Synthesis of 7-Deazatriacanthine and Restricted Rotation of Amino Groups in Pyrrolo[2,3-d]Pyrimidines

Abstract

Isopentenylation of 7-deazaadenine results in the formation of 7-deazatriacanthine (2a) and its corresponding isomer 5a. Chromatographic separation was difficult, but after Dimroth rearrangement of 5a into the exocyclic compound 3a, 7-deazatricanthine could be isolated. Similiar to its parent purine compound, 3a cyclizes in the presence of strong acid to give the tricyclic system 4. NMR data reveal that 7-deazatriacanthine exists as the amino tautomer 2a. Protonation of N-1 alkyl-ated 7-deazaadenine occurs at N-7 to give compound 6 which exhibits restricted rotation of the amino group. The rotational barrier was determined from temperature dependent proton NMR spectra and found to be about 70 kJmol^?1.