Practical Access to Trisubstituted Morita‐Baylis‐Hillman‐Type Products Through Copper‐Catalyzed Reductive Aldol Reactions of Alkynones

We report a convenient and diastereoselective copper‐catalyzed reductive alkynone aldol addition to aldehydes, with pinacolborane as stoichiometric reductant, that results in the generation of densely functionalized, trisubstituted (Z)‐enone products analogous to Morita‐Baylis‐Hillman adducts. We demonstrate that the reaction proceeds via an allenolborate, which we have characterized by NMR spectroscopy. A range of aromatic and aliphatic aldehydes can be reductively coupled with various non‐terminal alkynones. The reductive aldol reaction of alkynones with aldehydes provides trisubstituted Morita‐Baylis‐Hillman‐type products in high yields with complete (Z)‐selectivity. Using a copper‐catalysed reductive approach with pinacolborane as the reductive species, an allenolborate is formed in situ which reacts with aldehydes to give the corresponding aldol product.