A SUCCESSFUL ACID-PROMOTED ASYMMETRIC PICTET-SPENGLER REACTION OF N-A-BOC PROTECTED TRYPTOPHANS - EFFECT OF THE BOC GROUP ON REACTIVITY ANDSTEREOSELECTIVITY

N-a-BOC protected tryptophans 1 and 2 successfully undergo the Pictet- Spengler cyclization with a series of aldehydes in the presence of an acid catalyst (p-TSA) to furnish a mixture of trans and cis 1,2,3,4-te trahydro-beta-carbolines (3 and 4). During removal of the BOC function with TFA, the mixture of trans (3) and cis (4) isomers was enantiospe cifically converted into the trans N-a-H 1,2,3,4-tetrahydro-beta-carbo line 5, a key intermediate for the construction of N-a-H substituted s arpagine alkaloids.