Decandrinin, an unprecedented C9-spiro-fused 7,8-seco-ent-abietane from the Godavari mangrove Ceriops decandra

Summary Decandrinin (1), an unprecedented C9-spiro-fused 7,8-seco-ent-abietane, was obtained from the bark of an Indian mangrove, Ceriops decandra, collected in the estuary of Godavari, Andhra Pradesh. The constitution and the relative configuration of 1 were determined by HRMS (ESI) and extensive NMR investigations, and the absolute configuration by circular dichroism (CD) and optical-rotatory dispersion (ORD) spectroscopy in combination with quantum-chemical calculations. Decandrinin is the first 7,8-seco-ent-abietane.


Introduction
Ceriops decandra is a mangrove of the family Rhizophoraceae. It is widely distributed along the sea coasts of South Asia down to the southern pacific islands, and of Africa and Madagascar.
The genus Ceriops only consists of five mangrove plant species. Besides C. decandra, these are C. australis, C. pseudodecandra, C. tagal, and C. zippeliana [1][2][3][4]. In Indian traditional medicine, the bark of C. decandra have been used for the treatment of amoebiasis, diarrhea, hemorrhage, and malignant ulcers [5], making it rewarding to screen the bioactive compounds of this plant. Before our work, already 28 compounds had been isolated from C. decandra [6] (three pimaranes, four beyeranes, five kauranes, and 16 lupanes). Recently, some of us have reported on the isolation of eleven new diterpenes from this plant, named decandrins A-K [7], of which nine belong to the group of abietanes.

Results and Discussion
Decandrinin (1) was obtained as a colorless solid. Its molecular formula was established as C 20 H 28 O 4 by HRMS (ESI) (m/z 333.2053, calcd for [M + H] + , 333.2060). From this formula, it was suggested that 1 has seven degrees of unsaturation, of which four could be ascribed to one carbon-carbon double bond, one lactone carbonyl group, and two ketone groups, according to its 1 H and 13 C NMR data (Table 1); the molecule should thus be tricyclic. The NMR data and a DEPT experiment (  Figure 2).
HMBC correlations between H 3 -16/C-13, H 3 -17/C-13, and H-14/C-15 placed the above isopropyl group at C-13, while those from H-14 to C-9 and C-12 indicated the presence of a ∆ 13,14 double bond. HMBC correlations from H 3 -18, H 3 -19, and H 2 -2 to the carbon at δ C 213.1 suggested the location of a keto group at C-3, whereas those from H 2 -11 to the carbon at δ C 195.9 indicated that there was another keto group at C-8 ( Figure 2).
The NOE interactions for the two methyl groups at C-4 suggested that one methyl group is located at the same side as H-5, while the other one has the same orientation as Me-20. The NOEs between the two protons of the methylene at C-11 and Me-20 led to the conclusion that the carbonyl at C-8 is opposite to Me-20 ( Figure 3). If the carbonyl at C-8 was oriented in the same direction as Me-20 these NOEs would not be observed because there would be several atoms between the concerned protons ( Figure S9 in Supporting Information File 1). Therefore, the relative configuration of 1 was identified as shown in Figure 1.
The absolute configuration of 1 was assigned by CD and ORD spectroscopy in combination with quantum-chemical calculations. The conformational analysis of 1 by using RI-SCS-MP2/def2-TZVP//B97D/TZVP yielded six relevant conformers within the energetical range of 3 kcal/mol above the global minimum. For each of the six conformers thus identified, TDB2PLYP/def2-TZVP calculations were performed providing single UV and CD spectra, which were then summed up with Boltzmann weighting. The resulting averaged CD spectrum was corrected by a UV shift [26] of 13 nm and compared with the experimental CD curve (Figure 4). While the CD curve predicted for the 5R,9R,10S-configuration was nearly opposite to the one experimentally observed, the spectrum calculated for the 5S,9S,10R-enantiomer showed a good fitting with a moderate Δ ESI value of 58% [27]. To further corroborate the assignment of the absolute configuration of 1, ORD calculations were performed using the PBE0/cc-pVDZ//B97D/TZVP method. The ORD calculated for the 5S,9S,10R-configuration in the non-resonant region matched with the one observed experimentally ( Figure S10 in Supporting Information File 1). The good agreement of the experimental CD and ORD spectra with the ones calculated for the 5S,9S,10R-enantiomer revealed the absolute configuration of 1 to be as shown in Figure 4.
A plausible biogenetic precursor of decandrinin (1) might be the naturally more common β-diastereomer of 7,13-ent-abietadien-3-ol (2). Accordingly, its 3β-OH group would be oxidized to yield int A, whose C-9 would then be hydroxylated to afford int B. Oxidative cleavage at the ∆ 7,8 double bond of int B could yield int C. Finally, the lactonization of int C would give decandrinin (1) (Scheme 1).

Experimental General methods
Optical rotation values were recorded on a JASCO P-1020 polarimeter. CD spectra were recorded on a J-715 spectropolarimeter (JASCO, Gross-Umstadt, Germany). UV spectra were obtained on a Beckman DU-640 UV spectrophotometer. NMR spectra were recorded on a Bruker Avance 400 NMR spectrometer in CDCl 3 . High-resolution ESI mass spectra were performed on a Bruker maXis UHR-TOF mass spectrometer in positive ion mode. For column chromatography, silica gel

Extraction and isolation
The extraction and isolation procedures were in part identical to those described recently [7]: The chloroform extract (65.2 g) from air-dried bark (

Supporting Information
Supporting Information File 1 HRMS (ESI) and NMR spectra of decandrinin (1), NOE interactions for the B97D/TZVP-optimized structure diagnostic for the 9-epimer of decandrinin (1), and comparison of the calculated ORD with the experimental one.