An efficient synthesis of bicyclo[3.3.0]oct-2-en-4-ones and 2- Azabicyclo[3.3.0]oct-7-en-6-ones via β-amino-substituted α,β-unsaturated Fischer carbene complexes

The formal [3+2] cycloaddition of β-amino-substituted α,β-unsaturated Fischer carbenechromium complexes 2, which are easily prepared in a one-pot procedure from terminal alkynes 1 via an initially formed alkynylidene complex by a Michael-type addition of a secondary amine, with a variety of alkynes 3 afford the 1,2,5-trisubstituted 3-alkoxy-5-dialkylamino-1,3- cyclopentadienes 4 with an acetal-protected aldehyde or ketone carbonyl group in either the 5-substituent R¹ or the N-substituent R². Under acidic conditions, both the enol ether moiety in the 5-membered ring as well as the acetals in the side chains are hydrolyzed, and the resulting cyclopentenones 6 with carbonyl-group containing side-chains undergo facile intramolecular aldol reactions to give bicyclo[33.0]oct-2-en-4-ones 8 and 8- azabicyclo[3.3.0]oct-2-en-4-ones 12, respectively, in good to very good yields for most cases.