Vinyl Ketenes in Hetero-Diels-Alder Reactions

Significance

A route to optically enriched δ-lactones 3 via chiral tertiary amine-catalyzed hetero-Diels-Alder reaction is presented. α,β-Unsaturated acid chlorides 1 (slowly added via syringe pump) serve as precursors for vinyl ketenes 4, which are activated as enantiomerically pure zwitterionic ammonium dienenolates 5 by quinidine derivative 2a (2b for R¹ = i-Bu).Trapping of s-cis-5 with chloral as electron-poor dienophile furnishes δ-lactones 3 in moderate to good yields and good to high enantioselectivities. The Lewis acid co-catalyst Sn(OTf)₂ facilitates the dehydrohalogenation step and increases the overall efficiency leading to improved yields.