Three New Natural Cyclopentenedione Derivatives from Piper carniconnectivum

Valdir A. Facundo, Amanda L. Sá , Silane A. F. Silva, Selene M. Morais, Carlos R. R. Matos and Raimundo Braz-Filho* Departamento de Química, Universidade Federal de Rondônia, 78.900-000 Porto Velho RO, Brazil Departamento de Física e Química, Universidade Estadual do Ceará, 60740-000 Fortaleza CE, Brazil Setor de Química de Produtos Naturais, Universidade Estadual do Norte Fluminense, 28013-600 Campos RJ, Brazil


Introduction
Piperaceae is a tropical family that comprises many pharmaceutically important plants, useful in folk medicine and as bioproducer of essential oils.Piper carniconnectivum is a species found in the Amazon in the northern part of Brazil. 1 Results of an investigation of a specimen of Piper carniconnectivum collected in Porto Velho, Rondônia, Brazil, are reported in this paper.Three new natural cyclopentenediones (1-3) and the known coumarin xanthyletin (4) were isolated from the ethanol extract from roots.

Results and Discussion
The known coumarin xanthyletin (4) was identified by spectral data, involving mainly 1 H and 13 C NMR spectra and comparison with literature values. 2 The EIMS (Scheme 1) of 1 and 2 showed molecular peaks at m/z 238 daltons ([M] •+ ).The 1 H and 13 C (HBBD Scheme 1. Proposed fragmentation mechanisms of 1 and 2 (only peaks classified as principals).
and DEPT) NMR data (Table 1) coupled with the [M +• ] allowed us to deduce the molecular formula, C 15 H 10 O 3 for 1 and 2 (eleven degrees of unsaturation).This formula suggests the presence of a modified coumarin skeleton (C 9 H 6 O, chromenylidene-) linked to a C 6 H 4 O 2 moiety.
The results of the extensive application of 1D and 2D NMR spectral techniques in CDCl 3 and benzene-d 6 were also used to confirm the structures and to establish the 1 H and 13 C resonance assignments of 1 and 2 (Tables 1 and 2).3).At room temperature, a relatively rapid interconversion between the enol forms probably occurs and consequently average 1 H and 13 C NMR spectra are observed (Table 3).
The isomeric structures 1 and 2 are very similar and are clearly in agreement with the 1 H and 13 C NMR spectral data, revealing 1 H and 13 C chemical shifts with slight differences.The attribution of 1 H and 13 C chemical shifts of 1, present in the mixture (about 50% of each component) was based in a major γ effect (shielding) of the heterocyclic oxygen atom on the carbonyl carbon C-12, justifying the its minor 13 C chemical shift, as observed (e. g.) in the   a Number of hydrogens bound to carbon atoms deduced by comparative analysis of HBBD-and DEPT-13 C NMR spectra.Chemical shifts and coupling constants (J) obtained from 1D 1 H NMR spectrum. 1H-1 H-COSY spectrum was also used in these assignments.a Number of hydrogens bound to carbon atoms deduced by comparative analysis of HBBD-and DEPT-13 C NMR spectra.Chemical shifts and coupling constants (J) obtained from 1D 1 H NMR spectrum. 1H-1 H-COSY spectrum was also used in these assignments.] and one methyl (CH 3 ).The 1 H (1D and 2D 1 H-1 H-COSY) and 1 H-13 C-COSYn J CH (n=1, HMQC; n=2 and 3, HMBC) spectra are in agreement with these deductions (Table 1).The Econfiguration of the double bond between the carbon atoms CH-7 (δ H /δ C 7.79/143.56)and CH-8 (δ H /δ C 7.75/117.90)was defined by a coupling constant J 16.0 Hz observed in the 1D 1 H NMR spectrum.The 2D 1 H-1 H-COSY spectrum was used to confirm the spin-spin interaction ( 3 J H,H ) of these hydrogen atoms.The chemical shift of the olefinic quaternary carbon C-9 (δ C 168.11) was assigned on the basis of its heteronuclear long-range coupling with both hydrogens H-7 (δ H 7.79, 2 J CH ) and H-8 (δ H 7.75, 3 J CH ) and, consequently, the two ketone carbonyl groups (δ C 192.64, C-11, and 201.08,C-14) were located at a methylcyclopentene ring, which was confirmed by heteronuclear long-range of these carbons with H-13 and 3H-15 (Table 4).The NMR spectra only showed the presence enol tautomer 3 and/or 3a when the experiments were done in CDCl 3 , allowing to postulate the absence of the tautomers 3b, 3c and 3d (Scheme 4) based in the 1 H and 13 C chemical shifts of the signals observed in the 1 H and 13 C NMR spectra (Table 4).At room temperature, a relatively rapid interconversion between the tautomer forms probably occurs and consequently average 1 H and 13 C NMR spectra of 3 and/or 3a are observed (Table 4).
The results of the extensive application of 1D and 2D NMR spectral techniques were also used to confirm the structure and to establish the 1 H and 13 C resonance assignments of the natural products1, 2 (Tables 1 and 2) and 3 (Table 4), and of the derivative 5 (Table 3) obtained by catalytic reduction of the mixture of 1 and 2.
The biosynthesis of these new compounds has not yet been investigated.Structural examination of the compounds 1, 2 and 3 in view of biosynthetic arguments and application of a biosynthetic retroanalysis led us to suggest a biogenetic route.Cyclopentenoids are metabolites of varied biogenetic routes. 4Thus, the methylcyclopentene-1,3-dione ring present in the compounds 1, 2 and 3 can be postulated as a bioproduct of mevalonic acid, as summarized speculatively in Scheme 5.The C 6 -C 3 moiety (cinnamoyl type) is in accordance with the shikimate (shikimic pathway) route, 4 similar to the route of the bioformation of coumarins (e. g. xanthyletin, a prenylated coumarin, 4, which was also isolated of this plant) and other compounds isolated from Piper genus. 5he compound 3 may be postulated as precursor of 1 and 2 after it appropriated ortho-hydroxylation to produce the intermediary 2-hydroxy derivative.

Scheme 2 .
Scheme 2. Products obtained by catalytic reduction (H 2 /Pd/C) of the mixture of 1 and 2.