Synthesis 1-thioglycosides from 1-sulfenates

Abstract

Thioalkyl and thioaryl groups can be used to protect the anomeric position of sugars and can be activated to yield glycosyl donors I,‘. Thioglycosides can be activated by mercury(I1) sulfate 3, mercury(I1) chloride 4*5, phenyl mercury trifluoromethanesulfonate 6, mercury(II) benzoate 7, mercury01) nitrate *, copper trifluoromethanesulfonate 9, and lead011 perchlorate lo. Generation of glycosyl donors involves the conversion of thioglycosides into glycosyl bromides ‘i-l3 and glycosyl fluorides i4, and activation by use of methyl triflate 15-18, dimethyl(methyltio) sulfonium triflate 19, and nitrosyl tetrafluoroborate 20. Numerous methods are available for the preparation of thioglycosides 21*22, of which the most general is the S,2 reaction of alkali salts of thiophenols or alkanethiols with acetylglycosyl halides . 24 Acetylated alkyl l-thio-D-glucopyranosides can be obtained by reaction of 2,3,4,6-tetra-O-ace@-1-thio-/3-n-glucopyranose with alkyl bromides or iodides Z. In order to prepare 1-thioglycosides with non-participating groups, these compounds were deacylated and subsequently alkylated, usually with benzyl halides. The trichloroacetimidate method has been used to synthesise thio sugars and thioglycosides from sugar derivatives with HO-l unprotected . 26 A disadvantage of this procedure is the use of toxic and malodorous thiols and thiophenols. Carbohydrate sulfenates may be prepared 27 by the reaction of appropriate sugar derivatives with 2nitrobenzenesulfenyl chloride or 2,4-dinitrobenzenesulfenyl chloride in the presence of triethylamine, and we now report a new synthesis of aryl 1-thioglycosides by deoxygenation of monosaccharide sulfenates with triethyl phosphite. Thus, 2,3,4,6-tetra-0-methyl-l-O-(2-nitrophenylsulfenyl)-~-~-glucopyranose (la>, 2,3,4,6-tetra-O-benzyl-1-0-(2-nitrophenylsulfenyl)~-D-glucopyranose (2a), 2,3,4,6-tetra-0-ben~l-l-O-(2-nitrophenylsulfenyl)-~,~-~-galactopyranose (3a,