New bicyclic hemiacetals from the edible mushroom Ramaria madagascariensis

The fruiting bodies of R. madagascariensis (collected in Tibet, China) were supplied and identified by one of the authors (G. Liu). The voucher specimen (201004281) was deposited in the college of Agriculture and Biotechnology, Zhejiang University, China. The air-dried and powdered fruit bodies of R. madagascariensis (2.5 kg) were extracted with 95% EtOH (10 l × 3) at room temperature for 6 days. The EtOH extract was next partitioned between EtOAc and H₂O with the organic layer evaporated in vacuo to afford the EtOAc extract (45 g). The latter was subjected to a silica gel column chromatography eluted with increasing polarities of a mixture of CHCl₃ and MeOH, and seven fractions (Fr.1-Fr.7) were obtained on the basis of TLC analysis. Fraction 3 was separated on a silica gel column eluted with petroleum ether/EtOAc (10:1 to 0:1, v/v) to give five subfractions (Fr.3–1–Fr.3–5). Fr.3-3 was rechromatographed on Sephadex LH-20 (CHCl₃/MeOH, 1:1, v/v) followed by semipreparative reversed-phase HPLC (30% CH₃CN in H₂O over 10 min, 30–75% over 50 min) to afford compound 2 (5.7 mg, t_R 20.8 min). Compound 1 (4.6 mg) was obtained from Fr.3-4 by Sephadex LH-20 column with MeOH followed by repeated column chromatography on silica gel (CHCl₃/acetone, 10:1–2:1, v/v).

Compound 2 was isolated as a colorless powder. The molecular formula was determined by HRESIMS as C₁₈H₂₇NO₆, which was one oxygen atom less than 1. The NMR, UV and IR spectroscopic data for 2 were very similar to those of 1. The significant difference was in the carboxyamide chain with the absence of a hydroxymethyl group and the presence of the methyl group (CH₃-8′) located at C-7′ in 2. The above assignment was further supported by the HMBC correlations of H₃-8′ to C-6′ and C-7′, and H₂-7′ to C-6′ and C-8′. Detailed analysis of the 2D NMR data, in the same manner as described for 1, confirmed the identical bicyclic hemiacetal moiety. Therefore, 2 was identified as the analogue of 1, in which the terminal methyl group of the carboxyamide chain was dehydroxylated.

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