5,8-Dihydroxy-2-(4-methylpent-3-enyl)-1,4-naphthoquinone and its 2-4-methyl-1-(2-methylcrotonoyloxy)pent-3-enyl analogue (shikonin angelate) fromAlkanna hirsutissima

摘要

1334 J.C.S. Perkin I5,8- Di hydroxy-2- (4-methylpent-3-enyl) -1,4=napht hoqu inone and its 2-14-M et hyl-I -( 2-met hylcrotonoyloxy) pent -3-enyl] Analogue (S hi koninAngelate) from Alkanna hirsutissimaBy Mohammad Afza1"'t and Mikdad Tofeeq, Department of Chemistry, College of Science, University ofMosul, Mosul, IraqThe title compounds have been isolated and their structures established.Alkanna hirsutissima (Boragenaceae) grows abundantlyin Northern Iraq and its presence has also been reportedall along the Mediterranean coast and in the Syriandeserts. The Boragenaceae family is known to yieldshikonin (111) and its e~ters.l-~ We have now isolatedand characterised two metabolites of this plant whichhave not been reported previously.The crude pigment extract was fractionated on silicagel columns.Early fractions furnished a mixture oftwo compounds which could not be separated by t.1.c.They were eventually separated by repeated columnchromatography.The first pigment , C,H&,J was optically inactive andshowed a u.v.-visible absorption spectrum similar to thatof naphthazarin.6 Its i.r. spectrum showed bands at1660 and 1615 cm-l, expected for quinonoid C=C andhydrogen bonded C=O groups. Its n.m.r. spectrumshowed two one-proton singlets T -2.45 and -2-35,removed by D,O and assigned to chelated phenolichydroxy-functions. Aromatic (2H) and quinone proton(1H) signals appeared at T 2.7 and 3-1, respectively. Abroad triplet (1H) at T 4.8 could be attributed to anolefinic proton of the isopentenyl side chain and a multi-plet (4H) at T 7.5 indicated methylene protons coupledand deshielded by olefinic groups.Two singlets (each3H) at T 8.3 and 8-4 confirmed the presence of theisopentenyl side chain. The compound, therefore , hasstructure (I). This was supported by the mass spectrum,which showed the expected fragmentations involving theisopentenyl side chain. Compound (I) was also obtainedby removing the hydroxy-group from the side chain ofshikonin (111) with Raney nickel.O R0( 1 1 R - H(Ill R = MeCH: CMe.CO.0(El R=OHThe second pigment showed u.v.-visible and i.r.Its n.m.r. spectrumt Present address: Department of Chemistry, University Col-R. Mamima and S. Kuroda, Acta Phytochim., 1922, 1, 43.A. S. Romanova and A.I. Ban'Kovskii, Khim. prirod.A. S . Romanova, N. V. Tareeva, and A. I. Ban'Kovski,S. Miura, Syoyakugakzt Zasshi, 1963, 17, 45.absorptions similar to the first.lege of Swansea, Swansea SA2 8PP.Soedinenii, 1965, 266.Khim. prirod. Soedinenii, 1967, 71.showed 't -2.4 and -2.6 (each lH, s, exchangeable,phenolic OH), 2.9 (2H, s, aromatic), 3-1 (lH, d, J 2 Hz,quinone H) , 3-96 [ZH, distorted q, benzylic and olefinic(ester) protons] , 4.8 (lH, t, Me,C:CH) , 74-7.9 (8H,complex, C:C*CH, and MeCHXMe), and 8-02 (6H, 2s,Me&) , in agreement with structure (11) (shikoninangelate). This assignment was confirmed by alkalinehydrolysis to give shikonin (111) and angelic acid,identical with authentic samples. Alkanin angelate (theenantiomer of shikonin angelate) has been isolated byBrand et aZ.7EXPERIMENTALN.m.r.spectra were determined for solutions in CDCl,with a Varian A60 instrument. Mass spectra were measuredon an A.E.I. MS9 spectrometer a t 70 eV with sourcetemperature 250 "C. U.V. spectra refer to ethanolic solu-tions and i.r. spectra to solutions in chloroform. RF Valuesrefer to t.1.c. on Kieselgel G (Merck) in benzene-methanol-acetic acid (20 : 2 : 1 v/v). Light petroleum had b.p. 40-60 "C.Extraction and Purification of the Pigments.-Finely milledroots of Alkanna hirsutissima (1 kg) were extracted (Soxhlet)until no more pigment was removed. The extracts wereevaporated to give a red viscous oil (5.2 g), which was sub-jected to gradient elution on an acid-washed silica gelcolumn.Elution with light petroleum gave a red gumwhich was further purified on a series of columns (very slowelution). The resulting bright red gum crystallised from n-hexane as fine red needles (57 mg) of 5,8-dihydroxy-2-(4-n~ethyl~ent-3-enyl)-l,4-na~hthoquinone (I) , m.p. 92-93' ; RF0.77 (red spot) (Found: C, 70.5; H, 5.7%; Mf, 272.Cl6HI6O4 requires C, 70.6; H, 5.8%). Resonance stabilis-ation of an ally1 cation leads to @-fission * and this resultedin a base peak a t m/e 41 in the mass spectrum. A majorpeak at m/e 68 was a result of hydrogen transfer leading tothe formation of methylbutadiene cation radical and 5,s-hydroxy-3-methylnaphthoquinone.Later fractions from the original column containedimpure shikonin angelate (11).This was further purified bycolumn chromatography on acid-washed silica gel to furnish1- (5,8-dihydroxynaphthoquinon-2-y1) -4-methylpent-3-enyl 2-methylcrotonate (11) as a viscous oil (1.3 g), RF 0.77 (red spot)(Found: C, 68.0; HI 6.1%; M+ 370. C2,H2,06 requiresC, 68.1 ; HI 5.9%).A lkaline Hydrolysis of Shikonin Angelate (11) .-Thepigment (11) (500 mg) was shaken with aqueous ZN-sodium5 I. Morimoto, T. Kishi, S. Ikegami, and Y . Hurata, Tetra-hedron Letters, 1966, 4737.6 I. Singh, R. T. Ogata, R. E. Moore, C. W. J. Chang, andP. J. Scheuer, Tetrahedron, 1968, 24, 6053.8 K. Biemann, ' Mass Spectrometry; Organic ChemicalApplications,' McGraw-Hill, New York, 1962, p. 83.K. Brand and A. Lohmann, Bey., 1935, 68, 14871975 1335hydroxide (300 ml) at 20 "C for 25 h.The blue solution wasfiltered, cooled in ice, and acidified with dilute sulphuricacid. The red suspension was extracted with chloroform(3 x 150 ml), which was then extracted with saturatedaqueous sodium hydrogen carbonate. Acidification of thealkaline extract and extraction with ether gave an oil whichcrystallised from n-pentane as prisms of angelic acid, m.p.46-47" (lit.,Q 46"), identical (i.r. spectrum) with an authenticsample.The phenolic hydrolysis product was obtained as a redsolid by evaporation of the chloroform solution remainingafter alkaline extraction. Repeated crystallisation fromether-petroleum yielded shikonin as red prisms (150 mg),RF 0.44 (red spot), m.p. 147-148" (lit.,4 147") (Found: C,67.0; H, 5.5%; M+, 288. Calc. for C,,H,,O,; C, 66.7;H, 5.6%), identical with an independently isolated productfrom Jatrop ha glanduli fera.lOPartial Reduction of Shihonin.-Shikonin (200 mg) in dryethanol (150 ml) was heated under reflux in the presence ofRaney nickel (W-2; 350 mg) for 4 h. The product wasworked up in the usual way and chromatographed on anacid-washed silica gel column. Elution with light petroleumyielded a red gum which crystallised from n-hexane as fineneedles (35 mg), m.p. and mixed m.p. 92-93', identical(ir. spectrum) with the natural product (I), in all respects.We thank Dr. J. S. Davies, Department of Chemistry,University College of Swansea, for running the mass spectra,elemental analyses, and discussions. We also thankDr. S. M. Hussain, Department of Botany, University ofMosul, Iraq, for characterising the plant.[4/2612 Received, 16th Dacembev, 19741W. G. Young, K. T. Dillon, and H. J. Lucus, J . Amer. Chem.SOC., 1929, 51, 2528.lo J. A. Ballantine, Phytochemistvy, 1969,8, 1587