Stereoselective Synthesis of β-Anomeric 4‘-Thiaspirocyclic Ribonucleosides Carrying the Full Complement of RNA-Level Hydroxyl Substitution
摘要:
Stereoselective syntheses of a group of 4'-thiaspirocyclic ribonucleosides featuring both pyrimidine and purine classes and both possible configurations at C-5' are described. Use is made of the Pummerer reaction of substrates carrying an a-oriented 2,4-dimethoxybenzoyloxy substituent at C-2 in order to gain reliable stereocontrol via neighboring group participation. Irrespective of the S or R configuration of the pivotal sulfoxide intermediates, the nucleobase is captured from the beta-face. The competing process is formation of unsaturated sulfoxides, presumably via competing E2-type elimination. Although differences in reactivity between the two stereoisomeric series were noted, the common route has successfully given rise for the first time to desirable beta-anomeric sulfur-containing spiroribonucleosides with minimum formation of the alpha-anomers.
Stereoselective Synthesis of β-Anomeric 4‘-Thiaspirocyclic Ribonucleosides Carrying the Full Complement of RNA-Level Hydroxyl Substitution
摘要:
Stereoselective syntheses of a group of 4'-thiaspirocyclic ribonucleosides featuring both pyrimidine and purine classes and both possible configurations at C-5' are described. Use is made of the Pummerer reaction of substrates carrying an a-oriented 2,4-dimethoxybenzoyloxy substituent at C-2 in order to gain reliable stereocontrol via neighboring group participation. Irrespective of the S or R configuration of the pivotal sulfoxide intermediates, the nucleobase is captured from the beta-face. The competing process is formation of unsaturated sulfoxides, presumably via competing E2-type elimination. Although differences in reactivity between the two stereoisomeric series were noted, the common route has successfully given rise for the first time to desirable beta-anomeric sulfur-containing spiroribonucleosides with minimum formation of the alpha-anomers.