Divergent and Stereocontrolled Approach to the Synthesis of Uracil Nucleosides Branched at the Anomeric Position
摘要:
Electrophilic addition of NBS/pivalic acid (bromopivaloyloxylation) to 1-[3,5-bis-O-(tert-butyldimethylsilyl)-2-deoxy-D-erythro-pent- 1-enofuranosyl]uracil (2), readily accessible from O-2,2'-anhydro-uridine, furnished 1-[2-bromo-3,5-bis-O-(tert-butyldimethyylsilyl)-2-deoxy-1-(pivaloyloxy)-beta-D-ara-binofuranosyl]uracil (7) stereoselectively. This compound (7), having a leaving group at the 1'-position as well as 2'-beta-Br that could exert anchimeric assistance, serves as versatile intermediate for the stereocontrolled synthesis of various types of 1'-C-branched derivatives through nucleophilic substitutions by the use of organosilicon and organoaluminum reagents. The whole sequence constitutes the first example of the conversion of a naturally-occurring nucleoside to the analogues branched at the anomeric position.
Face-selective electrophilic addition (bromo-pivaloyloxylation) to 1-[3,5-bis-O-(tert-butyldimethylsilyl)-2-deoxy-D-erythro-pent-1-enofuranosyl]uracil (1), when combined with nucleophilic substitution using organosilicon or organoaluminum reagents, provides a new and highly divergent C-C bond forming method at the anomeric position.