Anomalous Reaction of Rh₂(OAc)₄-Generated Transient Carbonyl Ylides: Chemoselective Synthesis of Epoxy-Bridged Tetrahydropyranone, Oxepanone, Oxocinone, and Oxoninone Ring Systems

A series of symmetrical and unsymmetrical α,β-unsaturated ketones such as arylmethylidenecycloalkanones (16, 19), bis(arylmethylidene)cycloalkanones (21, 23, 27), bisdiphenylnonatetraenones (30), and bis(phenylpropenylidene)cycloalkanones (33, 38, 42) were synthesized and subjected to the rhodium(II)-catalyzed tandem cyclization−cycloaddition reactions with various α-diazo ketones. These reactions afforded spiro epoxy-bridged tetrahydropyranone, spiro epoxy-bridged oxepanone (41, 43), epoxyoxocin-4(5H)-one (35, 37), and epoxyoxonin-5(6H)-one (40) frameworks starting from relatively simple precursors. The regio- and stereochemistry and solid-state architecture arrangements of several products were characterized by single-crystal X-ray structure analysis. The cycloaddition of carbonyl ylides with the compounds having both CO and CC groups was found to be chemo- and regioselective. Interestingly, an unusual ring enlargement of cycloadducts 34, 36, and 41 derived from CO group addition was observed, affording epoxyoxocin-4(5H)-one and epoxyoxonin-5(6H)-one frameworks. Examples for the tandem cyclization−cycloaddition−ring enlargement reaction were also described. The rhodium(II)-generated carbonyl ylides behaved anomalously, furnishing the cycloadducts as a result of the cycloaddition of carbonyl ylides to CO group despite the presence of CC groups.