Synthesis of 5-Hydrazino-2-cyclopentenone Derivatives by a Gold(I)-Catalyzed Cycloisomerization/Hetero-Diels-Alder/Ring-Opening Tandem Reaction of Enynyl Acetates

A highly efficient, one-pot synthesis of ring-fused 5-hydrazino-2-cyclopentenone derivatives is achieved by a gold(I)-catalyzed cycloisomerization/hetero-Diels-Alder/ring-opening tandem reaction of suitable enynyl acetates. By mixing the latter with a dialkylazodicarboxylate in the presence of a gold(I) catalyst, the 1,3-acyloxy migration/Nazarov cyclization process leads to dienyl acetate intermediates which are trapped by the heterodienophile present in situ. This provides strained intermediates which undergo highly regioselective ring opening by a retro aza-Michael reaction promoted by traces of water, eventually yielding the target compounds. Six- and seven-membered ring-fused cyclopentenones and piperidine- and tetrahydropyran-fused cyclopentenones bearing a pendant hydrazino functionality on a bridgehead carbon atom can be obtained in high yield (68-96%) by this approach.