Busaliol and Busalicifol, Two New Tetrahydrofuran Lignans from Bupleurum Salicifolium

-Three tetrahydrofuran lignans [l-3} were obtained from the leaves of Bupleu17lm salici!olium, of which busaliol {l} and busalicifol {2} are novel. Their structures were determined by spectral and chemical methods. Also obtained in this investigation were a number of other lignans, coumarins, a polyacetylene, and a triterpenoid, all of known structure. Bupleurum salicifolium Sol!. ex Lowe (Umbelliferae) is endemie to the Canary lslands and is a speeies highly speeialized in the biosynthesis of seeondary metabolites derived from shikimie aeid (1-7). This paper repores the isolation and strueture elucidation of two new tetrahydrofuran lignans, busaliol [l} and busalieifol [2} from B. salicifolium. Although the biologieal aetivity of these lignans has yet to be evaluated, other tetrahydrofuran lignans have been shown to be CNS stimulants and to aet synergistieally with the inseetieide pyrethrum (S-lO). Several metabolites were also isolated from the EtOH extraet of the leaves after repeated ehromatography on Si gel and Sephadex LH-20; inclusive of the polyaeetylene, SS -heptadeea-2(Z)-9(Z)-diene-4 ,6diyne-1,S-diol (11); the triterpenoid, berulin; six coumarins eomprised of 6, 7 ,Strimethoxyeoumarin, herniarin, seopoletin, seoparone, and limettin (1216); and nineteen lignans, eonsisting of bursehernin, matairesinol dimethy 1 ether, kaerophyllin, guayadequiol, pluviatolide, Me:tJI .. I ' .. I 7" 6 , 1 R,=H, R,=R4 =R5 =OH, R¡=OMe 2 R,=OEt, R,=R4=R5=OH, R¡=H 3 R,=H, R,=R4=R5=OH, R¡=H 4 R,=H, R,=R4 =R5 =OAc, R¡=OMe 5 R,=OEt, R,=R4=R5=OAc, R¡=H guamaroline, bupleurol, matairesinol, epipinoresinol, ()-aretigenin, ()-nortrachelogenin, thujaplieatin methyl ether, guayarol, salieifolin, isosalieifolin, (-)epinortrachelogenin,2-hydroxythujaplieatinmethyl ether, 2,5-dehydrothujaplieatinmethyl ether, and 3-(2 ,4-dihydroxy3 -methoxybenzy 1)-4-( 4-hydroxy3methoxybenzyl)tetrahydrofuran [3} (2, 5,7,17-21). Repeated ehromatography on Si gel and SephadexLH-20 of the EtOH extraet of the leaves of B. salicifolium gave three tetrahydrofuran lignans [1-3} along with the other eonstiruents of known strueture listed above. Compound 3 was identified as 3-(2,4-dihydroxy-3-methoxybenzyl)-4 -( 4 -hyd roxy3 -methoxybenzyl)tetrahydrofuran whieh was isolated previously from Tinospora cordifolia Miers (Menispermaceae) (21). Compound l was isolated as an oil, showing a positive optieal rotation and a [Mt at miz 390.1679, eonsistent with the molecular formula C21H26Ü7' lts uv (234, 2S0 nm)and ir(3420, 3012,1612, 1514 em -1) data indieated its aromatie nature and the presenee of one or more hydroxy groups. The IH-nmr speetrum of l (Table 1) exhibited the general features of tetrahydrofuran lignans (21-23) and was similar to that of 3. A multiplet at 8 2.45, double doublets at 8 2.40, 2.74, and a multiplet at 8 2.60 were attributed to H-S', H-70., H-713, and HS, respeetively, while four double doubletsat8 3.4S, 3.52, 3.S2,and4.09were assigned to H-90.,13 and H-9' 0.,13. SigTABLE 1 'H-Nmr Data ofCompounds 1-5'