roup; (xxi) a guanidino group; (xxii) a benzyloxy group; (xxiii) a guanidinophenylthiomethyl group; (xxiv) a morpholinosulfonylphenoxymethyl group; (xxv) a pyridyloxymethyl group; or (xxvi) (1,1-dioxothiazol-3-yl)carbonyl group; or (b) when Y is a sulfur atom, R=hu 2 represents (i) a hydrogen atom; (ii) an alkyl group of one to four carbon atoms; (iii) an alkoxyl group of one to four carbon atoms;