A Retro-Pictet-Spengler Reaction

The Pictet-Spengler route to tetrahydro-P-carbolines is frequently used in indole alkaloid synthesis, and much attention has been devoted in recent years to the development of enantiospecific Pictet-Spengler reactions and the factors which influence the diastereochemistry at C-l and C-3 (c.f 1). The diester la was prepared from 2 as 1 1 mixture of diastereomers and heated in 2% ethanolic hydrogen chloride for 3 hours in an attempt to effect epimerisation at C-l and increase the amount of trans isomer. The product, however, was 2, which was isolated in 76% yield. Heating of the cis and trans diesters 1 separately in ethanolic hydrogen chloride for 3 hours also gave 2. By comparison, when a cis, trans mixture (39 61) of lb was stirred in a mixture of methylene chloride and trifluoroacetic acid at room temperature for 90 minutes the trans diester was obtained in 96% yield. 

Suggest mechanisms to account for the degradation of 1 into 2, and for the epimerisation of cis-lb to trans-lb. 

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