Tri- and Tetrameric Proanthocyanidins with Dentin Bioactivities from Pinus massoniana

Abstract

Guided by dentin biomechanical bioactivity, this phytochemical study led to the elucidation of an extended set of structurally demanding proanthocyanidins (PACs). Unambiguous structure determination involved detailed spectroscopic and chemical characterization of four A-type dimers (2 and 4−6), seven trimers (10−16), and six tetramers (17−22). New outcomes confirm the feasibility of determining the absolute configuration of the catechol monomers in oligomeric PACs by one-dimensional (1D) and two-dimensional (2D) NMR. Electronic circular dichroism as well as phloroglucinolysis followed by mass spectrometry and chiral phase high-performance liquid chromatography (HPLC) analysis generated the necessary chiral reference data. In the context of previously reported dentin-bioactive PACs, accurately and precisely assigned 13 C NMR resonances enabled absolute stereochemical assignments of PAC monomers via (i) inclusion of the 13 C NMR γ-gauche effect and (ii) determination of differential 13 C chemical shift values (Δδ C ) in comparison with those of the terminal monomer (unit II) in the dimers 2 and 4−6. Among the 13 fully elucidated PACs, eight were identified as new, and one structure (11) was revised based on new knowledge gained regarding the subtle, stereospecific spectroscopic properties of PACs.

Rare A-Type, Spiro-Type, and Highly Oligomeric Proanthocyanidins from Pinus massoniana

“…The two 4β→6 linkages were deduced from the chemical shifts of C-6 (δ C 110.5 in the G-ring and δ C 111.0 in the M-ring) and ECD evidence. Units II and IV were both elucidated as epicatechin by the singlet signals of II-H-2 and IV-H-2 and the upfield shifted C-2 (δ C 78.7 in F-ring and δ C 79.9 in the L-ring) to the corresponding carbon in proanthocyanidin A1 ( 4 ) (δ C 81.6) . Phloroglucinolysis confirmed the absolute stereochemical assignments.…”

“…A NOESY correlation between II-H-3 and III-H-6 indicated a 3,4- trans configuration in the F-ring, and thus II-C-3 was S-configured . A comparison of the tendency of ECD curves of 1 in the diagnostic region 280–300 nm with those of proanthocyanidin A2 ( 5 ) and proanthocyanidin A5 ( 7 ) revealed that 1 is closely mirror-symmetric to 7 (Figure B). Thus the partial structures of units II and III in 1 were assigned to be enantiomeric to those in 7 .…”

“…Units II and IV were both elucidated as epicatechin by the singlet signals of II-H-2 and IV-H-2 and the upfield shifted C-2 (δ C 78.7 in F-ring and δ C 79.9 in the L-ring) to the corresponding carbon in proanthocyanidin A1 ( 4 ) (δ C 81.6) . Phloroglucinolysis confirmed the absolute stereochemical assignments. An analysis of the reaction products used a combination of chiral-phase high-performance liquid chromatography (HPLC) and MS (Figure S24) to verify that 4 and 5 are the basic components of 3 .…”

“…Pinuspirotetrin (2) represents the first PAC with a heterodimeric framework consisting of a spirotype dimer connected with an A-type dimer. Also characterized in underivatized form and exhibiting dentin bioactivity, pinumassohexin (3) was elucidated as a hexameric PAC with mixed A + B-type interflavanyl linkages (IFLs), assembled from the known tetramer, peanut procyanidin E, 8 and the dimer, epicatechin−(2β→O→7,4β→8)−epicatechin (5), 8,9 via a 4β→6 bond. The structure elucidation of 1−3 utilized 1D/2D NMR, electric circular dichroism (ECD), and 13 C NMR γ-gauche effects as well as phloroglucinolysis, a recently established toolbox for absolute configurational assignments in PACs.…”

“…The structure elucidation of 1−3 utilized 1D/2D NMR, electric circular dichroism (ECD), and 13 C NMR γ-gauche effects as well as phloroglucinolysis, a recently established toolbox for absolute configurational assignments in PACs. 8 The molecular formula C 45 H 34 O 18 of 1 was determined using the 13 S1) showed the characteristic 1 H AMX patterns of the three 1,3,4-trisubstituted aromatic B-, E-, and H-rings. Additionally, a small J coupling (<1 Hz) between H-2 and H-3 suggested the terminal unit to be 2,3-cisconfigured (ent)-epicatechin.…”