Hetaryl onium salts

In an early example, Mukaiyama and coworkers used hetaryl onium salts for nucleoside synthesis. The active hetaryl onium salt is generated in situ from the reaction of 2-chloro-3-ethylbenzoxazolium tetrafluoroborate 77 and the glycosyl acceptor. With benzimidazole as glycosyl acceptor, the resulting 2-(l-benzimida-zoyl)benzoxazolium tetrafluoroborate 78 was obtained. The reaction between the hetaryl onium salt 78 and hemiacetal donor 1 occurs at 60 °C to activate the hemiacetal and thereby reveal the glycosyl acceptor. This procedure led to the formation of nucleoside 80 with exclusive 1,2-trans selectivity [139]. The nucleoside 81 was similarly prepared. Alternatively, 2-fluoro-l-methylpyridinium tosylate 79 directly... 

In an extension beyond hetaryl onium salt promoted hemiacetal activation, Ishido and coworkers have reported the carbodiimide activation of hemiacetals [141]. In the method (Scheme 3.13), the hemiacetal donor 1 is treated with a carbodiimide electrophile 83 and copper(I) chloride to provide glycosyl isourea intermediate 85. Highly susceptible to hydrolysis, the isourea 85 was not isolated but could be detected by 13C NMR and IR spectroscopy [142,143], Accordingly, the reaction between intermediate 85 and the glycosyl acceptor (NuH) provides glycoside product 3, along with urea by-product 84. 

---