STEREOSELECTIVE RING-OPENING OF ELECTRONICALLY EXCITED CYCLOHEXA-2,4-DIENONES - CAUSE AND EFFECT

The two conformers of a cyclohexa-2,4-dienone with different substitue nts at C(6) on irradiation are believed to undergo ring opening stereo specifically affording a mixture of two configurationally isomeric die ne-ketenes (and descendents thereof). Exceptions are generally found f or 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 i nteract through space with the ring C=O group. In these cases, only on e of the two anticipated diene-ketenes (and descendents thereof) is pr oduced. 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 c ompound classes accessible to diene-ketenes are seven-membered carbocy cles (by intramolecular aldolization of the zwitterion of appropriatel y substituted, transiently formed diene-(N,O)-ketene acetals) and beta -lactams (by Staudinger reaction).