An improved method for manufacture of simvastatin of formula (I) in high purity. The process for preparation of compound (I) comprises ##STR00001## the steps of agitating a compound of formula (II), ##STR00002## wherein R is hydrogen, NH.sub.4.sup.+ or an alkali metal in an organic solvent and in an inert atmosphere at a temperature of between 27.degree. C. to 40.degree. C. in the presence of a weak acid followed by neutralization with an organic base and obtaining compound of formula (I) in high purity and substantially free of impurities through a step of isolation and crystallization.