Switchable synthesis of glycosyl selenides or diselenides with direct use of selenium as the selenating agent

Symmetrical diglycosyl-selenides or diselenides can be readily prepared with high chemoselectivity by direct use of elementary selenium as a cheap selenating agent (reduced in situ by sodium borohydride), and glycosyl iodides (or other halides) as reactive glycosyl donors. The chemoselectivity of the synthesis is critically affected by both the presence or not of triethyl phosphite, and the sodium borohydride to selenium stoichiometric ratio. Taking advantage of indications from mechanistic studies, the scope of the strategy was further extended to the synthesis of unsymmetrical diglycosyl selenides. Preliminary biological experiments evidenced an interesting antiproliferative effect of galactosyl diselenide against some tumor cell lines, otherwise negligible with the corresponding selenide.