Pongaflavanol: a Prenylated Flavonoid from Pongamia pinnata with a Modified Ring A

A new prenylated flavon-4-ol with a modified ring A, which we have named pongaflavanol (1), was isolated from the stem bark of Pongamia pinnata along with the known compound tunicatachalcone (2). The structure of compound 1 was elucidated on the basis of spectroscopic data.


Introduction
Pongamia pinnata (Linn) Pierre (Leguminosae, Papilionacea; synonym: Pongamia glabra Vent; is a medium sized glabrous tree (common names: pongam, Indian beech) that grows in the litoral regions of South Eastern Asia and Australia.All parts of this plant have been used as crude drugs for the treatment of tumors, piles, skin diseases, wounds and ulcers [1].Its extracts possess significant antidiarrhoeal, antifungal, antiplasmodial, antiulcerogenic, antiinflammatory and analgesic activities [2][3][4][5][6].Previous phytochemical investigation of this plant indicated the presence of abundant prenylated flavonoids such as furanoflavones, furanoflavonols, chromenoflavones, furanochalcones, and pyranochalcones [1,[7][8][9].During our continuing study of this plant, two prenylated flavonoid derivatives with a modified ring A, pongaflavanol (1) and tunicatachalcone (2) [10], were isolated from its stem bark.Pongaflavanol was a new compound and its structure was elucidated on the basis of spectroscopic data interpretation.To the best of our knowledge compound 1 represents the first example of a naturally occurring prenylated flavan-4-ol with a modified ring A, while compound 2 was isolated for the first time from this particular plant source.

Results and Discussion
Compound 1, a brown paste, gave a molecular ion [M + ] at m/z 408.2310 in the HREIMS, indicating a molecular formula C 26 H 32 O 4 (calcd.408.2301).Together with HMQC spectra, the 1 H-and 13 C-NMR spectra (  13 C-NMR spectral data of 1 with those of griffonianone C [11] and tunicatachalcone suggested that the two downfield carbons, C-5 and C-9, were oxygenated olefinic carbons.This was confirmed by the HMBC spectra.In the H 1 -H 1 COSY spectra both the correlations of H-2 (δ H 4.88) with H-3 (δ H 2.45-2.48,2.04-2.12)and H-3 with H-4 (δ H 4.96) were observed, which indicated the presence of a CH-CH 2 -CH substructure.In the HMBC spectra, observed correlations from H-2 to C-1′ (δ C 139.5) and C-2′, 6′ (δ C 125.9) indicated that C-2 was connected with C-1′ of the aromatic ring.H-3 a correlated to C-10 (δ C 108.4) and H-4 correlated to C-9 (δ C 166.7) and C-10.This suggested the presence of the double bond between C-9 and C-10, which was connected to C-4 (δ C 62.2).A HMBC correlation from H-2 to C-9, chemical shifts of C-2 and C-9 and HRMS data disclosed the presence of an oxygen bridge between C-2 and C-9.Thus, the structures of the B and C rings of compound 1 were established.The HMBC correlations from H-1′′ (δ H 2.55-2.67,m, 2H) and 1′′′ (δ H 2.53, m, 2H) to C-7 (δ C 200.1), C-8 (δ C 57.2), C-9, from H-1′′ to C-1′′′ (δ C 38.3), and from H-1′′′ to C-1′′ (δ C 38.1) indicated that the two 3-methylbut-2-enyls attached to the saturated alicyclic quaternary carbon C-8, as well as C-7 and C-9 attached to C-8 respectively.The olefinic proton (H-6) correlated with C-8, C-7, C-5 (δ C 173.5), and C-10.This suggested that C-5 was connected to C-10, and C-6 (δ C 97.1) was connected to C-7.The location of the methoxyl group on C-5 was revealed by the HMBC correlation between the methoxyl protons and C-5.The E configuration of the trisubstituted double bond between C-5 and C-6 was determined by the presence of a NOESY correlation between the methoxyl group protons and the olefinic one (H-6), thus completing the assignment of the structure of ring A. The molecular structure of compound 1 contained two chiral carbon atoms, C-2 and C-4.The stereochemical relationship was established on the basis of proton chemical shifts and the associated coupling constants in the 1 H-NMR spectra.The chemical shifts and coupling constants of the protons of ring C are fully compatible with either the half-chair or sofa conformation of the heterocyclic ring in which the 2-aryl group is in the equatorial position.The H-3 b was the high-field proton of the proton pair at C-3, which suggested that H-3 b was in the axial orientation.The value of J 2,3b (12.2 Hz) is so large that it can only arise from a transdiaxial coupling, which suggested that H-2 is axial and the 2phenyl group is equatorial.The large value of J 4,3b (9.4 Hz) requires H-4 to be quasi-axial [12].Therefore, it was reasonable to assign a 2,4-cis configuration to compound 1, which was thus identified as 4-hydroxy-5-methoxy-8,8-bis(3-methylbut-2-enyl)-2-phenyl-3,4-dihydro-2H-chromen-(8H)-one, for which we propose the trivial name pongaflavanol.

CHI A1
Isomerization reaction a b

General
Optical rotations were measured with a Jasco 1020 polarimeter.NMR spectra were obtained on a Bruker AVANCE 500 spectrometer (operating at 500 MHz for 1 H-NMR, 125 MHz for 13 C-NMR).EIMS and HREIMS spectra were recorded on a Finnigan MAT TSQ 700 mass spectrometer.UV spectra were obtained in a Beckman DU-640 UV spectrophotometer.Semipreparative HPLC was carried out using a Waters Nova-pack HR C18 column (19×300mm) on a system comprised of a Waters 600E Multisolvent Delivery System and a Waters 996 Photodiode Array Detector.

Plant Material
Stem bark of Pongamia pinnata was collected in October 2002 from Hainan Province, Southern China.The material was identified by Prof. Si Zhang, Guangdong Key Laboratory of Marine Materia Medica, South China Sea Institute of Oceanology, Chinese Academy of Sciences.A voucher specimen is deposited at the herbarium of the South China Sea Institute of Oceanology (No. GKLMMM005).