NMR and quantum-chemical study of the stereochemistry of spiroepoxides obtained by oxidation of (Z)-3-arylidene-1-thioflavan-4-ones

Epoxidation of (Z)-3-arylidene-1-thioflavan-4-ones (1) yielded trans,eis (2 ) and trans,tuans (3) isomers. The structure and signal assignments were el ucidated by extensive application of one- and two-dimensional H-1 and C-13 NMR spectroscopy. The conformational analysis was achieved by the applicati on of (3)J(C,H) couplings and ab initio MO calculations. Both the preferred ground-state conformers (envelope-A conformations) obtained as global mini ma of the HF ab initio structures and the C-13 chemical shifts calculated b y the GIAO method from the global minima structures of the tuans,cis and tr ans,tuans isomers are in agreement with the experimentally obtained NMR res ults. Copyright (C) 2001 John Wiley & Sons, Ltd.