TRANSANNULAR REACTIONS OF CYCLOALKENES, CYCLOALKADIENES AND CYCLOALKATRIENES .16. PHOTOCYCLIZATION OF CYCLONONENE AND CYCLOUNDECENE

Irradiation of cyclononene (7b) and cycloundecene (7d) in pentane afforded cis-bicyclo[4.3.0]nonane (10b) and cis-bicyclo[6.3.0]undecane (10d), respectively. Small amounts of the fragmentation products 1-undecene (14d) and 1-undecyne (15d) were also obtained from cycloundecene (7d). The photobehavior of the series of medium- and large-ring alkenes 7a-e is compared. Aside from E reversible Z isomerization, the principal pathway in each case involves rearrangement of the pi,R(3s) excited state to the cycloalkylidene intermediate 8, which undergoes predominant, if not exclusive, 1,5-transannular insertion to afford the corresponding cis-bicyclo[n.3.0]alkane (10). The cycloalkylidenes 8 formed in this way exhibit behavior somewhat different from those generated by other methods. The possible contributory role of carbene intermediates derived from pi,R(3s) excited states in E revesible Z photoisomerization of acyclic alkenes is also discussed.