Stereoselective Ring Opening of Electronically Excited Cyclohexa‐2, 4‐dienones: Cause and effect

Abstract

The two conformers of a cyclohexa‐2, 4‐dienone with different substituents at C(6) on irradiation are believed to undergo ring opening stereospecifically affording a mixture of two configurationally isomeric diene‐ketenes (and descendents thereof)‐ Exceptions are generally found for those dienones with one C and one O substituent or even with two C substituents, if one of them carries a polar group at a site able to interact through space with the ring CO group. In these cases, only one of the two anticipated diene‐ketenes (and descendents thereof) is produced. A thorough investigation of the photochemistry of a series of structurally different cyclohexa‐2, 4‐dienones on analytical as well as on preparative scale extends our mechanistic knowledge of the various routes from diene‐ketenes into a variety of compound classes. Novel compound classes accessible to diene‐ketenes are seven‐membered carbocycles (by intramolecular aldolization of the zwitterion of appropriately substituted, transiently formed diene‐(N, O)‐ketene acetals) and β‐lactams (by Staudinger reaction).

A Solvolytic C−C Cleavage Reaction of 6-Acetoxycyclohexa-2,4-dienones: Mechanistic Implications for the Intradiol Catechol Dioxygenases

“…The acyclic ester products 3a and 3b are at the same oxidation state as the starting materials but correspond to the addition of 2 equiv of methanol, and their formation requires a reaction mechanism involving cleavage of the C−C bond between the oxygenated carbons C-1 and C-6, analogous to intradiol catechol cleavage. The same reaction products were observed when the reaction was carried out in the dark, thus ruling out a photochemical reaction, which has been observed for other 2,4-cyclohexadienones …”

Substituent and solvent influence on the nature of the oxyallyl intermediate in the rearrangement of bicyclo[3.1.0]hex-3-en-2-ones

“…It has been suggested that oxyallyl compounds are intermediates in both thermal and photochemical rearrangements of bicyclo[3.1.0]hex-3-en-2-ones to phenols [25][26][27][28][29][30][31][32][33][34][35]. Some time ago, we studied the reaction pathway for the prototype rearrangement of bicyclo[3.1.0] hex-3-en-2-one (1a) to phenol (3a) in the gas phase at the CASSCF/CASPT2 level [36].…”

Photochemistry of 3-Substituted Bicyclo[3.1.0]hex-3-en-2-ones. Regioselective Synthesis of Ortho-Substituted Phenols by Pauson−Khand Reaction

“…Besides these natural products, several compounds with the same bicyclic structure have been prepared, most commonly by photorearrangement of cyclohexenones, phenols, or cyclohexadienones . The latter transformation, originally found in the conversion of santonin to lumisantonin, has been the subject of extensive mechanistic studies , Much less information is known about the photochemical rearrangement of bicyclo[3.1.0]hex-3-en-2-ones into phenols, which was observed by Eastman in 1959 when he reported that the irradiation of neat umbellulone ( 1 ) gave thymol ( 2 ) (Scheme ). The difficult availability of these bicyclic compounds has reduced the study of their photochemical behavior to few cases .…”