| Studies on the total syntheses of Salvia miltiorrhiza Bunge (Ladiatae) abietanoid o-quinones are described. The 3-methylbenzo (1,2-b) furan-4,5-dione (88) was prepared in a three step sequence in excellent yield. The utility of 88 as a dienophile in intermolecular Diels-Alder reactions was examined and the cycloadditions were found to proceed in good to excellent yield with high pressure and ultrasound promotions.; The ultrasound promoted Diels-Alder reactions of 88 with appropriately substituted vinylcyclohexene derivatives has led to the syntheses of tanshinone IIA (2), nortanshinone (23), tanshinone IIB (4), methyltanshinonate (5), tanshindiol A (24), tanshindiol B (25), and hydroxytanshinone IIA (27), active constituents of the Chinese traditional medicine, Dan-Shen, prepared from the roots of S. miltiorrhiza Bunge. Methyltanshinquinone (216), the hydrogenation product of the natural product methylenetanshinquinone (17), also an active constituent of S. miltiorrhiza, was similarly prepared. The effect of ultrasound in promoting the cycloadditions parallels that of high pressure, and improves the regioselectivity in favor of the desired isomers. These results revealed that the original structural assignments of tanshindiol B (25) and tanshindiol C (26) were reversed. Thus, tanshindiol B is the cis-diol (formerly assigned as the trans-diol) and tanshindiol C is the trans-diol (formerly assigned as the cis-diol).; Asymmetric synthesis of methyltanshinonate (5), tanshinone IIB (4), ({dollar}-{dollar})-tanshindiol B (25) and (S)-(+)-hydroxytanshinone IIA (27) are described. Both enantiomers of 5 were synthesized in high chemical yields and high enantiomeric excesses. The absolute stereochemistry of natural 4 and 5 was thus confirmed by total synthesis as the C-4 (S)-configuration. All the {dollar}sp{lcub}13{rcub}{dollar}C NMR spectra of the above synthetic natural products were assigned using selective INEPT and HETCOR experiments.; A study of solvent and temperature effects on ultrasound promoted Diels-Alder cycloadditions is described. The results showed that both rate and regioselectivity of the cycloadditions were affected by solvent, concentration and temperature.; The structure of a new abietanoid pigment, epi-danshinspiroketallactone (28b), from S. miltiorrhiza, was also determined by NMR spectroscopy. |
