Iridoid synthase that catalyzes the first step in generation of the iridoid ring scaffold using the linear monoterpene (6E)-8-oxogeranial as substrate (PubMed:29091815). Iridoids comprise a large family of distinctive bicyclic monoterpenes that possess a wide range of pharmacological activities, including anticancer, anti-inflammatory, antifungal and antibacterial activities (Probable). Catalyzes the conversion of the linear monoterpene (6E)-8-oxogeranial to (S)-8-oxocitronellyl enol, a precursor of nepetalactones, which are metabolites that are both insect-repellent and have euphoric effect in cats (PubMed:29091815). (S)-8-oxocitronellyl enol + NADP(+) = (6E)-8-oxogeranial + H(+) + NADPH (S)-8-oxocitronellyl enol + NAD(+) = (6E)-8-oxogeranial + H(+) + NADH Belongs to the short-chain dehydrogenases/reductases (SDR) family.