| Abstract: |
Abstract: The corrposition of the products of reaction of l-(2,3-anhydro-5-0-benzoyl-fHD-lyxofuranosyl)uracil (l) with NH4N3 was studied by a reverse-phase HPLC system which was found to separate the 3-azido-arabino 2 and 2-azido-xylo 3 isomers that were formed. The use of a 10:1 ratio of NH.N3 to 1 in refluxing EtOH was found to minimize ring opening at C-2 (7%). The higher stereoselectivity of ring opening produced by using a large excess of NH4N3 was suppressed by conducting the reaction in DMF. Preventing the escape of the NH3 by-product only resulted in debenzoylation. The isolation of pure, crystalline 3 was achieved by reverse-phase preparative HPLC. Separation from the arabino isomer was also effected by debenzoylation and selective acetonide formation with the xylo isomer, which allowed facile isolation of the latter by normal phase chromatography. Hydrolysis of the acetonide 7 provided unprotected 2-azido-xylo nucleoside 6, which was also obtained by NaOMe treatment of 3. The mechanistic basis for the stereo-selectivity of epoxide opening is discussed. © 1989, Taylor & Francis Group, LLC. All rights reserved. |
