311  US 9,323,150 B2  312  pentyloxyethyl group, a cyclopentyloxypethyl group, a  cyclohexyloxyethyl group,  and a cyclohexyloxypethyl  group.  As the alkoxycarbonyl group, there can be mentioned, for  example, a linear or branched alkoxycarbonyl group having 2  to 21 carbon atoms, such as a methoxycarbonyl group, an  ethoxycarbonyl group, an n-propoxycarbonyl group, an  i-propoxycarbonyl group, an n-butoxycarbonyl group, a  2-methylpropoxycarbonyl group, a 1-methylpropoxycarbo- nyl group, and a t-butoxycarbonyl group.  As the cycloalkoxycarbonyl group, there can be men› tioned, for example, those having 4 to 21 carbon atoms, such  as a cyclopentyloxycarbonyl group and a cyclohexyloxycar› bonyl group.  As the alkoxycarbonyloxy group, there can be mentioned,  for example, a linear or branched alkoxycarbonyloxy group  having 2 to 21 carbon atoms, such as a methoxycarbonyloxy  group, an ethoxycarbonyloxy group, an n-propoxycarbony› loxy group, ani-propoxycarbonyloxy group, ann-butoxycar- bonyloxy group, and a t-butoxycarbonyloxy group.  As the cycloalkoxycarbonyloxy group, there can be men› tioned, for example, those having 4 to 21 carbon atoms, such  as a cyclopentyloxycarbonyloxy group and a cyclohexyloxy› carbonyloxy group.  The cyclic structure that may be formed by the bonding of  the two R$_{15}$s to each other is preferably a 5- or 6-membered  ring, especially a 5-membered ring (namely, a tetrahy› drothiophene ring) formed by two bivalent R$_{15}$s in coopera- tion with the sulfur atom of general formula (ZI-lA).  The cyclic structure may have substituents. As such sub› stituents, there can be mentioned, for example, a hydroxyl  group, a carboxyl group, a cyano group, anitro group, an alkyl  group, a cycloalkyl group, an alkoxy group, an alkoxyalkyl  group, an alkoxycarbonyl group, an alkoxycarbonyloxy  group and the like. A plurality of such a substituents may be  introduced in this ring structure. The plurality of such sub› stituents may be bonded to each other to thereby form a ring  (for example, any of aromatic or nonaromatic hydrocarbon  rings, aromatic or nonaromatic heterocycles, and polycyclic  condensed rings each comprised of a combination of two or  more of these).  It is especially preferred for the R $_{15 }$to be a methy 1 group, an  ethyl group, or the bivalent group allowing two R$_{15}$s to be  bonded to each other so as to form a tetrahydrothiophene ring  structure in cooperation with the sulfur atom of the general  formula (ZI-lA).  Substituents may further be introduced in the alkyl group,  cycloalkyl group, alkoxy group and alkoxycarbonyl group  represented by R$_{13}$ ,  and the alkyl group, cycloalkyl group,  alkoxy group, alkylsulfonyl group and cycloalkylsulfonyl  group represented by  Rw  As such a substituent, there can be  mentioned, for example, a hydroxyl group, an alkoxy group,  an alkoxycarbonyl group, a halogen atom ( especially, a fluo- rine atom) or the like.  Specific examples of the cation moieties in the compounds  represented by general formula (ZI-lA) will be given below.  c5-g-~o  HO-o-~o  HO~:~  H3C  H3C  d  \}  f~  H3C-!~OH  HO:P-o  CH30  C12H2sO-o-~o  0  HO~~  H3C  -continued 