Novel skeleton sesquiterpenoids isolated from guava leaves

Abstract A chemical investigation of the plant Psidium guajava L., collected in Guangdong province, afforded two novel skeleton sesquiterpenoids 1 and 2. Compound 2 also known as isocaryolan-9-one was a new natural product. The structure of the novel compound 1 was determined as guavacid A by various spectroscopic methods. A possible biosynthetic pathway for 1 and 2 was proposed. Graphical abstract

because of its obvious bioactivities such as anti-tumour (Yang et al. 2007;Fu et al. 2010;Shao et al. 2010;Gao et al. 2012;Shao et al. 2012;ukwueze et al. 2015). To find potentially hypoglycaemic secondary metabolites from P. guajava L., this study was undertaken to investigate the chemical constituents of the guava leaves collected from south of China. One novel skeleton sesquiterpenoid 1 together with a new natural product 2 was obtained as a result. Compound 2 also known as isocaryolan-9-one could be synthesised by the allylic alcohol treated with TCNe at room temperature (Colado et al. 1998;Racero et al. 2000). Interestingly, the isocaryolane skeleton of 2 exists in the psiguadials B as a sesquiterpenoid moiety (Shao et al. 2010). In this study, the novel structure of compound 1 was elucidated by the combination of eI-MS, eSI-MS, 1d and 2d NMR techniques including 1 h NMR, 13 C NMR, hSQC, hMBC, NOeSY. Furthermore, the possible biosynthetic pathway of 1 and 2 was also proposed.

Results and discussion
Compound 1 was obtained as colourless needle-like crystals in Pe-etOAC. The molecular mass was deduced to be 236 amu according to molecular ion peak at m/z 236 ([M] + ) in its eI mass spectrum and the giving quasi-molecular ion peak at m/z 235 ([M-h] − ) in its eSI mass spectrum. Analysis of the 1 h NMR data revealed three methyls at δ h 1.01, 1.01, 0.95 (each 3h, s). Connecting with the hSQC, four quaternary C-atoms including one C=O group (δ C 184.5, 52.6, 41.9, 33.5), two Ch groups (δ C 46.0, 48.9), six Ch 2 groups (δ C 48.9, 45.4, 37.2, 34.3, 29.5, 25.8) and three Me groups (δ C 30.5, 22.0, 20.3) were observed in its 13 C NMR spectrum. The above spectral evidence revealed that the molecular formula of 1 was C 15 h 24 O 2 (Ω = 4) suggesting a tricycle sesquiterpenoid.
The relative configuration of compound 1 was elucidated by analysis of its NOeSY data. In the NOeSY spectrum, key correlations ( Figure S11) between h-5 (δ h 1.55) and h-15 (δ h 1.01)/ h-11β (δ h 1.95) suggested that these protons were cofacial and assigned as β-orientation. On this basis, an unambiguous structure of compound 1 was showed in Figure 1.
Bicyclocaryophyllene-type (-)-caryophyllene oxide intermediate 3 have been isolated as a major product from the leaves of P. guajava L. (Meckes 1996). By opening the ether ring, 3 could be transformed into intermediate 4 which was also reported in the guava leaves (ekundayo 1991). under acidic condition and acting in Markovnikov fashion, 4 can produce intermediate 6 which would further be transformed into intermediate 7 by an alkyl migration model. Then, 1 and 2 can be generated from 7 and 6 by oxidation, respectively. Therefore, a plausible biosynthetic pathway for compounds 1-2 could be proposed through this route (Scheme 1).

Plant material
The leaves of P. guajava L. were collected in September 2012 in Maoming, Guangdong province, P.R. China, and identified by associate professor Bo-Ru Liao. A voucher specimen (No. 2012-9) was deposited in the key Laboratory of Natural Active Products Research Center at South China Agricultural university, Guangzhou, P.R. China.

Conclusions
To the best of our knowledge, compound 1 was a novel compound. Although the isocaryolane skeleton of compound 2 has been obtained by rearrangement of (-)-trans-caryophyllene with several electrophilic reagents (Colado et al. 1998), both the sesquiterpenoid skeletons of 1 and 2 were reported for the first time from the natural products.

Supplementary material
The NMR and MS spectra of the two compounds are available online as supplementary material.

Disclosure statement
No potential conflict of interest was reported by the authors.