This paper presents a method for deriving valid relational syllogisms with comparative relations, called DMcr. We first introduce some basic notions and notation for comparative relations, predicates, quantifiers, scope dominance, scopelessness and syllogisms. We next describe DMcr in detail and prove that DMcr is sound. We then discuss how we can extend DMcr so as to derive more valid syllogisms, including the use of existential assumptions, and restriction to finite domains. Finally, we point to several possible directions for future studies on relational syllogisms.