From protopines to berbines: synthesis of 1-methoxystylopine and its N-metho salts from coulteropine

The transformation of protopines into berbines under improved conditions has been used to synthesize 1-methoxystylopine. Coulteropine, the main alkaloid from Romneya coulteri, was used as the starting protopine to accomplish the stereocontrolled synthesis of both cis and trans N-methyl-1-methoxystylopinium salts. The results of ab initio calculations (B3LYP/6-31G∗∗) which are consistent with experimental data, sustain the influence of the C-1 substituent on both the conformational equilibrium of berbines and the rate of N-methylation.