To investigate the stereoselectivity of the hydride transfer catalyzed by CrISY and AmISY, we initially analyzed reactions with model substrates lacking the 8-oxo group. These substrates can undergo enzymatic reduction, but the missing aldehyde moiety prevents subsequent cyclization (supplemental Fig. S2). With commercially available citral, a mixture of geranial (E-isomer) and neral (Z-isomer), as a substrate, CrISY yielded exclusively S-citronellal in a stereoconvergent fashion. In contrast, AmISY produced a 6:4 mix of R- and S-citronellal (supplemental Fig. S2c). To more closely reflect the structure of the physiological ISY substrate 8-oxogeranial, we synthesized geranial with low neral content (2.5%, supplemental Fig. S2a) by oxidation of geraniol with Dess-Martin periodinane. With this substrate, AmISY showed high stereoselectivity (supplemental Fig. S2d, 89% R). The R selectivity of AmISY observed here strongly supports the proposed biosynthesis of antirrhinoside via R-selective reduction of 8-oxogeranial. AmISY therefore appears to be the first example of an epi-iridoid synthase.