Patent ID: 7569575

Claim:
A method for the synthesis of a compound of formula IV wherein X is O; Z is S; B is a nucleobase; R 3 is selected from —R H , —N 3 , —NR H R H* , —NR H C(O)R H* , —C(O)NR H R H* , —OR H , —OC(O)R H , —C(O)OR H , —SR H , —SC(O)R H , and tri(C 1-6 -alkyl/aryl)silyloxy; each R H and R H* independently being selected from hydrogen, optionally substituted C 1-6 -alkyl, optionally substituted aryl, and optionally substituted aryl-C 1-6 -alkyl; A 4 and A 5 independently are selected from C 1-6 -alkylene; and R 5 is selected from iodo, bromo, chloro, C 1-6 -alkylsulfonyloxy, cyclohexanesulfonyloxy and cyclopentanesulfonyloxy, each optionally substituted with one or more halogen, and arylsulfonyloxy optionally substituted with one or more substituents selected from nitro, halogen, C 1-6 -alkyl, and C 1-6 -alkyl substituted with one or more halogen; said method comprising the following steps: treating an intermediate of the formula I: wherein X, B, R 3 , A 4 , and A 5 are as defined above; R 2 is selected from iodo, C 1-6 -alkylsulfonyloxy optionally substituted with one or more halogen and arylsulfonyloxy optionally substituted with one or more substituents selected from nitro, halogen, C 1-6 -alkyl, and C 1-6 -alkyl substituted with one or more halogen; R 3 and R 2 may together form an epoxide and R 4 and R 5 independently are as defined for R 5 above, or R 4 and R 5 together constitutes a tetra(C 1-6 -alkyl)disiloxanylidene group; with a nucleophile selected from Na 2 S and potassium thioacetate, so as to substitute R 2 , and effecting ring-closure between the C2′ and C4′ positions under the influence of lithium hydroxide in a polar aprotic solvent so as to yield the compound formula IV.