Ytterbium(III) trifluoromethanesulfonate for specific activation of n-pentenyl orthoesters in the presence of acid-sensitive functionalities

Various Lewis acids have been examined as agents which react with N-iodosuccinimide to release iodonium ion for activating n-pentenylorthoester (NPOE) donors, particularly for coupling to acceptors that have acid-labile protecting groups. Although many of the reagents do not tolerate cyclic acetals, BF3·Et2O does. However, the latter cleaves p-methoxybenzyl (PMB), making it possible to use this Lewis acid to simultaneously promote coupling of NPOEs and remove PMB groups. Of several transition metal salts investigated, ytterbium triflate is outstanding in its ability to promote NPOE coupling with complete preservation of acid-labile protecting groups.