Iridoid synthase that catalyzes the first step in generation of the iridoid ring scaffold using the linear monoterpene (6E)-8-oxogeranial as substrate. 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. (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. Highly divergent. Was originally thought to catalyze the entire reaction from (6E)-8-oxogeranial to nepetalactol including a cyclase step, but new results have shown that the cyclase is a different enzyme and this enzyme exclusively catalyzes a reduction step.