X. Xu
1984
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Journal
Acta Chimica Sinica
Abstract
The arteannuin degradation product, a norsesquiterpenoid lactone 2, and its stereoisomer 11 were synthesized from 10R(+)-citronellal 3 by the reaction sequence shown in the Chinese,Text. Keto ester 7 was obtained from 3 through cyclization, hydroboration. oxidation and esterification. Reaction of 7 with ethyl formate was accompanied by simultaneous transesterification to give β-formyl keto ester 8. Reaction of 8 with methyl vinyl ketone afforded diketoaldehyde 9. Treatment of 9 with NaOH caused the cleavage of formyl group with simultaneous cyclization to afford α, β-unsaturated keto carboxylate 10. Acidification of 10 with dilute HCl gave a mixture of the desired norsesquiterpenoid lactone 2 accompanied by a small amount of its stereoisomer 11. When Ba(OH)_2 was used in place of NaOH, 11 was isolated as the main product. Chromatographic separation on silica gel gave pure 2 and 11 as well as an α, β-unsaturated keto acid 12. Treatment of 2, 11 and 12 with triethyloxonium fluoroborate afforded the stereoisomeric α, β-unsaturated keto esters 13, 14 and 15, respectively. Compounds 2 and 11 showed different Cotton effects (CE) in the spectral range of the n-π~* transition of the carbonyl group (Fig. 1). Since 2 showed a positive CE, and 11 showed a negative CE, a cis-A/B ring fusion and a trans-A/B ring fusion might be assigned to 2 and 11 respectively. The experimental results are in consistence with that predicted from the octant rule. Owing to the negative CE in the spectral range of the n-π~* transition of the lactone group, a cis-B/C ring fusion of 2 and 11 might be assigned. The results are also in agreement with the prediction from the sector rule and chirality rule of lactone. Irradiation of 11-H and 7-H caused 16.4% and 13.9% enhancement of the signal of 5-H_e, respectively, Hence, the trans-A/B ring fusion and cis-B/C ring fusion of 11 were further confirmed. The stereochmistry of compound 2 was established ultimately by X-ray diffraction analysis. (Fig. 2). Compounds 13, 14 and 15 all exhibited positive CE, but with different amplitudes, thereforo 1-H was α-oriented. As a result, the absolute configuration of the lactone rings of 2 and 11 is 6α, 7α-and 6β 7β-respectively. On the basis of coupling constant of 7-H and 11-H obtained from decoupling technique, the configuration of 11-methyl group of 2 and 11 were both assigned as β, (2, J_(7H,11H)=12 Hz, φ~160°, 11, J_(7H,11H)=6 Hz, φ~30°).