PHOTOISOMERIZATION OF BICYCLE [4.2.0]OCTADIENES TO TRICYCLO[4.2.0.0(2,5)] OCTENES - APPLICATION TO THE SYNTHESIS OF [N]LADDERANES

A new approach for the extension of [n]adderanes is described which in volves formation of an end-fused bicyclo[4.2.0]octadiene and its photo isomerisation to the related tricyclo[4.2.0.0(2,5)] octene as the ladd erane extension step. The protocol is illustrated with the synthesis o f [3]ladderane (9) from an unsubstituted cyclobutene and nor[5]laddera ne (13) from an ester activated cyclobutene. The exo-stereochemistry o f (13) is confirmed by reference to intermediate (23) for which a sing le crystal X-Ray structure analysis was obtained.