Rhodium-Catalyzed Cross-Aldol Reaction: In Situ Aldehyde-Enolate Formation from Allyloxyboranes and Primary Allylic Alcohols

摘要

The aldol reaction is one of the most fundamental carbon-carbon bond-forming reactions. A cross-aldol reaction between two different aldehydes, in principle, provides the most straightforward step- and redox-economical access to polyketides. Numerous modern aldol methods. however, utilize ketones, thioesters, esters, and other carboxylic acid derivatives as donors to circumvent the problems inherent to aldehyde-aldehyde cross-aldol reactions. Thus, additional multistep transformations of aldol products, including protection and redox processes, are required to generate β-hydroxy-protected aldehydes. In the cross-aldol reaction between two different aldehydes, chemoselective activation of one aldehyde as a donor and the other aldehyde as an acceptor is difficult, and often affords mixtures of homo- and heteroaldol products (Scheme 1a).