ASYMMETRIC INTRAMOLECULAR DIELS-ALDER CYCLOADDITIONS OF 2-PYRONE-3-CARBOXYLATES AND SYNTHESIS OF VITAMIN-D-3 A RING PHOSPHINE OXIDE

Intramolecular Diels-Alder cycloadditions of 2-pyrone-3-carboxylates w ith trans-vinyl silaketal groups tethered via a chiral, non-racemic 1, 3-butanediol auxiliary proceeded in unexpected stepwise cycloadditions through ionic intermediates to provide cis-disubstituted bicylolacton es. The ratio of two isomers, exo and endo, was 5 to 1, and each isome r was found to be diastereomerically pure (>99% de). Their relative an d absolute stereochemistries were determined by H-1 NMR spectroscopy a nd confirmed by X-ray crystallography of minor, endo-adduct 9. The maj or exo-adduct was successfully transformed to (-)-2-butyl substituted A-ring phophine oxide 16, a key element for the synthesis of 2-butyl v itamin D-3.