Configurational and Conformational Studies of Quinolizidine and Beta-Carboline Moieties in the Corynanthe-Tryptamine Alkaloids

Configurational and conformational assignments of 11 Corynanthe-Tryptamine alkaloids (usambarane skeleton) were performed based on the correlation of the high-level calculated and experimental ¹H and ¹³C NMR chemical shifts. For some compounds, the reassignment of a number of individual signals together with spectral assignment of experimentally unresolved peaks was suggested. The different conformations of the C/D quinolizidine ring system appear strictly dependent of the structure of the side chain (ethyl-, vinyl- or ethylidenic); in the latter case, the configuration (E or Z) of the 19-20 double bond of the ethylidenic chain is determinant to establish a cis- or a trans-quinolizidine system of rings C/D. The conformation is also influenced by the equatorial or axial conformation of (C15) substituents.