Process for purifying lovastatin and simvastatin with reduced levels of dimeric impurities

A process for a statin selected from the group consisting of Lovastatin or Simvastatin comprising:

• a) dissolving or suspending the statin in the solvent comprising 1-70% or an alcohol selected from the group consisting of Methanol, Ethanol, i-Propanol, n-Propanol, i-Butanol, n-Butanol, t-Butanol and a mixture thereof;
• and a second solvent selected from the group dichloromethane, dichloroethane, chloroform, carbon tetrachloride, acetonitrile, petroleum ether, heptane, hexane, cyclohexane, acetone, butyl-methyl keton, methyl acetate, ethyl acetate, propyl acetate, i-butyl acetate, n-butyl acetate, t-butyl acetate, methyl formate, ethyl formate, propyl formate;
• and a mixture of said solvents;
• b) adding a mild base selected from the group aliphatic mono- or di- or triamines, aromatic amines, ammonium hydroxide, ammonia gas, an aqueous solution of any of the foregoing, and mixtures thereof;
• c) treating of the solution;
• and
• d) isolating lovastatin or simvastatin from said solvent mixture by crystallization or precipitation, at about −20° C. to about +25° C.