The present invention provides intermediates of formula IIb and IIc useful for the preparation of Simvastatin: ##STR1## wherein R.sub.1 and R.sub.2 are each independently an alkyl, heteroalkyl, aryl or heteroaryl moiety, or R.sub.1 and R.sub.2, taken together, form a heterocyclic moiety containing 5-8 atoms; wherein each of the foregoing alkyl and heteroalkyl moieties may be linear or branched, substituted or unsubstituted, cyclic or acyclic or saturated or unsaturated, and each of the foregoing heterocyclic, aryl and heteroaryl moieties may be substituted or unsubstituted.