Diastereoselective, Catalytic Access to Cross‐Aldol Products Directly from Esters and Lactones

High oxidation‐state carbonyl coupling partners including esters and lactones were reacted with enones to give aldol‐type products directly using two‐fold organoborane catalysis. This new retrosynthetic disconnection to aldol‐type products is compatible with enolisable coupling partners, without self‐condensation, and couples the high reactivity of secondary dialkylboranes with the stability of pinacolboronic esters. Excellent chemoselectivity, substrate scope (including those containing reducible functionalities and free alcohols) and diastereocontrol were achieved to access both the syn‐ and anti‐aldol‐type products. Mechanistic studies confirmed the two‐fold catalytic role of the single secondary borane catalyst for boron enolate formation and formation of an aldehyde surrogate from the ester or lactone coupling partner.