L-Tryptophan esters

The most interesting report with respect to preparative applications concerns a ligase antibody obtained by immunization against phosphodiester hapten 5 (Scheme 2) [22]. This antibody catalyzes the coupling of an N-acylated aminoacid 4-nitrophenylester such as 6 with an L-tryptophan ester or amide such as 7 with useful selectivity and rate. 

Bailey and coworkers have explored the stereoselectivity of the Pictet-Spengler reaction with L-tryptophan esters [330]. They found that use of TFA could effect kinetically-controlled cyclization. For 1,3-disubstituted p-carbolines, the preference was for the cA-1,3 product (diequatorial). For 1,2,3-trisubstituted (A-benzyl) analogs, the kinetic preference is for cw-l,3-diaxial product. A particularly useful derivative was obtained by converting the ester group to cyanomethyl. Originally the cyanomethyl derivative was cyclized via the enamide prepared from methyl propiolate, and gave a 3 1 cis trans ratio [331]. 

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