Prenylindole Alkaloids from Raputia praetermissa (Rutaceae) and their Chemosystematic Significance

O extrato diclorometano do caule de Raputia praetermissa levou ao isolamento de quatro compostos novos, 4-desóxi-raputindol C (1), raputimonoindol A-B (2, 3) e hexadecanil 2-hidróxi4-metóxi-cinnamato (5), juntamente com os alcalóides 5-(4-metóxi-metilfuran-2-il)-1H-indol (raputimonoindol C), furoquinolinos maculosidine, robustine, evolitrine e dictamnine. O estudo do extrato hexano levou ao isolamento de N-metil-4-metóxi-quinolin-2(1H)-ona, skimmianina, cicloartenona, sitosterol, stigmasterol e sitostenona. Os alcalóides antranílicos isolados indicam que o gênero possui afinidade química relevante com aqueles da tribo Cusparieae, mas difere de Neoraputia devido à ausência de alcalóides prenilindois neste último, cujas espécies foram anteriormente incluídas em Raputia.

Compound 1, C 26 H 26 N 2 (HREIMS), was identified as a bisindole alkaloid.The presence of two indole nucleus was suggested by an UV absorption maximum at 328 nm, an IR band at 3426 cm -1 (NH), and 1 H NMR signals for N-H protons at d 8.00 and 8.04 (brs, no correlation in the HSQC spectrum) (Table 1), which in the COSY experiments showed cross peaks with the 1 H signals at d H 7.14 (dd, 3.2, 2.5 Hz), 6.50 (ddd, 3.2, 2.5, 1.0 Hz), 7.11 (dd, 3.0, 2.5 Hz) and 6.44 (ddd, 3.0, 2.5, 1.0 Hz), respectively.These signals were then assigned to 2N-H (d 8.00 and 8.04), 2H-a (d H 7.14 and 7.11) and 2H-b (d H 6.50 and 6.44) of the indole rings, respectively.HMBC cross peaks (Figure 2 led to the assignment of a 5',6'-dialkylindole system.Moreover, the observed cross peaks between the 1 H signals at d H 4.07 (H-6), 2.43 (H-5a) and 1.81 (H-5b), and the 13 C signals for C-5', C-6' and 48.5 (quaternary carbon, C-3) suggested a 3,3,6-trisubstituted cyclopentyl fused to the indole ring at C-5' and C-6'.An isobutene group was identified from the 1 H NMR signals at d H 5.22 (dsep, 9.0, 1.0), 1.82 (br s, 3H), and 1.79 (br s, 3H), assigned to olefinic proton and methyl groups, respectively.This was supported by the HMBC experiments which showed correlations from the olefinic proton at d H 5.22 to the methyl carbons at d c 25.9 and 18.3.The isobutene group was connected at C-6 due to coupling of H-6 to the olefinic proton at d H 5.22.A methyl group must be connected at C-3 due to the observed HMBC cross peaks between its 1 H signal at d 3H 1.59 and the 13  attachment of the ethylenyl bridge to C-3 and C-5'' was evidenced from HMBC correlations H-1 (d H 6.13)/C-3, C-4'' and H-2 (d H 6.40)/C-3, C-6'.These spectral characteristics are in agreement with those published for raputindole C (6, Figure 3), isolated from Raputia simulans. 14The main difference observed in the1 H and 13 C NMR spectra (Tables 1 and 2) of compound 1, when compared to those of 6, was the replacement of the signal for an oxymethylene by a methyl singlet (C-4, d 3H 1.59, d C 27.1).The relative stereochemistry of compound 1, named as 4-deoxyraputindole C, was determined from gNOESY experiments.The nOes of H-2, H-1, and H-5a, coming from H-6, indicated that H-6 and the ethylenylindole system are on the same side of the five-membered ring, whereas nOe between H-5b and CH 3 -4, required the methyl group to be anti (b) to H-6, and syn to the isobutene chain.
Compounds 2-4 showed the spectral characteristic of a 5-substituted-1H-indole alkaloid (Tables 1, 2).The 13 C NMR spectrum (Table 2) revealed resonances for C-2 to C-7a in close agreement with those for the corresponding carbons in the structural unit 5-substituted-1H-indole of compound 1, except for C-5 whose chemical shift was affected by a different substituent.Elemental analysis and MS indicated the molecular formula to be C 18 H 21 NO for compound 2, requiring the presence of an aliphatic chain of 10 carbons and one oxygen.A heterocyclic oxolane was identified from the 1 H NMR spectrum, which showed a proton at d     The 1 H NMR resonances for compound 4, when compared to those of 2, showed low field shifts for the disubstituted furan ring potons.In compound 4, the main difference observed in the 1 H NMR, when compared with  13 C NMR spectral data are consistent with 4 being 5-(4-methoxymethylfuran-2yl)-1H-indole, which has previously been isolated from Raputia simulans Kallunki. 19However, the isolation of 5-(4-methoxymethylfuran-2-yl)-1H-indole was cited without spectroscopic data in an congress whose abstracts were published in Planta Medica Proceedings. 19Thus, its spectroscopic data are cite here for the first time, and it was named raputimonoindole C.

Plant material
Raputia praetermissa was collected in the Forest Reserve Adolpho Ducke, Amazonas, Brazil, and identified by J. R. Pirani (Department of Botany, University of São Paulo).A voucher specimen (189865) is deposited in the Herbarium of the Instituto Nacional de Pesquisa da Amazônia (INPA), Manaus, AM (Brazil).

Table 1 .
1H NMR data for 1-4 Elemental analysis and MS indicated the molecular formula to be C 14 H 11 NO 3 and C 14 H 13 NO 2 for compounds 3 and 4, respectively, requiring the presence of an aliphatic chain of 6 carbons (C 6 H 5 O 3 and C 6 H 7 O 2 , respectively) and indole nucleus (C 8 H 6 N).Their 1 H NMR resonances, when compared to those of 2, showed low field shifts for the disubstituted furan ring potons.In C 71.5).As for compound 1, the relative stereochemistry of 2 was deduced from gNOESY experiments.The nOes of the H-4 and H-6, coming from H-3', indicated that H-3' and the indole system must be on the same side of the oxolane ring.The above data confirmed the structure of 2, here named as raputimonoindole A.

3 ,
was the replacement of the resonance for a carbomethoxy group by two 1 H singlets at d 2H 4.11 (d C 66.0) and d 3H 3.44 (d C 57.6) from a methoxymethylene group.This was supported by the mass spectrum which showed a fragment at m/z 197 [C 14 H 13 NO 2 -H 2 CO] +• .These signals together with the mass and derivative showed the spectral characteristic of a trans a,b-unsaturated carboxyl functional group (d Hb 7.61, d, J 15.9, d Cb 144.5; d Ha 6.29, d, J 15.9, d Ca 115.7; COOR 167.3).In addition, the 1 H NMR showed signals for one methoxyl group at d H 3.92 (s, 3H; d C 55.9), three aromatic hydrogens at d H 7.07 (dd, 8.1, 1.7 Hz), 7.03 (d, 1.7 Hz) and 6.91 (d, 8.1 Hz), clearly indicating the aromatic ring to be 1,2,4-trisubstituted.From the HMBC experiments, the cross peaks observed between the signal of methoxyl group at d 3H 3.92 with d C 146.7, and the 1 H signal at d H 7.61 (H-3') with d C 147.9 but not with d C 146.7, indicated the presence of a 2-hydroxy-4-methoxy-cinnamic acid derivative.The 13 C NMR spectrum revealed resonances for an aliphatic chain of sixteen carbons, one being attached to carboxylate as indicated by the HMBC cross peak between the 1 H signal at d H 4.18 and the