Phenylsulfanylation of 3 ',4 '-Unsaturated Adenosine Employing Thiophenol-N-Iodosuccinimide Leads to 4 '-Phenylsulfanylcordycepin: Synthesis of 4 '-Substituted Cordycepins on the Basis of Substitution of the Phenylsulfanyl Leaving Group

[摘要]：Upon reaction of the 3',4'-unsaturated adenosine derivative 2 with N-iodosuccinimide (NIS) and thiophenol, an unexpected electrophilic hydrophenylsulfanylation proceeded to provide 4'-phenylsulfanylcordycepin 7 in 79% yield with the ratio 7a/7b = 6.6/1. A study of the reaction mechanism revealed that hydrogen iodide (HI) generated from NIS and PhSH acted as an active species. On the basis of a deuterium experiment using PhSD, initial protonation occurred at the beta face of the double bond to furnish the beta-pi complex III, which underwent anti addition of PhSH as a major pathway. Nucleophilic substitution of N(6)-pivaloylated 9 with various alcohols in the presence of N-bromosuccinimide (NBS) gave the respective 4'-alpha-alkoxycordycepins 15a-21a as the major stereoisomers. Use of DAST in place of an alcohol gave the 4'-alpha-fluoro analogue 23a stereoselectively. Radical-mediated carbon carbon bond construction was also applicable to 7, giving 4'-alpha-allylcordycepin (24a) and 4'-alpha-cyanoethylcordycepin (25) derivatives.

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