Total Synthesis of Gypsetin, Deoxybrevianamide E, Brevianamide E, and Tryprostatin B: Novel Constructions of 2,3-Disubstituted Indoles

Abstract

A concise and efficient total synthesis of the acyl-CoA:cholesterol acyltransferase inhibitor gypsetin (1) is described. The route features a straightforward method for the introduction of a reverse prenyl group into the C2-position of an N-phthaloyl-protected tryptophan (11). The total synthesis of gypsetin was completed by the dimethyldioxirane-promoted double-oxidative cyclization of a prefashioned diketopiperazine (19). Total syntheses of deoxybrevianamide E (24) and brevianamide E (25) following similar procedures are also described. The reaction of nucleophiles with in situ-generated 3-chloroindolenines provides a route to 2,3-disubstituted indoles from 3-substituted precursors. Indications of the scope and limitations of such reactions are provided. A total synthesis of tryprostatin B (41), a diketopiperazine derived from an l-tryptophan derivative (bearing a prenyl group at the α position of the indole) and l-proline, was accomplished. The key step involved the introduction of the prenyl function onto a protected tryptophan congener (11). A route for the prenylation of ketones with virtually no competitive reverse prenylation is also provided.