Circular dichroism of indole alkaloids. Part 1. Vincane derivatives

Abstract

Of the seven c.d. bands found between 320 and 180 nm in vincane derivatives the two at longest wavelengths (¹L_b and ¹L_a transitions) depend mainly on the chirality of rings C and E (i.e. the configuration at C-3). The increment of a 14-substituent (3α-compounds: negative for the ¹L_b and positive for the ¹L_a c.d. band) is independent of the nature and stereochemistry of the substituent. 14α-Substituents contribute positively at ca. 230 nm to the c.d. of 3α-vincane derivatives, 14β-substituents negatively; at ca. 250 nm, however, both give positive contributions. The c.d. increments of individual groups at C-14 over the whole wavelength range (in acetonitrile solution) are additive even for 14-gem-disubstituted products. Because of the complex nature of the c.d. between 250 and 200 nm the stereochemistry at C-14 of 3α-vincane derivatives can be deduced only by comparison of their c.d. curves with ‘synthetic’ ones.