The N-Acyliminium Pictet−Spengler Condensation as a Multicomponent Combinatorial Reaction on Solid Phase and Its Application to the Synthesis of Demethoxyfumitremorgin C Analogues

l -Tryptophan immobilized on polystyrene−Wang resin was sequentially reacted with an aldehyde and Fmoc-amino acid chloride. This generates a transient N -acyliminium species which undergoes Pictet−Spengler condensation to give a mixture of cis and trans tetrahydro-β-carbolines. Removal of the Fmoc protecting group, with concomitant diketopiperazine formation, results in cyclative cleavage of the desired products from the resin.