Catalyzes the conversion of syn-copalyl diphosphate to stemodene. 9alpha-copalyl diphosphate = diphosphate + stemod-13(17)-ene Binds 3 Mg(2+) ions per subunit. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Catalyzes the committed step in the biosynthesis of the stemodane family of diterpenoid secondary metabolites, some of which possess mild antiviral activity. Also produces stemod-12-ene and stemar-13-ene as minor products. Belongs to the terpene synthase family. According to PubMed:16256063, this sequence exists in both japonica and indica subspecies and was submitted by the authors as a japonica cultivar nipponbare sequence. However, the corresponding sequence cannot be found in the currently available japonica nipponbare genome or japonica expressed sequence tag data, but matches perfectly in indica genome sequence. It is therefore annotated as indica sequence.