Cycloaddition of 4‐Phenyl‐4 H ‐1,2,4‐triazole‐3,5‐dione (PTAD) to 7‐Alkylidene‐2,3‐benzonorbornadienes

A series of 7‐alkylidene‐2,3‐benzonorbornadienes 2a – f was prepared from the corresponding fulvenes 1a – f by reaction with benzyne. The cycloaddition of 2 with PTAD was investigated. While the phenyl derivative 2a was quite unreactive and afforded traces of the [2 + 2] cycloadduct 3a , the phenyl methyl system 2b led to the ene product 3b in fair yield. The diphenyl and bis‐(4‐chlorophenyl) cases 2d and 2e , respectively, led to a mixture of the rearrangement urazoles 3d and 3e and the double PTAD [4 + 2] cycloadducts 4d and 4e in form of the syn/syn and syn/anti diastereomers. Analogously, the unsymmetrically substituted 7‐(diarylmethylene)benzonorbornadienes 2c (X = Ph, Y = p ‐MeOPh) and 2f (X = Ph, Y = p ‐NO 2 Ph) gave similar products, except that the possible diastereomeric rearrangement urazoles were formed ( E , Z ‐ 3c and E , Z ‐ 3f ). In the case of 2c the oxidized monocycloadduct 5c was also isolated in low yield. The ratio of rearrangement urazoles 3 to [4 + 2] cycloaddition products 4 increased with increasing electron withdrawal of the substituents on the aryl groups, i.e. it was largest for p ‐nitrophenyl and smallest for p ‐methoxyphenyl. NOE studies and X‐ray analyses (of 3b , of 4d with NMe instead of NPh, and of 5c ) were essential to assign the stereochemistry of these complex products.