General and Modular Synthesis of Isomeric 5-Substituted Pyridin-2-yl and 6-Substituted Pyridin-3-yl <i>C</i>-Ribonucleosides Bearing Diverse Alkyl, Aryl, Hetaryl, Amino, Carbamoyl, and Hydroxy Groups
作者:Martin Štefko、Lenka Slavětínská、Blanka Klepetářová、Michal Hocek
DOI:10.1021/jo200949c
日期:2011.8.19
A general modular and practical methodology for preparation of diverse 5-substituted pyriclin-2-yl and 6-substituted pyridin-3-yl C-ribonucleosides was developed. Regioselective lithiation of 2,5-dibromopyridine proceeded at position 5 or 2 depending on the solvent, and the resulting bromopyridyl lithium species underwent additions to TBS-protected ribonolactone and follow-up transformations to corresponding acetylated hemiketal intermediates 7 and 10 that were diastereoselectively reduced to give either 5-bromopyridin-2-yl or 6-bromopyridin3-yl silyl-protected C-ribonucleosides 8 or 11 in 68% and 77% overall yields as pure beta-anomers. These bromopyridyl C-nucleoside intermediates were then subjected to a series of palladium-catalyzed cross-coupling reactions, aminations, aminocarbonylations, and hydroxylations to give a series of protected 1 beta-(5-alkyl-, 5-aryl-, 5-amino-, 5-carbamoyl-, and 5-hydroxypyridin-2-yl)-C-ribonucleosides 13a-i and beta-(6-alkyl-, 6-aryl-, 6-amino-, 6-carbamoyl-, and 6-hydroxypyridin-3-yl)-C-ribonucleosides 15a-i. Deprotection of silylated nucleosides by Et3N center dot 3HF, TBAF, or TFA gave a series of free C-nucleosides 14a-i and 16a-i.