Remarkable behaviour in acid hydrolysis of the cycloadducts formed between allyl bromide and 1,2,4-trichloro-3,5,5-trimethoxycyclopentadiene

Two isomeric compounds (2a and 2b) are formed on addition of allyl bromide to 1,2,4-trichloro-3,5,5-trimethoxycyclopentadiene (1). The striking difference in behaviour of these two isomers on acid catalysed solvent addition can be explained by assuming a stabilization of the intermediate carbenium ion by the near endo-CH₂Br group in 2a. The stabilization of this cation from the endo-side forces subsequent nucleophiles to attack from the exo-direction, resulting in an overall cis-exo addition to 2a. trans-Additions on the contrary are observed for 2b. Screening by the bromomethyl grouping of the intermediate cation formed from 2a gives a satisfactory explanation for the formation of the observed reaction products and for the sigmatropic rearrangement observed in poorly nucleophilic media. The structures of parent compounds 2a and 2b are confirmed from the different reaction products. The structure 4a, as previously reported⁴ to be formed under similar reaction circumstances, is revised and an alternative structure 4'a is proposed.