Reexamination of products and the reaction mechanism of the chalcogeno-Baylis-Hillman reaction: Chalcogenide-TiCl4-mediated reactions of electron-deficient alkenes with aldehydes

Reactions of p-nitrobenzaldehyde (4) with methyl vinyl ketone (5) were cond ucted in the presence of TiCl4 and dimethyl sulfide (3) or selenopyranone 6 . When the raw product was purified by column chromatography on silica gel, alpha-chloromethyl aldol 8 was obtained as a mixture of diastereoisomers 8 a and 8b. In contrast, purification of the raw product by preparative TLC o n silica gel gave alpha-methylene aldol 7. The mechanism for the formation of alpha-chloromethyl aldol 8 and diasteroselection for the syn-isomer 8a a nd anti-isomer 8b are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.