PREPARATION OF ALKYL-SUBSTITUTED INDOLES IN THE BENZENE PORTION .11. TOTAL SYNTHESIS OF (6R,8S)-HERBINDOLE-A, (6R,8S)-HERBINDOLE-B, (6R,8S)-HERBINDOLE-C, (6R,8S)-CIS-TRIKENTRIN-A, (6R,8S)-CIS-TRIKENTRIN-B, (6R,8R)-TRANS-TRIKENTRIN-B, AND (6R,8R)-ISO-TRANS-TRIKENTRIN-B - DETERMINATION OF THE ABSOLUTE STRUCTURES OF NATURAL HERBINDOLES AND TRIKENTRINS

The key intermediate, (3R,5S)-3-5-dimethyl-1-cyclopentenylmethanol (13) for chiral syntheses of herbindoles and trikentrins, was prepared from the known Dields-Alder adduct 12. Employing pertinent methodologies developed in the previous model study (preceding paper), the title herbindoles 31, 34, and 38 as well as trikentrins 4, 6, 41, and 47 having (6R,8S)- or (6R,8R)-absolute configuration of the dimethyl groups on the 1,6,7,8-tetrahydrocyclopent[g]indole moiety were synthesized starting from 13 by way of precursor compounds 24b, 27a, and 27b for the crucial indole cyclization reaction. The following results were also obtained. (i) The proposed chemical structures of three herbindoles were confirmed. (ii) The absolute structures of herbindole A and trans-trikentrin B were established to be 1 and 7 by directly comparing optical properties between synthetic materials and the natural products. (iii) The absolute structures of herbindole B (2) and herbindole C (3) were assumed by analogy with 1. (iv) The absolute structures of cis-trikentrin B (6) and iso-trans-trikentrin B (8) were estimated by a circular dichroism (CD) analysis study. (v) Our previous proposal for the absolute configuration of cis-trikentrin A (4) was further confirmed.