A TANDEM CYCLIZATION-ONIUM YLIDE REARRANGEMENT-CYCLOADDITION SEQUENCE FOR THE SYNTHESIS OF BENZO-SUBSTITUTED CYCLOPENTENONES

A new annulation sequence leading to benzo-substituted cyclopentenones is effected by treating o-alkynyl-substituted alpha-diazoacetophenones containing tethered heteroatoms with Rh(II) carboxylates. The reaction involves addition of the initially formed keto carbenoid onto the acetylenic pi-bond to give a rearranged vinyl carbenoid. Sulfonium ylide formation occurred both intra- and intermolecularly when the reaction was carried out in the presence of a sulfide. In the case where an ether oxygen was present on the backbone of the vinyl carbenoid, the resulting oxonium ylide underwent a [1,2]- or [2,3]-shift to give the rearranged product. These cyclic metallocarbenoids were also found to interact with a neighboring carbonyl or oximino pi-bond to produce carbonyl or azomethine ylides. The 1,3-dipoles generated in this manner were trapped with dimethyl acetylenedicarboxylate. The domino transformation was also performed intramolecularly by attaching a trapping agent directly to the carbonyl group. Incorporation of an amido carbonyl on the alkyne side chain was found to dramatically alter the course of the tandem annulation reaction. A novel rearranged cycloadduct was formed in high yield whose structure was elucidated by X-ray crystallography. The mechanism for its formation involves the opening of a transient intermediate oxabicyclo[2.2.1]heptane followed by a Wagner-Meerwein rearrangement.