Abstract
In spite of exclusive lithiation at the 8-position of 9-(2,3,5-tris-O-TBDMS-β-d-ribofuranosyl)-6-chloropurine (2) with LDA, subsequent quenching of its lithiated species with Bu3SnCl (or TMSCl) results in the formation of 2-substituted products. Under optimized reaction conditions, where LTMP was used as a lithiating agent, 9-(2,3,5-tris-O-TBDMS-β-d-ribofuranosyl)-6-chloro-2-(tributylstannyl)purine (11) was formed in quantitative yield. Several experiments carried out to verify the reaction mechanism suggested that an anionic stannyl (or silyl) transfer from the 8- to the 2-position had been involved. Manipulation of the 2-tributylstannyl group in 11 and in its adenine counterpart (22) has disclosed a new entry to 2-substituted purine nucleosides. This chemistry was briefly applied to the synthesis of the 2-fluoro analogue of neplanocin A.
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