Biosynthesis of the brevianamides. An ab initio study of the biosynthetic intramolecular Diels-Alder cycloaddition

An ab initio study of the transition structures for the intramolecular Diels-Alder cycloaddition of the aza diene 6 to give brevianamides A, 1, and B, 2, has been carried out with analytical gradients at ab initio 3-21G and 6-31G* basis sets within Hartree-Fock procedures. The correlation effects have been estimated by using the perturbational approach at MP2 and MP3 levels, The geometry, electronic structure, and transition vector component are qualitatively model independent in this study. An analysis of the geometries and energies of the corresponding transition structures provides an explanation for the fact that brevianamide A, 1, is biosynthesized in larger quantities than brevianamide B, 2, while their respective epimers at C7 are not formed.