Diazo Tetramic Acids Provide Access to Natural-Like Spirocyclic Δ α,β -Butenolides through Rh(II)-Catalyzed O-H Insertion/Base-Promoted Cyclization.

3-Diazotetramic acids were found to be valid substrates for the recently discovered approach toward natural-like Δ ^α,β -spirobutenolides via Rh(II)-catalyzed O-H insertion into propiolic acids followed by base-promoted intramolecular Michael addition. The target Δ ^α,β -spirobutenolides were obtained in generally high yields and, in the case of chiral 5-monosubstituted 3-diazotetramic acids, high diastereoselectivity. The synthesis of Δ ^α,β -spirobutenolides that we report here was virtually insensitive to the structure of the propiolic acids though it was somewhat sensitive to the structure of the 3-diazotetramic acids, thereby demonstrating quite a large scope. Thus, a new class of α-diazocarbonyl compounds suitable for the realization of the approach outlined above was identified.