Relative Stereochemistry of a Diterpene from Salvia cinnabarina.

In previous papers we described the antispasmodic, hypotensive and antibacterial activities of the new secoisopimarane diterpenoid 3,4-secoisopimara-4(18),7,15-triene-3-oic acid (1) isolated from Salvia cinnabarina M. Martens and Galeotti, whose relative stereochemistry could be only partially determined on the basis of NMR spectroscopic techniques since 1 did not give suitable crystals for X-ray diffraction analysis. In order to confirm these partial results and to obtain the complete relative stereochemistry of 1 we prepared derivatives of the carboxylic group since this group was the most suitable for synthesis of derivatives without changes in the configuration of the rings. We prepared the methylester 2 (oil) and then the hydrazide 3 by treatment of 2 with hydrazine hydrate. Since compound 3 did not give the suitable crystals too, we prepared the acetone derivative 4, which finally gave suitable crystals for X-ray diffraction analysis. Since compounds 1, 2, 3 and 4 showed in the 13C NMR signals practically identical for 19-carbons of the diterpenoid moiety and only the substituted C-3 showed different δ-values, it may be inferred that the same relative stereochemistry is maintained in all compounds. The relative stereochemistry obtained by X-ray diffraction analysis showed that H-5 and H-9 are axial and on the opposite side of the 20 axial methyl group; moreover, the vinyl group at C-13 is equatorial and the 17 axial methyl group is on the same side of the 20 axial methyl group.