Cyclic sulfonium salt, method for production of cyclic sulfonium salt, and glycosidase inhibitor

A method for the production of a cyclic sulfonium salt, comprising a step for esterifying a pentose selected from D-xylose, D-ribose, D-arabinose, D-lyxose, L-ribose, L-arabinose and L-lyxose and a derivative thereof to form a cyclic sulfate ester of a heptitol with a protected hydroxy group, as represented by general formula (2):

• (wherein R1 and R2 are each a hydrogen atom or a hydroxy group-protective group, in which the hydroxy group-protective group comprises a cyclic acetal-protective group selected from —C(CH3)2—, —CH(CH3)— and —CHAr— (wherein Ar is a phenyl group or a substituted phenyl group), an ether-type protective group comprising an alkoxyalkyl group as represented by —CH2OR3 (wherein R3 is —CH2OCH3 or —CH2CH2OCH3) or a silyl ether-type protective group as represented by SiR43 or SiR42R5 (wherein R4 and R5 are each an alkyl group as represented by —CH3 or —C(CH3)3 or an aryl group as represented by —Ph);
• a coupling step for coupling the resulting cyclic sulfate ester of the heptitol (2) with a thiosugar as represented by general formula (7′):
• (wherein R3 is hydrogen atom or a hydroxy group-protective group comprising a cyclic acetal-protective group selected from −C(CH3)2—, —CH(CH3)— and —CHAr— (wherein Ar is a phenyl group or a substituted phenyl group), an ether-type protective group comprising an alkoxyalkyl group as represented by —CH2OR3 (wherein R3 is —CH2OCH3 or —CH2CH2OCH3) or a silyl ether-type protective group as represented by SiR43 or SiR42R5 (wherein R4 and R5 are each an alkyl group as represented by —CH3 or —C(CH3)3 or an aryl group as represented by —Ph)
• to yield a cyclic sulfonium salt with the protected hydroxy group as represented by general formula (8′):
• and a step for deprotecting the hydroxy group-protective group of the hydroxy group-protected cyclic sulfonium salt to yield a cyclic sulfonium salt as represented by general formula (1)