Iridoid synthase that catalyzes the first step in generation of the iridoid ring scaffold using the linear monoterpene (6E)-8-oxogeranial as substrate (PubMed:28701463). Reduces 8-oxogeranial, generating an unstable product that is subsequently cyclized into several possible products, either non-enzymically or by dedicated cyclases (PubMed:28701463). 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 (PubMed:28701463). (S)-8-oxocitronellyl enol + NADP(+) = (6E)-8-oxogeranial + H(+) + NADPH (S)-8-oxocitronellyl enol + NAD(+) = (6E)-8-oxogeranial + H(+) + NADH (R)-8-oxocitronellyl enol + NADP(+) = (6E)-8-oxogeranial + H(+) + NADPH kcat is 0.72 sec(-1) with (6E)-8-oxogeranial as substrate. Expressed in leaves. Belongs to the short-chain dehydrogenases/reductases (SDR) family. Highly divergent.