Solid-Phase Intramolecular N-Acyliminium Pictet−Spengler Reactions as Crossroads to Scaffold Diversity

A novel solid-phase intramolecular Pictet−Spengler reaction is presented. The approach utilizes masked aldehyde building blocks protected as their N-Boc-1,3-oxazinanes for the clean generation of solid-supported aldehydes. When exposed to simple acidic treatment, the aldehyde functionality is rapidly released and becomes susceptible to nucleophilic attack from an amide nitrogen of the peptide backbone, which results in the formation of a highly reactive cyclic N-acyliminium ion. Subsequently, a quantitative and highly stereoselective Pictet−Spengler reaction takes place by attack of the indole from a neighboring tryptophan, thus appending two new N-fused rings to the indole moiety. Extension of this intramolecular reaction to substituted indoles, and other reactive heterocycles, such as furane and thiophenes, provides a convenient and rapid access to a range of pharmacologically interesting tri- and tetracyclic scaffolds. Finally, the reaction products may conveniently be released from the solid support (PEGA) by cleavage of the base-labile linker (HMBA).