STEREOSTRUCTURE OF ISOMERIC (±)-1-THIOFLAVANONE 1-OXIDES

Elimination, ring-contraction, and fragmentation reactions of 1-thioflavanone 1-oxides

Epimeric thioflavanone sulfoxides (2b) were selectively transformed into thioflavone (1a), thioaurone (3a), and di(2-cinnamoylphenyl) disulfide (4). Disulfide 4 can be recyclized into thioaurones (3a–c) and thioflavanones (2a,5) with heterolysis of the S—S bond. The 3-p-anisylidene sulfoxide analog of 2b (6) transforms, with fragmentation, into 4'-methoxythioaurone 3b.Key words: chalcone, ring contraction, sulfoxides, thioaurones, thioflavonoids.

Chemo- and diastereoselectivity in the dimethyldioxirane oxidation of 2,3-dihydro-4H-1-benzothiopyran-4-ones and 4H-1-benzothiopyran-4-ones. Unusual reactivity of 4H-1-benzothiopyran-4-one 1-oxides

The oxidation of the 1-thiochromanones 1-3 by dimethyldioxirane (DMD) produced the corresponding sulfoxides 4-6 or sulfones 7-9; their relative amounts depended on the amount of oxidant used. A low diastereoselectivity was observed in the sulfoxidation of the 2-substituted 1-thiochromanones 2 and 3, due to the small steric differentiation during the DMD attack. An unusual reactivity pattern was found in the DMD oxidation of the 1-thiochromones 10-12, in that the sulfoxides 13-15 were more reactive toward the electrophilic oxidizing agent than the corresponding sulfides. The observed anomaly may be explained in terms of transannular stabilization of the transition structure (TS) for the sulfone formation, promoted through favorable conformational effects in the sulfoxide. Higher sulfoxide/sulfone ratios were found in solvents of greater hydrogen bond donor capacity, which is in accordance with the postulated stabilizing effect.