Abstract
Imidazo [1, 2-a] indole-spirolactone 13, the principal ring system of tryptoquivalines, was successfully synthesized by a new method via oxidative double cyclization of 1-(N-alkoxy-carbonylalanyl)-3-indolepropionic acids (5) with N-bromosuccinimide, which were synthesized by acylation of benzyl 3-indolepropionate (10) with N-alkoxycarbonyl alanine p-nitrophenyl ester (12). The use of KF, 18-crown-6 and (iso-Pr)2NEt in acetonitrile lead to high yields of benzyl 1-(N-methoxycarbonyl-α-methylalanyl)-3-indolepropionates (4). The removal of the protecting group of 13c provided 2. The mechanisms of these reactions are discussed.
