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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 19, Number 9
BKCSDE 19(9)
September 20, 1998 

Asymmetric Intramolecular Dels-Alder Cycloaditions of 2-Pyrone-3-Carboxylates and Synthesis of Vitamin D3 A Ring Phosphine Oxide
Cheon-Gyu Cho, Gary H. Posner
Intramolecular Diels-Alder cycloadditions of 2-pyrone-3-carboxylates with 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 bicylolactones. The ratio of two isomers, exo and endo, was 5 to 1, and each isomer was found to be diastereomerically pure (>99% de). Their relative and absolute stereochemistries were determined by 1H NMR spectroscopy and confirmed by X-ray crystallography of minor, endo-adduct 9. The major exo-adduct was successfully transformed to (-)-2-butyl substituted A-ring phophine oxide 16, a key element for the synthesis of 2-butyl vitamin D3.
957 - 961
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