NOVELTY - Preparation of zidovudine (I) involves reacting 5'-O-pivaloyl-2,3'-anhydrothymidine (IV) with sodium azide in presence of ammonium chloride in an aprotic solvent at 90-120 degrees C, cooling the reaction mixture to 20-30 degrees C, adding water to the reaction mixture to obtain a precipitate, reacting 5'-O-pivaloyl-3'-azido-3'-deoxythymidine (V) with aqueous alkali metal hydroxide solution in methanol at 25-40 degrees C, charcoalizing to obtain a charcoalized solution, evaporating to dryness and crystallizing the crude zidovudine.    USE - Preparation of zidovudine used as antiretroviral drug for treating HIV and AIDS.    ADVANTAGE - The method is environmentally-friendly and enables simple, safe and efficient preparation of zidovudine with excellent feasibility, and high yield and high purity, and removes organic and inorganic impurities. The solvent used in preparation of zidovudine is recoverable and reusable.    DETAILED DESCRIPTION - Preparation of zidovudine of formula (I) involves reacting 5'-O-pivaloyl-2,3'-anhydrothymidine of formula (IV) with sodium azide in presence of ammonium chloride in an aprotic solvent at 90-120 degrees C, cooling the reaction mixture to 20-30 degrees C, adding water to the reaction mixture to obtain a precipitate, reacting 5'-O-pivaloyl-3'-azido-3'-deoxythymidine of formula (V) with aqueous alkali metal hydroxide solution in methanol at 25-40 degrees C, adjusting pH of the reaction mixture to 6-7.5, charcoalizing to obtain a charcoalized solution, evaporating the charcoalized solution to dryness to obtain crude zidovudine and crystallizing the crude zidovudine with water. Zidovudine has HPLC purity of 99.5% or more.Piv=pivaloyl.    An INDEPENDENT CLAIM is included for preparation of 5'-O-pivaloyl-2.3'-anhydrothymidine (IV) used as intermediate for preparing compound (I) involves reacting 3-thymidine with pivaloyl chloride in pyridine at 0-10 degrees C to obtain 5'-O-pivaloylthymidine of formula (II), in-situ reacting 5'-O-pivaloylthymidine (II) with mesyl chloride at 25-30 degrees C, distilling off pyridine to obtain a residue, adding water to the residue and heating the mixture to 60-75 degrees C, cooling the mixture to 25-30 degrees C, isolating the crude 5'-O-pivaloyl-3'-O-mesylthymidine by filtration, recrystallizing the crude 5'-O-pivaloyl-3'-O-mesylthymidine using toluene as solvent to obtain 5'-O-pivaloyl-3'-O-mesylthymidine of formula (III), reacting 5'-O-pivaloyl-3'-O-mesylthymidine (III) with an alkali metal hydroxide in 3-8C aliphatic alcohol at 35-55 degrees C, isolating crude 5,-O-pivaloyl-2,3'-anhydrothymidine (IV) from reaction mixture and purifying the crude 5'-O-pivaloyl,2,3-anhydrothymidine (IV), treating with dichloromethane and toluene. The HPLC purity of 5'-O-pivaloyl-2,3'-anhydrothymidine (IV) and 5'-O-pivaloyl-3'-O-mesylthymidine is 99% or more and 97% or more respectively.    Ms=mesyl.