Gold(I)-Catalyzed\nRautenstrauch/Hetero-Diels–Alder/Retro-aza-Michael\nCascade Reaction for the Synthesis of α‑Hydrazineyl-2-cyclopentenones

A one-pot synthesis of ring-fused, α-hydrazineyl-2-cyclopentenone\nderivatives is achieved by a gold(I)-catalyzed Rautenstrauch/hetero\nDiels–Alder/ring opening tandem reaction of suitable propargyl\nesters. By mixing the latter with a dialkylazodicarboxylate in the\npresence of a gold(I) catalyst, the 1,2-acyloxy migration/cyclization\nprocess (Rautenstrauch reaction) leads to cyclopentadienyl ester intermediates\nwhich are trapped by the heterodienophile present in situ. This provides\nstrained intermediates which spontaneously undergo highly regioselective\nring opening by a retro aza-Michael reaction promoted by the gold(I)\ncatalyst, eventually yielding the target compounds. Six- and seven-membered\nring-fused cyclopentenones bearing a pendant α-hydrazineyl moiety\ncan be obtained in moderate to excellent yield (50–98%) by\nthis approach, with a minimal erosion of the initial optical purity\nwhen using enantioenriched substrates.