Synthesis of 7-Halogenated 8-Aza-7-deaza-2′-deoxyguanosines and Related Pyrazolo[3,4-d]pyrimidine 2′-Deoxyribonucleosides

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Abstract
The synthesis of 7-bromo and 7-iodo derivatives of 8-aza-7-deaza-2′-deoxyguanosine (2, 3) as well as the halogenated 4-alkoxy derivatives 4 a - c and 5 a - c is described. Glycosylation of the halogenated pyrazolo[3,4-d]pyrimidine anions of 7 a -c or 8 a - c with 2-deoxy-3,5-di-O-(p-toluoyl)-α-d-erythro-pentofuranosyl chloride (9) yields regioisomeric glycosylation products, the N(1)-isomers 10 a - c and 11 a - c as well as the N(2)-compounds 12 a - c. The latter isomers lose their halogen during the glycosylation in the presence of non-anhydrous KOH. Anhydrous conditions (NaH) furnished 10 c, 11 c together with the halogenated N(2)-isomers 13 a,b. Compounds 10 a - c, and 11 a - c were deprotected and converted to the 4-alkoxy nucleosides 4 a - c and 5 a - c. The N(1)-nucleosides 4 c and 5 c were hydrolyzed to give the 7-bromo or 7-iodo derivatives of 8-aza-7-deaza-2′-deoxyguanosines 2 and 3. Different from regular 2′-deoxyribonucleosides the sugar moiety of pyrazolo[3,4-d]pyrimidine 2′-deoxyribonucleosides shows a preferred N-type pucker (3T2) in solution, a conformation which is also detected in the solid state.

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