Aldol Reactions of Germyl Dienolates With Precise Control Over Regioselectivity and Perfect Diastereoselectivity

Enolates are essential nucleophiles for the formation of CC bonds. In particular, dienolates, i.e., π‐extended enolates that contain a conjugated additional CC bond alongside the enolate moiety, are valuable nucleophiles for the synthesis of π‐conjugated hydroxy ketones. Alas, selective aldol reactions of dienolates remain challenging because they can yield up to eight possible regioisomeric ( α ‐ or γ ‐position) and stereoisomeric ( syn or anti ) aldol adducts. Here, we present the synthesis of Ge dienolates via the (4 + 1) cycloaddition of allenones and GeCl 2 together with an investigation of their reactivity. Facilitated by the rigidity of their cyclic structures, the Ge dienolates undergo highly diastereoselective aldol reactions to furnish either syn or anti products. The key role of the solvents in determining the regioselectivity is also demonstrated. The solvent‐dependent reversibility of the α ‐addition provides precise control over the regioselectivity, permitting the selective formation of either α ‐products or γ ‐products. Notably, the regio‐ and diastereoselectivity can be governed by the choice of solvent and the presence of Lewis acids. Our method can be applied to construct the 5‐hydroxy‐1,3‐dicarbonyl framework, which is difficult to obtain from previously reported dienolates. This study thus not only provides new insights into dienolate chemistry but also establishes a versatile platform for the diverse synthesis of structurally distinct molecules.