Intramolecular trapping of an oxacarbene intermediate in the photochemical ring expansion of a cyclopentanone

Mechanisms of Intramolecular Rearrangements of Cyclic Siloxycarbenes

“…Cyclic alkoxycarbenes have been known to undergo a number of intramolecular rearrangements, including 1,2-H migration, ring contraction, and decarbonylation. − Particular attention has been paid to the mechanism of ring contraction not only because it is an intriguing problem but also because it is closely related to the photochemical ring expansion of cycloalkanones. − Experimental evidence seems to indicate that this reaction occurs via both a concerted mechanism involving an anion-like shift from oxygen to the “vacant” carbene p orbital and a stepwise fragmentation-recombination mechanism involving an acyl−alkyl biradical intermediate. − Decarbonylation is also believed to involve initial fragmentation to an acyl−alkyl biradical, which thereafter loses carbon monoxide to afford a dialkyl biradical. − The latter biradical then collapses to the appropriate cycloalkane product.…”

Ninhydrin as a Source of Naphthoquinone Acetals and Benzocycloheptenetrione Derivatives, and the Trapping of an Oxacarbene Derived from a Cyclopentanone

“…Ring expansions of cyclopentenone derivatives leading to pyran systems via oxacarbene intermediates are rare. The scarce reports include reactions of cyclocamphanone,22,27 a stereochemical study using 3‐oxacyclopentanones,28 photolysis of a spirocyclopropyl‐anellated cyclopentanone in a synthesis of thromboxane A 2 29 and a similar step in the synthesis of pederol,30 and, finally, a photochemical ring expansion starting from a camphor derivative 31. To the best of our knowledge the photolysis leading to rac ‐ 16 is not only a rare case in that a cyclopentenone ring is expanded, but also the first case of such a reaction leading to an orthoester; it is possible that the formation of side product 4 of the irradiation of 2 can be explained along similar lines.…”