DITERPENOIDS FROM NOLANA ELEGANS

M.CRISTINA CHAMY, M.PIOVANO AND J.A.GARBARINO^*

Depto. de Química, Universidad Técnica Federico Santa María, Casilla 110-V, Valparaíso ,
Chile. e-mail: juan.garbarino@qui.utfsm.cl

(Received: March 6, 2002 - Accepted: July 3, 2002)

ABSTRACT

Three labda-diterpenes have been isolated from the aereal parts of Nolana elegans. Their structures were elucidated by high resolution spectroscopic methods as labd-8a ,15-diol 1, 15-hydroxy-labd-8(17)-ene 2 and 3b -acetoxy,15-hydroxy-labd-8(17)-ene 3.

KEY WORDS : N.elegans, Nolana paradoxa ssp. atriplicifolia, Nolana section Nolana, Nolanaceae, diterpenes, labdane diterpenes.

RESUMEN

De las partes aéreas de Nolana elegans se aislaron tres diterpenos de esqueleto labdano. Sus estructuras fueron determinadas por espectroscopía de alta resolución como: labda-8a ,15-diol 1, 15-hidroxi-labd-8(17)-eno 2 y 3b -acetoxi,15-hidroxi-labd-8(17)-eno 3.

PALABRAS CLAVES: N.elegans, Nolana paradoxa ssp. atriplicifolia, Nolana sección Nolana, Nolanaceae, diterpenos, labdanos.

INTRODUCTION

The genus Nolana is distributed in the desert and semidesert coastal zones of Perú and Chile, belonging to the Nolanaceae family with ca 18 species arranged into two sections, Alona y Nolana ¹⁾.

Very little is known about the chemistry of the genus, 3 species of Nolana section Alona, has been reported to produce sesquiterpenes²⁾ and a series of highly oxigenated labdanes diterpenes^3-5). A specie from the genus Nolana sect. Bargemonthia has been reported to produce diterpenes of the kaurene type⁶⁾. This paper describes the isolation and structural elucidation of three labdane diterpenes present in Nolana elegans, a typical member of the Nolana section Nolana¹⁾, also known as Nolana paradoxa ssp. atriplicifolia ⁷⁾

Materials and Methods

Melting points were determined on a Kofler hot-stage apparatus and are uncorrected. IR spectra were recorded on a Nicolet Impact 420 spectrophotometer. ¹H and ¹³C NMR were recorded in CDCl₃ solutions in a Bruker AV-400 MHz, (Lab. NMR ¾ V Región), with TMS as int. stand. Low mass spectra were taken at 70 eV (probe) in a Shimadzu QP-2000. Silica Gel (200-300 mesh) was used for CC and silica HF-254 for TLC. Spots were detected on TLC by heating after spraying with 25% H₂SO₄ in H₂O.

Plant Material

The aerial parts of Nolana elegans, were collected in Copiapó, III Region, Chile , in September 1997, and autenticated by professor Aldo Mesa. A voucher specimen is deposited at the Herbarium of the Natural Product Laboratory of Universidad Técnica Federico Santa María (# 97015).

Extraction and Isolation of the products

The aerial parts of N. elegans, (1500 g) were extracted at room temp. successively with petrol and CH₂Cl₂ for 72 h each. The solvents were removed in vacuo to yield 20 g (petrol extract) and 5 g (CH₂Cl₂) extract of a syrupy residues. The extracts were subjected separately to chromatography over silica gel column (400 g) and eluted with mixts of petrol and EtOAc of increasing polarity. Frs. (125 ml) were combined based on TLC and ¹HNMR (60 MHz) analysis, ulterior purification was achieved by repeated CC on silica gel.

15-hydroxy-labd-8a -ol : crystals, mp:65-67ºC. IR(KBr) n _max (cm^-1) : 3500, 2930 ¾ 2870, 1470, 1390-1370, 1220, 1090-1050, 960- 900. ¹HNMR d (ppm): 3.60 (2H, dt, J= 6.0, 12.0 Hz, H-15), 2.56 (2H, brs,OH) ,1.09 (3H, s, Me-17), 0.86 (3H, d, J=6.0 Hz, Me-16) 0.81(3H, s, Me-18) and 0.74 (6H, s, Me-20 and Me-19).

¹³CNMR d (ppm): 39.7(C-1), 18.4(C-2), 40.2(C-3), 33.2(C-4), 56.0(C-5), 20.5(C-6), 44.3(C-7), 74.4(C-8), 61.6(C-9), 38.9(C-10), 21.9(C-11), 41.9(C-12), 29.9(C-13), 39.9(C-14), 60.8(C-15), 20.1(C-16), 23.9(C-17), 33.4(C-18), 21.6(C-19), 15.4(C-20). MS [70 eV] (%Rel. Int.) : 310 [M⁺, (C₂₀H₃₈O_2, ] (6.1), 293 [M⁺ - OH] (25.7), 292 [M⁺ - H₂O], (7.3), 277[M⁺ - H₂O-Me] (11.2), 239 (12.1), 221 (5.3), 192 [C₁₃H₂₄], (5.2), 191[M⁺ - H₂O-C₆H₁₃O], (10.1), 177[192-Me](26.4), 157(43.6), 151(14.5), 143(14.7), 137(29.9), 125(28.7), 124(15.7), 123 (40), 121(14.5), 111(22.4), 109(52.1), 107(13.4), 101(1.9), 99(11.1), 95(56.7), 93(14.3), 83(65.2), 82(21.8), 81(55.5), 79(13.2), 71(73.1), 69(100), 57(37.6), 55 (61.5).

15-hydroxy-labd-8(17)-ene: oil, IR(KBr) n _max (cm^-1) :3000-2830, 1630, 1470-1360, 1020, 995-895.¹HNMR d (ppm):4.81 (1H,brs,H-17), 4.51(1H,brs, H-17’), 3.60 (2H, m, H-15), 0.89 (3H, d, J=6.1Hz, Me-16), 0.86 (3H, s, Me-18), 0.80(3H, s, Me-19) and 0.67(3H, s, Me-20).

¹³CNMR d (ppm) : 37.3(C-1), 19.4(C-2), 42.2(C-3), 33.4(C-4), 55.5(C-5), 24.4(C-6), 39.1(C-7), 148.8(C-8), 57.1(C-9), 39.9(C-10), 21.6(C-11), 38.4(C-12), 29.9(C-13), 40.2(C-14), 61.1(C-15), 19.7(C-16), 106.2(C-17), 33.6(C-18), 22.6(C-19), 14.1(C-20). MS [70 eV] (%Rel. Int.) : 292[M⁺, C₂₀H₃₆O](23.3), 277[M-Me](13.3), 205[M-C₅H₁₁O](4.8), 195(13.7), 191[M-C₆H₁₃O](18.6), 177(33.3), 175(15.1), 165(18.5), 163(14.1), 143(30.0), 137(52,4), 135[C₁₀H₁₅] (98.2), 121(29.5), 111(23.1), 109(71.2), 107(32.1), 101(1.5), 95(80.5), 93(30.0), 85(46.6), 83(63.0), 81(81.2), 69(100), 71(59.1), 67(36.9), 55(67.5).

3b -acetoxy,15-hydroxy-labd-8(17)-ene: yellow oil , IR(KBr) n _max (cm^-1) :3080,3000-2830, 1740,1630,1470-1360,12401150,1020, 995-895. ¹HNMR d (ppm):4.81 (1H,brs,H-17), 4.49(1H,brs,H-17’),4.48 (1H,dd,J=5.2,10.5 Hz, H-3) 3.63 (2H, m, J=4.5Hz,H-15), 2.02(3H,s,OAc),0.87 (3H,d,J=6.2Hz,Me-16), 0.84 (3H,s,Me-18), 0.82(3H, s, Me-19) and 0.68(3H,s,Me-20). ¹³CNMR d (ppm) : 36.7(C-1), 24.3(C-2), 80.8(C-3), 38.0(C-4), 54.7(C-5), 23.8(C-6), 38.0(C-7), 147.9(C-8), 56.6(C-9), 39.2(C-10), 21.0(C-11), 36.1(C-12), 29.9(C-13), 40.1(C-14), 61.1(C-15), 19.4(C-16), 106.8(C-17), 28.2(C-18), 16.5(C-19), 16.5(C-20).,171.1(OCOCH₃) ,21.3(OCOCH₃).

MS [70 eV] (%Rel. Int.): 350(4.0), 319 (11.2), 305 (25.4), 290 [M-CH₃COOH](25.8), 275(8.0), 249(20.5), 193(9.0), 175(22.1), 152(26.0), 135(100), 107(27.5), 101(10.6), 95(75.5), 83(62.9), 69(98.5), 55(56.7).

RESULTS AND DISCUSSION

The dicloromethane extract of the aereal part of Nolana elegans, was subjected to column chromatography on silica gel using increasing proportions of ethylacetate in petrol to afford b -sitosterol, labd-8a -15-diol 1, 15-hydroxy-labd-8(17)-ene 2 and 3b -acetoxy -15hydroxy-labd-8(17)-ene 3 .

The molecular formula C₂₀H₃₈O₂ was deduced for 1, from a combined evaluation of the ¹H and ¹³C NMR spectra and by low resolution MS ( M⁺ at m/z 310). Compound 1 showed IR absorption at 3500 cm-1 (O-H), 1220 (C-O) and 1090-1050 (C-O).

The main structural features shown by ¹H and ¹³CNMR were consistent with a labdane skeleton^8,9), bearing a tertiary hydroxyl group at C-8, a hydroxymethylene group attached at C-15, four tertiary methyl groups and a secondary methyl group (C-16). The configuration of the hydroxyl group at C-8 can be assigned from considerations of the ¹³CNMR spectra where an equatorial hydroxyl group has a deshielding effect on C-17 of 6.5 ppm compared to the axial, C-17 resonating at d 24.0¹⁰⁾ instead of d 30.5 ppm⁷⁾ . In addition C-9 is shielded by 2.6 ppm when the hydroxyl is equatorial (d 61.4 vs 58.8 ppm). Since the ¹³CNMR spectrum of 1 shows C-17 at 23.9 ppm and C-9 at 62.3 ppm, the hydroxyl at C-8 is equatorial. So compound 1 is shown to be labda- 8a , 15-diol. This natural diol was isolated before from Ricinocarpus muricatus¹¹⁾ and from Oxylobus glanduliferus (Sch-Bip) Gray¹²⁾.

Compound 2 was isolated as a yellow oil, whose mass spectrum ( [M⁺] at 292 ) and ¹³C NMR data were consistent with a molecular formula of C₂₀H₃₆O. The ¹HNMR was characteristic of labdane type diterpenes with an exomethylene group d 4.51 and 4.81 ppm ( each 1 H, brs, H-17 and H-17’) and three cuaternary methyl groups at d 0.86, 0.80 and 0.67 ppm ( each 3H, s). Furthermore the ¹HNMR spectrum suggested the presence of a primary hydroxyl group( d 3.60, m, H-15) and a secondary methyl group (d 0.89, 3H. d, J=6.1 Hz, H-16). The peak in the MS at m/z 191 corresponded to the loss of C₆H₁₃O (m/z 101) i.e. the side chain of a labdane diterpene in which the primary hydroxyl group an the Me-CH were located. The assignments of the ¹³CNMR spectral signal of 2 (see Experimental) made on the basis of the observed multiplicities and by comparison with reported ¹³C NMR spectral data of related compounds ^13-15) confirmed all the above results and defined the proposed structure as 15-hydroxy-labda-8(17)-ene.This compound was obtained synthetically¹⁶⁾ prior to our work.

Compound 3 was isolated as an oil. The ¹HNMR spectrum of compound 3 suggested a structure closely related to diterpenoid 2, with an identical side chain, and an exomethylene group on C-8. The ¹HNMR spectrum showed also a one proton double-doublet (d 4.48 ppm) which was attributed to the axial proton geminal to a secondary acetoxy group( d _H 2.02 ppm). This acetyl group must be placed between a tetrasubstituted sp³ carbon and a methylene grouping (J_aa=10.5 Hz, J_ae= 5.2 Hz).Thus the new diterpenoid must be a 15-hydroxy-labda-8(17)-ene with an equatorial acetoxyl group on the C-3 position.Thus compound 3 is 3b -acetoxy,15-hydroxy-labda-8(17)-ene. The corresponding 3b ,15-dihydroxy-labda-8(17)-ene has been obtained from Araucaria imbricata¹⁷⁾ and from Moldenhawera nutans¹⁸⁾.

From a taxonomic point of view, there are two subspecies of Nolana paradoxa, which are morfologically very similar, so the chemical studies of these species is important, because meanwhile Nolana paradoxa ssp paradoxa has 13-hydroxy-4,11(12)-cadinadien-14-oic acid and 8a ,15-dihydroxy-labdane as its principal metabolites¹⁹⁾, Nolana elegans has only diterpenes .

ACKNOWLEDGMENTS

We are grateful to Fundación Andes and Fondo Nacional de Desarrollo Regional, for the acquisition of the NMR equipment, to professor Aldo Mesa for identification of the plant material.

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