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 gamma-substituted-beta,gamma-unsaturated alpha-ketophosphonates is described. A substrate-dependent switch in diastereoselectivity was observed, with gamma-aryl alpha-ketophosphonates providing preferentially the syn-dihydropyranone-phosphonates and gamma-methyl alpha-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.