Enantioselective NHC-Catalysed Formal [4+2] Cycloaddition of Alkylaryl­ketenes with β,γ-Unsaturated α-Ketophosphonates

NHC-mediated enantioselective formal [4+2] cycloadditions of alkylarylketenes with γ-substituted-β,γ-unsaturated α-ketophosphonates is described. A substrate-dependent switch in diastereoselectivity was observed, with γ-aryl α-ketophosphonates providing preferentially the syn -dihydropyranone-phosphonates and γ-methyl α-ketophosphonates favouring the anti -dihydropyranone-phosphonate. In addition, ketene generation in situ retained high levels of stereoselectivity and led to improved product yields when compared with the corresponding two-step procedure.