No abstract

The reaction of the key compound 14 with methyl 4‐formyl‐butanoate (9) or with 5‐benzoyloxypentanal (13) gave the D‐seco‐aspidospermane derivatives 17 and 18, respectively. Compound 17 was indirectly and 18 was directly converted to (±)‐20‐deethylvincadifformine (7) and (±)‐20‐deethyl‐20‐epivincadifformine (8). Epimerization occurred in both cases. The thioxo compound 23 was used for the synthesis of (±)‐20‐deethyltabersonine (28) and (±)‐20‐deethyl‐3‐oxotabersonine (27). Oxidative ring transformation of the (±)‐20‐deethylvincadifformine (7) gave (±)‐16‐deethylapovincamine (29).

The individual epimerization steps of vincadifformine (1), deethylvincadifformine isomers (2,3), their synthetic intermediates (4–13) as well as that of simpler compounds with D‐secoaspidospermane skeleton (15–17) were studied a) in protic medium (boiling deuteroacetic acidic) and b) under reductive (with sodium borodeuteride in boiling acetic acid, or with sodium borohydride in boiling deuteroacetic acid) conditions.