Benzocycloheptenones photoisomerise to strained transbenzocycloheptenones which undergo smooth /-4+2 7 cycloaddition with a variety of dienes.
The adducts with cyclopentadiene can be readily transformed to trans fused 5-7-6 ring systems while cycloaddition with isoprene and 2-trimethylsilyloxy-l,3-butadiene produces directly trans fused 6-7-6 ring systems.
The remarkable discovery by coreyl and Eaton 2 have demonstrated that 2-cycloheptenone on photoirradiation isomerises to highlY strained trans-cycloheptenone which reacts with cyclopentadiene and furan to afford trans adducts.
After this observation, 2,6-cyclOheptadienone 3 and benzocycloheptenones 4 have been reported to exhibit photoisomerisation. To date, these extraordinarilY reactive trans species have never been employed in any synthetic endeavor. Our interest in this area of research arises out of the necessity for a direct synthetic route to the lessaccessible trans fused cycloheptanoid compounds especially the 5-7 one, as this structural unit is incorporated in many sesqui-and diterpenes:,6 A plethora of elegant strategies for access to the cis 5-7 rings 7 have emerged in the last decade, while only a limited number of multistep strategies for the trans fused 5-7 rings 8 have been reported.