. . . . "oxypropyl, 2,3-dihydroxypropyl, 2-hydroxy-3-methoxypropyl, 2,3,4,5,6-pentahydroxyhexyl, 1,1-bis(hydroxymethyl)ethyl, 2-(2-hydroxyethoxy)ethyl, 2-methoxyethyl, 1-methyl-2-hydroxyethyl, 3-hydroxypropyl, 3-methoxypropyl and 1,1-bis(hydroxymethyl)-2-hydroxyethyl.Preferably, R4, R5 and R6 are a hydrogen atom, a methyl group and an alkyl group which may have 1 to 3 substituents selected from hydroxy gro" . . . . .