of the three synthesised compounds (3a) exists as a mixture of two rotational isomers on the basis of variable temperature nmr spectroscopy.
HudsonH.R., in The Chemistry of Organophosphorus Compounds, Vol. 1. Primary, Secondary and Tertiary Phosphines, Polyphosphines and Heterocyclic Organophosphorus (III) Compounds, (F.R. Hartley Ed:), Wiley, New York, 1990, pp. 386–472.