Compounds represented by the general formula (I), as well as intermediates for the synthesis of thereof: ##STR1## where R.sup.1 is a hydrogen atom or an acyl group; R.sup.2 is a lower alkyl group; R.sup.3 is a hydrogen atom or a lower alkyl group; R.sup.4, R.sup.5 and R.sup.6, which may be the same or different, are a hydrogen atom or an optionally substituted alkyl, alkenyl, alkynyl or aryl group; R.sup.2 and R.sup.4, when taken together, may form a 5-membered ring; R.sup.5 and R.sup.6, when taken together, may form a cycloalkyl group or a heterocyclic group in which at least one methylene on the ring of a cycloalkyl group is substituted by an oxygen atom, a sulfur atom or an alkyl-substituted nitrogen atom, provided that R.sup.6 is not present if the ring formed by R.sup.2 and R.sup.4 taken together is a benzofuran ring. The compounds represented by the general formula (I) have a highly selective anti-oxidative activity and are useful as therapeutics of ischemic diseases such as arteriosclerosis and myocardial infarction.