Chiroptische Eigenschaften und Äquilibrierungen stereoisomerer Yohimbane und ihrer 17‐Ketoderivate

The chiroptical properties (CD. and ORD.) of four stereoisomeric yohimbanes (1–4) and the corresponding 17‐keto derivatives 5–8 have been studied. The aromatic and the ketonic Cotton effects have been considered in terms of the appropriate Sector and Octant Rules, respectively. Positively charged nitrogen makes an ‘anti‐octant’ contribution to the ketonic Cotton effect. The chiroptical properties, the NMR. spectra and dehydrogenation reactions with mercuric acetate together indicate the presence of a C/D‐trans‐quinolizidine system in all the compounds studied, except those of the pseudo series (3 and 7), in which a C/D‐cis‐quinolizidine system is present. An explanation of the unexpectedly slow dehydrogenation of the 3‐epi‐allo compounds 4 and 8 is suggested. Conformational analysis of the results of equilibration experiments on pairs of 3‐epimeric compounds (in both glacial acetic acid at 145° and in K‐t‐butylate at 211°) leads independently to conclusions which agree with those drawn from the physical methods. The great differences between the equilibrium positions of the free bases and the protonated forms are due to the presence in the latter cases of oriented ion pairs R3N+H…︁−O2CCH3.