Catalytic, Asymmetric Vinylogous Mukaiyama Aldol Reactions of Pyrrole- and Furan-Based Dienoxy Silanes: How the Diene Heteroatom Impacts Stereocontrol

Denmark's chiral bisphosphoramide/silicon tetrachloride system performs as an excellent Lewis base-Lewis acid catalyst for the vinylogous Mukaiyama aldol reaction of pyrrole- and furan-based dienoxy silanes with aromatic and heteroaromatic aldehydes. This asymmetric methodology provides a powerful synthetic entry to a variety of delta-hydroxylated gamma-butenolide-type frameworks with high efficiency and valuable margins of regio-, diastereo-, and enantioselectivity. Notably, the nature of the heteroatom within the vinylogous dienoxy silane donor heavily impacts the diastereocontrol, with syn-configured aldol adducts emerging from pyrroles bearing electron-withdrawing N-protecting groups (Boc, Ts, and Cbz) and anti-configured adducts prevailing when furan- or N-alkyl/alkenylpyrrole donors are involved.