Pergamon 5,6-DIHYDRO-wPYRONES FROM SYNCOLOSTEMON ARGENTEUS LYNNE A. COLLETT, MICHAEL T. DAVIES-COLEMAN and DOUGLAS E. A. RIVHI * Dcpurtmcnt of Chemistry, Rhodes University. Grahamstown 6140. South Africa (Rrceicrd 7 October 1997) Key Word Index~Syncolostemon argenteus; Lamiaceae; 5,6-dihydro-a-pyrones: syn- argentolides A- E. Abstract ~The chemical structures of five new cc-pyrones, synargentolides A-E, isolated from Svncolosterwon argenteus, have been established as 6R-[4R,SR,6Striacetyloxy- I Eheptenyll-5,6-dihydro-2H-pyran-2- one, 6R-[5,6S-diacetyloxy-l,2-dihydroxy-3E-heptenyl]-5,6-dihydro-2H-pyran-2-onc, 6R-[5,6S-diacetyloxy-l,2- dihydroxy-3E-heptenyI]-5S-acetyloxy-5,6-dihydro-2H-pyran-2-one, 6R-[5,6Sdiacetyloxy- I ,2-dihydroxy-3fG heptenyl]-5-hydroxy-5,6-dihydro-2H-pyran-2-one and 6%[4,6,%diacetyloxy- I ,5-dihydroxy-2Gheptenyl]-SS- acetyloxy-5,6-dihydro-2H-pyran-2-one, respectively, based on spectral chiroptical and chemical evidence. .i_‘; 1998 Elsevier Science Ltd. All rights reserved INTHODUCTION In continuation of our search for new 6-substituted 5.6-dihydro-a-pyrones from the Southern African genus Syncolostenron [I], we have examined S. arg- enteus from the Qngoya forest in the Kwazulu-Natal midlands of South Africa. RESULTS AND DISCUSSION Column chromatography of an acetone extract of the dried aerial parts of S. argenteus gave oleanolic acid as the major constituent, as has been observed for S. parvr$orus [I], S. rotundijolius [2] and S. denxi- flows [3]. A fraction elutcd by hexane-EtQAc (I : I) was further purified by flash chromatography and HPLC to afford the cc-pyrone, synargentolide A (1). Similar purification of a more polar (EtQAc) fraction provided synargentolides B-E (2-5). The molecular formula of 1, the major compound, w as established as CIYHz408 by HREI mass spec- trometry and its IR spectrum is consistent with the presence of an cc&unsaturated d-lactone (v,,, 1735 cm ‘) and a trans-disubstituted double bond (v,,,, 940 cm ‘). Although ‘%I NMR spectroscopy (Table 1) only accounted for 17 of the 18 carbon atoms, it w as clear from an HMQC experiment that an oxymethine carbon signal was concealed beneath the CDCI, sol- vent peak. From the chemical shifts of the ‘%Z signals, it was apparent that 1 contained four carbonyl car- bons (6 163.8, 170.0, 170.1 and 170.2). arising from *Author to whom correspondence should be addressed. three acetate moieties and the carbonyl carbon of the 5,6-dihydro-a-pyrone ring, and four vinylic carbons (6 121.6, 128.3. 130.9 and 144.5) from an endocyclic and an exocyclic double bond. These data were sup- ported by the ‘H NMR spectrum (Table 2) which revealed signals for the c(- and P-protons of the x- pyrone ring (ci 6.01 and 6.84), two further vinylic pro- tons (;i 5.64 and 5.71) four oxymethine protons (6 4.86, 4.96, 5.08 and 5.15) and three acetate methyl singlets (ii l.YY, 2.02 and 2.12). All seven degrees of unsaturation implied by the molecular formula are thus accounted for. The f?-configuration of the exocyclic double bond in 1, suggested from the IR data, was established using spin-decoupling NMR experiments. Irradiation of the 3’-methylene protons reduced the signal for H-2’ from a double triplet, which overlapped with the H-l’ res- onance, to a doublet with J, ,? = 15.7 Hz, indicative of a rrans-disubstituted double bond (J = 9-l I Hz for a cb-olefin [4]). Synargentolide A has four chiral centres. A positive n -+ rr* Cotton effect in the CD spectrum (n,,, = 265 nm, AC = +3.5) confirmed the expected (6R)-stereo- chemistry observed in all the 6-substituted 5,6-dihy- dro-a-pyrones thus far isolated from the Lamiaceae [l]. The configurations of the remaining 4’,5’ and 6’ asymmetric carbons were established as follows. Syn- argentolide A (1) was saponified and the acetonides prepared by filtering an acetone solution of saponified 1 through a column of Amberlyst 1.5 resin [S]. The acetonides. obtained in poor yield, were separated by HPLC to afford compounds 6 and 7 as the major and minor products, respectively. Although the chemical shift of the C-8’ quaternary carbon (6 108.6) in the 6.51