Reactions of p-nitrobenzaldehyde (4) with methyl vinyl ketone (5) were conducted 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 8a and 8b. In contrast, purification of the raw product by preparative TLC on silica gel gave alpha-methylene aldol 7. The mechanism for the formation of alpha-chloromethyl aldol 8 and diasteroselection for the syn-isomer 8a and anti-isomer 8b are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.