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

A new annulation sequence leading to benzo-substituted cyclopentenones is effected by treating o-alkynyl-substituted alpha-diazoacetophenone s containing tethered heteroatoms with Rh(II) carboxylates. The reacti on involves addition of the initially formed keto carbenoid onto the a cetylenic pi-bond to give a rearranged vinyl carbenoid. Sulfonium ylid e formation occurred both intra- and intermolecularly when the reactio n was carried out in the presence of a sulfide. In the case where an e ther oxygen was present on the backbone of the vinyl carbenoid, the re sulting oxonium ylide underwent a [1,2]- or [2,3]-shift to give the re arranged product. These cyclic metallocarbenoids were also found to in teract with a neighboring carbonyl or oximino pi-bond to produce carbo nyl or azomethine ylides. The 1,3-dipoles generated in this manner wer e trapped with dimethyl acetylenedicarboxylate. The domino transformat ion 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 th e tandem annulation reaction. A novel rearranged cycloadduct was forme d in high yield whose structure was elucidated by X-ray crystallograph y. The mechanism for its formation involves the opening of a transient intermediate oxabicyclo[2.2.1]heptane followed by a Wagner-Meerwein r earrangement.