A useful methoxyvinyl cation equivalent: α-t-butyldimethylsilyl-α-methoxyacetaldehyde.

Abstract Described are the synthesis and application of α- t -butyldimethylsilyl-α-methoxyacetaldehyde as a formal methoxyvinyl cation equivalent. Addition of Grignard reagents to the title aldehyde, followed by treatment of the intermediate β-hydroxysilanes with KH, gives good yields of largely Z -methoxyvinylated products. Assuming a Peterson-like elimination mechanism, one can infer that the Grignard addition proceeds with high syn selectivity. These results are consistent with a chelation control model involving coordination to the α-methoxy group in the title aldehyde rather than an alternative stereoelectronic Felkin–Anh-type model. It must be noted that a steric Felkin–Anh model also accounts for the observed stereochemistry. All told, the title reagent can be employed to efficiently append a Z -configured methoxyvinyl group to an appropriate R–M species, in two steps.