Catalytic Cyclization Reactions of Huisgen Zwitterion with α-Ketoesters by in Situ Chemoselective Phosphine Oxide Reduction

The first examples of catalytic cyclization reactions between the Huisgen zwitterion and α-ketoester derivatives are reported. The use of phenylsilane with a substoichiometric amount of bis(p-nitrophenyl)phosphate/diisopropylethylamine proved to be crucial for the in situ reduction of the phosphine oxide formed during the reaction. The optimized protocol is applied to alkyl or aryl ketoesters, furnishing either the corresponding cycloadducts or the hydrazone derivatives, depending on the substitution patterns of the substrates, in moderate to good yields (up to 80% yield, 18 examples).