The present invention provides two synthetic routes for the preparation of Temsirolimus (compound 1b and analog of Temsirolimus 1a). The first route includes the synthesis of CCI-779 by directly reacting rapamycin (4b) or Prolyl-rapamycin (4a) with substituent-2,2-bis(methoxy) propionic acid anhydride(11) in the presence of an organic base, followed by deprotection to give CCI-779 or Proline CCI-779. The second route includes a process involving a reaction of rapamycin-OH-31-sily ether (4d) or Prolyl-rapamycin-OH-31-sily ether (4c) with substituent-2,2-bis(methoxy) propionic acid anhydride(11) in the presence of an organic base and followed by subsequent hydrolysis step to obtain the desired CCI-779 or Proline CCI-779.Compound 11, as described in this invention, is stable at room temperature, cost effective and ease of processing.