合成路线1

13) Later on Carroll and Kotian modified Shen's route by replacement of N-carbomethoxypyrrole with N-(t-carbobutoxy)pyrrole in order to avoid the drastic reaction condition in the deprotection step. However, there was the problem of competing retro Diels-Alder reaction of the cycloadduct (L) and dechlorination. Therefore, the selective reduction of the least substituted double bond of (L) was carried out first. Desulfonation and simultaneous reduction of (LI) with amalgam obtained a 1:2 mixture of exo-(LII) and endo-(LII) analogous to Shen's report. The endo-isomer was epimerized to the exo-isomer in 46% yield using Fletcher's procedure. Finally, deprotection of the BOC group under mild conditions produced (I) in nearly quantitative yield.