Photochemical Synthesis of Azepinoindoles and Azocinoindoles from N-Chloroacetylindolylethylamines, and a Mechanistic Study based on the Correlation between Quantum Yields and Calculated Frontier Electron Densities of Indole Radicals

• Published in: Chemical & pharmaceutical bulletin Vol. 28; no. 3; pp. 900 - 909
• Main Authors: NARUTO, SHUNJI, YONEMITSU, OSAMU
• Format: Journal Article
• Language: English
• Published: Tokyo The Pharmaceutical Society of Japan 01.01.1980; Japan Science and Technology Agency
• Subjects: indole-1-radical
• ISSN: 0009-2363; 1347-5223
• DOI: 10.1248/cpb.28.900

On irradiation in 50% aqueous ethanol, the N-chloroacetyl derivatives of seven isomeric indolylethylamines gave the corresponding azocinoindole derivatives through photocyclization at the ortho and peri positions. Quantum yields for product formation with 2537 Å light showed the relative reactivity at each position of the indole ring to be in the order 3>6, 4>7, 2>5>1. In order to identify the reactive intermediary species and to establish the mechanism of the photocyclizations, frontier electron densities of several species of indole were calculated by CNDO/2 and INDO. The quantum yields for product formation from 1-substituted indole derivatives correlated fairly well (correlation coefficient r=0.82) with the singly occupied molecular orbitals (SOMO) electron densities (ED) of the 1-methylindole radical cation, but there was a better correlation (r=0.90) between the quantum yields for product formation from 1-unsubstituted compounds and the SOMO-ED of the indole-1-radical. These results show that the mechanism of the photocyclization of N-chloroacetylindolylethylamines involves both indole radical cations and indole-1-radicals as reactive intermediary species.