Partial synthesis process for preparing (+) -vincamine and related indolic alkaloids

A process for preparing vincamine and other related indolic alkaloids represented by the general formula (I):

• ##STR4## wherein:
• (a) when X=Y=H, R'=--OH, R=--COOCH.sub.3, formula (I) represents vincamine,
• (b) when X=Y=H, R'=--COOCH.sub.3, R=--OH, formula (I) represents 16-epivincamine,
• (c) when X+Y=a double bond, R'=--OH, R=--COOCH.sub.3, formula (I) represents .DELTA..sup.14 -vincamine,
• (d) when X+Y=a double bond, R'=--COOCH.sub.3, R=--OH, formula (I) represents .DELTA..sup.14 -16-epivincamine,
• comprising the steps of:
• a compound of the general formula (II):
• ##STR5## wherein:
• (e) when X=Y=H, formula (II) represents vincadifformine, and
• (f) when X+Y=a double bond, formula (II) represents tabersonine, is used as a starting substrate;
• a reaction medium, comprising a low molecular weight alcohol, a trialkyl phosphite as a reducing agent, and a photosensitizer, is prepared;
• said substrate is added to said reaction medium, said trialkyl phosphite being present in a stoichiometrical excess with respect to said substrate;
• the thus formed reaction mixture is subjected to gaseous oxygen blowing under substantially ambient temperature and pressure conditions and at the same time said mixture is subjected to irradiation from a light source in order to activate oxygen in situ, for a time not exceeding 150 minutes, to obtain oxidation of said substrate and a resulting reduction;
• the treated reaction mixture is acidified, alkalized and extracted;
• and finally the extraction residue is subjected to fractional crystallization and/or column chromatography in order to separate distinct products represented by said general formula (I) in the pure state.