The absolute stereochemistry of a diterpene from Ballota aucheri Christopher A. Gray, Douglas E.A. Rivett, Michael T. Davies-Coleman* Department of Chemistry, Rhodes University, Grahamstown, 6140, South Africa Received 7 October 2002; received in revised form 25 February 2003 Abstract The semi-synthetic transformation of hispanolone, isolated from Ballotaafricana, into 6b-hydroxy-15,16-epoxylabda-8,13(16),14- trien-7-one has established an ent-labdane absolute stereochemistry for a diterpene metabolite originally isolated from B. aucheri. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Ballota africana; Ballota aucheri; Lamiaceae; Diterpene; Furanolabdane; Hispanolone; Hispanone; 6b-Hydroxy-15,16-epoxylabda- 8,13(16),14-trien-7-one; 6a-Hydroxy-15,16-epoxylabda-8,13(16),14-trien-7-one; 6b-Acetoxy-15,16-epoxylabda-8,13(16),14-trien-7-one; 6a-Acetoxy- 15,16-epoxylabda-8,13(16),14-trien-7-one 1. Introduction The genus Ballota (Lamiaceae) consists of about 33 species growing mainly in the Mediterranean region and adjoining Asia Minor (Codd, 1985). Phytochemical investigations of about a third of these species have revealed that furanolabdane diterpenes are the pre- dominant natural products (recently reviewed by Seidel et al., 1999). The indigenous southern African species, Ballota africana, is a good source of the furanolabdane, hispanolone (1), and we have demonstrated that this compound is a useful starting point for semi-synthetic transformations (Davies-Coleman and Rivett, 1993). Recently we have used the dehydrated analogue of 1, hispanone (2), as a model compound to investigate the preparation of 6b,7a-hydroxylated labd-8-enes. Inter- estingly, one of the compounds prepared during this investigation, 6b-hydroxy-15,16-epoxylabda-8,13(16),14- trien-7-one (3), is a natural product originally isolated from B. aucheri by Rustaiyan et al. (1995). However, although the NMR data of synthetic 3 was compatible with those reported for the natural product, the sign of the optical rotation of 3 was opposite to that of the naturally occurring diterpene. In this paper we report the synthesis and spectroscopic data for 3, its acetylated derivative 4 and their C-6 epimers (5 and 6). 0031-9422/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0031-9422(03)00156-0 Phytochemistry 63 (2003) 409–413 www.elsevier.com/locate/phytochem * Corresponding author. Tel.: +27-46-603-8264; fax: +27-46-622- 5109. E-mailaddress: m.davies-coleman@ru.ac.za(M.T.Davies-Coleman).