Preparation of a ε-caprolactonic diterpenoid derivate by unexpected oxidative cleavage/lactonization of 2-oxoaustroeupatol

From aerial parts of Austroeupatorium inulifolium was isolated the ent-nor-furano triol labdane austroeupatol 1. The compound 1 was treated with IBX showing an unexpected selectivity at the potentially oxidizable sites of the substrate yielding the 2-oxoaustroeupatol (2) and 2,19-dioxoaustroeupatol (3). The treatment of 2 with sodium periodate yields a heterocyclic derivative (ε-caprolactone derivate 4) formed by oxidative cleavage and unexpected intramolecular attack of the hydroxymethylene (C-19) oxygen to the ketonic carbon (C-2). A plausible mechanistic pathway for the obtention of compound 4 is proposed. Graphical Abstract Supplementary Information The online version contains supplementary material available at 10.1007/s13659-022-00343-2.


Introduction
Furano diterpenoids of labdane-type are representative products of Eupatorium inulifolium [1][2][3], now named Austroeupatorium inulifolium [4]. The substantial biological activity of these metabolites is responsible for the interest in developing methods for total synthesis of these compounds from terpenoids and chemical modifications of several available metabolites [5]. Austroeupatol (1) is an ent-nor-furano triol of labdane-type obtained in good yield from aerial parts of Austroeupatorium inulifolium

Open Access
Natural Products and Bioprospecting   [1,2]. Due to its functionalization and relative highly abundance of compound 1 is an interesting substrate to study molecular conversions to generate novel derivatives with potential biological activity and to broaden the understanding of the structure-reactivity relationships. Here we report the generation of an unexpected ε-caprolactonic diterpenoid derivate (4) obtained by oxidative cleavage/lactonization of 2-oxoaustroeupatol (2). A plausible mechanism proposed to explain the formation of compound 4 is briefly discussed.

Results and discussion
The austroeupatol (1) was isolated as a white solid from methanol extract of the dried uncrushed leaves and stems of A. inulifolium. Structure 1 was established based on detailed study of the spectroscopic data (see Additional file 1). Treatment of compound 1 (300 mg) with IBX generated 2-oxoaustroeupatol 2 (34%), and 2,19-dioxoaustroeupatol 3 (3%). In previous research we have reported the isolation and description of compound 1, and its selective oxidation with IBX [2]. The plausible explanation of IBX's selectivity on 1, is focused on the formation of a conformer stabilized by intramolecular hydrogen bonds [2], which reduce the reactivity of the methylene alcohol (C-19), and methine alcohol at C-3. Upon treatment of ketone diol 2 (33 mg) with NaIO 4 , due to the absence of the glycol moiety in the substrate (2), the oxidative cleavage of the bond C-2/C-3 should not occur. However, the formation of a new product [4 (21%)] was detected (Scheme 1). Data obtained from the analysis of 1 H and 13 C NMR (see Additional file 1) spectra of compound 4 allowed us to establish the molecular formula C 19 H 26 O 4 , which requires eight degrees of unsaturation (one degree of unsaturation more than the starting material 2). This is consistent with the detection in the 13 C NMR spectrum of two new peaks whose chemical shifts correspond to an ester [δ C : 173. The oxidative cleavage and subsequent formation of the heterocyclic ring can be justified by a concerted mechanism, in which periodate anion is coupled to substrate 2 through the transition state depicted in Scheme 2 and is assisted by intramolecular attack at C-2 of the C-19 hydroxy group. This hypothesis was strengthened by a pilot test in which compound 3 was used in the same reaction conditions with NaIO 4 and no change was detected (Scheme 1); therefore, the presence of hydroxy group at C-19 is mandatory.

Conclusion
The methanolic extract from the aerial parts of A. inulifolium is an important source of austroeupatol (1). Due to its high abundance and its high functionalization, compound 1 is an appropriate substrate to continue performing exploratory chemical reactions focused on enriching the understanding of the structure-reactivity relationships. On the other hand, we have described the procedure to obtain and elucidate a ε-caprolactonic diterpenoid derivate (4) from austroeupatol (1). Finally, a plausible mechanism to explain the formation of novel compound 4 has been proposed.

General procedures
All moisture sensitive reactions were carried out in flame-dried glassware under argon atmosphere. The solvents used, tetrahydrofuran (THF) and dichloromethane (CH 2 Cl 2 ), were treated immediately before use with the equipment MBRAUN Solvent Purification System. Dimethylformamide (DMF) was distilled on CaH 2 under argon, prior to use. Evaporations were conducted under reduced pressure at temperatures less than 40 °C, unless otherwise noted. Column chromatography was carried out under positive pressure, using 40-63 µm silica gel (Merck) as adsorbent and the indicated solvents in each case. Melting points were determined with a Fisher-Johns instrument and they have not been corrected. Optical activity was measured in CHCl 3 on 60 Hz-Steeg & Reuter G.m.b.H. polarimeter. IR spectra were recorded on a Perkin-Elmer FT-1725X spectrophotometer as film or KBr pellets. 1D and 2D NMR spectra, in the indicated solvents, were acquired with a Bruker-Avance DRX-300 instrument and calibrated using residual solvent peak as an internal standard. Mass spectra were recorded on a Hewlett-Packard Spectrometer, model 5930A (70 eV). Analytical thin layer chromatography (TLC) was developed on 0.25 mm layers of silica gel plates HF 254 (Merck) and spots were visualized by spraying with a mixture of water (235 mL), ammonium molybdate (12 g), ammonium cerium nitrate (0.5 g) and concentrated sulfuric acid (15 mL), and then heating with air flow at 100 °C for a few seconds.

Plant material
The plant material (leaves and stems) was collected at "sector el Arenal, Municipio Libertador, Estado Mérida". The plant was identified as

Extraction and isolation procedure
The dried uncrushed leaves and stems (ca. 6 kg) was extracted with dichloromethane at room temperature for 5 min. The material was dried under hood and extracted with methanol. The solutions from both extractions were filtered and then concentrated under vacuum in a rotary evaporator at temperatures no higher than 40 °C. The extracts obtained weighed ca. 400 g (dichloromethane extract) and ca. 1616 g (methanol extract). Austroeupatol (1) was isolated as a white solid (9 g) in the fraction eluted with hexane-EtOAc 8: