Alpha-glucosidase inhibitors and their synthesis from a natural source

A process for the preparation of semi-synthetic 3-O-alkyl or aryl esters of (−) mesquitol, the said process comprising steps of:

• a) drying the wood of Dichrostachys cinerea in shade,
• b) powdering the shade dried wood of step (a),
• c) extracting the powder of step (b) with petroleum ether, followed by halogenated hydrocarbon solvent to obtain the plant extracts and a plant residue,
• d) soaking the plant residue of step (c) in methanol, filtering and concentrating the methanol extract to obtain a residue,
• e) purifying the residue of step (d) over silica gel column by eluting with mixture of organic solvent,
• f) obtaining pure (−) mesquitol,
• g) adding to (−) mesquitol of step (f) anhydrous potassium carbonate, a ketonic solvent, benzyl halide and refluxed the mixture under nitrogen atmosphere for a time period of 2 hour to 8 hour,
• h) filtering the mixture of step (g), washing the residue with ketonic solvent, combining the filtrate and the wash, evaporating under reduced pressure to obtain a residue,
• i) purifying the residue of step (h) to obtain pure terra-O-benzyl-mesquitol of formula (2),
• j) treating the compound of formula (2) of step (i) with N,N′-Dicyclohexyl carbodiimide (DCC) required aliphatic acid in an anhydrous methylene chloride under nitrogen atmosphere followed by addition of 4-dimethyl arminopyridine, stirring the mixture for 6 h to 18 h at room temperature, filtering the mixture, washing the residue with methylene chloride, combining the filtrate and washing to obtain methylene chloride solution,
• k) washing the methylene chloride solution of step (j) with water, drying over anhydrous sodium sulphate, filtered, evaporating the solvent to obtain a residue,
• l) purifying the residue of step (k) to obtain the required 3-O-alkyl esters of tetra-o-benzyl (−) mesquitol of formulae 3a to 3g.
• m) providing a cooled solution of compound (2) in anhydrous methylene chloride along with triethyl amine, flushing nitrogen,
• n) adding required benzoyl chloride to step (m) mixture stirring the mixture for 2 h to 6 h at an ambient temperature,
• o) adding water to the reaction mixture of step (n), extracting with methylene chloride, separating methylee chloride layer and aqueous layer,
• p) drying methylene chloride layer of step (o) over anhydrous sodium sulphate, filtering and evaporating the solvent to obtain a residue,
• q) purifying the residue of step (p) to obtain 3-O-aryl ester of tetra-o-benzy (−) mesquitol of formula 3h to 3k,
• r) stirring the compound of step (l) or step (q) in an alcoholic solution in the presence of palladium charcoal and hydrogen for a time period of 4 h to 8 h at room temperature, and
• s) filtering the mixture of step (r), removing the solvent from the filtrate to obtain the required 3-O-alkyl or aryl esters of (−) mesquitol of general formula (4).