a I wherein X is a sulfur atom and Y is a vinylene group which may be substituted by lower alkyl group, that is, the compounds of this invention shown by formula Ic ##STR25## wherein R.sub.1,R.sub.2 and R.sub.3 have the same meaning as above and R.sub.6 represents a hydrogen atom or a lower alkyl group, can be produced by reacting the compound shown by formula Id ##STR26## wherein R.sub.1, R.sub.2