The compound represented with general formula (I): ##STR2## wherein R is an ethyl group, is known according to the description of Japanese Examined Patent Publication No. 63-61302; and the compound wherein R is a straight chain, branched or cyclical hydrocarbon group having 3 to 10 carbon atoms, or a straight chain or branched hydrocarbon group having 1 to 5 carbon atoms substituted with an aromat