Chapter 3 Nuphar Alkaloids

Intramolecular iminium ion-sulfide charge-transfer association: a recurring theme in the study of thiaspirane alkaloids

“…The band's appearance was reversible; it disappeared on basification but reappeared on reacidification. The diamine alkaloids 1 and 3 (Rx = R2 = R3 = R4 = H) in acid revealed no such band nor did the ß-thio tertiary amines 7 and 8.3 The -hydroxy hemiaminal 9, as its iminium perchlorate in 95% ethanol, produced only short-wavelength end absorption («220 8000) in the region (220-235 nm) characteristic of simple iminium ions.4 Subsequently, Dr. Richard Hammer, working in my laboratory, observed that a-and ß-thioiminium perchlorates (10)(11)(12)(13)(14) in acetonitrile gave weak bands (e 170-1000) in the 275-295-nm region where the spectra of 15 and 16 were transparent.5 All these UV observations were consistent with charge-transfer (CT) behavior,6 789which had been demonstrated earlier in a similar case-the intramolecular interaction of divalent sulfur with carbonyl in thiacyclic ketones.7-9 However in the present instance, the ground-state association involves redistribution of charge which is distinguished from separation of charge involved in previous CT cases.…”