| http://www.w3.org/ns/prov#value | - 2 combine with each other to complete a saturated 5- or 6-membered ring.More desirably, R.sub.1 is a hydrogen atom or an alkyl group containing an amino group or a heterocyclylic group as a substituent, while R.sub.2 is an alkyl group containing 1 to 10 carbon atoms which may have a substituent or an aryl groupcontaining 6 to 12 carbon atoms which may have a substituent; or R.sub.1 and R.sub.2 are
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