3″-Hydroxymethyl-butyrolactone II from Aspergillus sp

Abstract In continuation of the search for new compounds from the terrestrial fungus Aspergillus sp., one new butyrolactone, 3″-hydroxymethyl-butyrolactone II (1) was isolated. The chemical structure of 1 was confirmed by extensive 1D and 2D NMR and HR-ESI mass data analysis, and by comparison with literature data. The absolute configuration was also determined by ECD calculations. Graphical Abstract


Results and discussion
Compound 1 was obtained as a moderately yellow oil. The HR-ESI-MS gave the m/z 409.0889 [MþNa] þ (calcd. 409.0899), which suggested the molecular formula as C 20 H 18 O 8 . IR (KBr) t max (cm À1 ) of 3345 and 1685 indicated the presence of hydroxyl and carbonyl group of 1. In the 1 H-NMR spectrum, one group of p-substituted benzyl proton signals at d H 7.51 (2H, d, J=8.8 Hz, H-2 0 , 6 0 ) and 6.88 (2H, d, J = 8.8 Hz, H-3 0 , 5 0 ) were observed. Moreover, proton signals at d H 6.79 (1H, d, J = 2.0 Hz, H-2 00 ), d H 6.51 (1H, d, J = 8.4 Hz, H-5 00 ), and d H 6.47 (1H, dd, J = 8.4, 2.0 Hz, H-6 00 ) were ascribed to one 1,2,4-trisubstituted benzyl group. Also, one methoxyl group at d H 3.74 (3H, s) was also recognized in the 1 H-NMR spectrum. The 13 C-NMR spectrum gave two carbonyl signals at d C 168.4 and 170.2, along with one methoxyl carbon signals (d C 53.9) and fourteen sp 2 carbon signals, which could be assignable to the aforementioned two benzyl groups elucidated by the proton signals and one double bond. All these fragments derived from the 1 H and 13 C NMR signals accounted for 11 of 22 unsaturation degrees for compound 1, which indicated that one more ring moiety was present in 1 so as to satisfy the 22 unsaturation degrees. As the chemical shifts of the two carbonyl groups were all below 173.0 ppm, the ring moiety was considered to be a lactone by comparing the carbon signals at d C 168.4, 138.5, 128.1, and 85.2 to those of the butyrolactone II derivatives in the literature (Haroon et al. 2013;Nong et al. 2014), and 1 was also revealed as a butyrolactone II derivative. And the butyrolactone II skeleton of 1 was further confirmed by HMBC spectrum, in which key correlations from the methoxyl group to C-6, from H-2 0 to C-3, from H-5 to C-3/C-2 00 /C-6 00 were given, substantiating the presence of a butyrolactone II skeleton.
While all these NMR data resembling those of the known butyrolactone II (Haroon et al. 2013;Nong et al. 2014), there was still one oxygen-substituted methylene group (d H 3.74/d C 58.6 determined by HSQC correlation) that could not be assigned to the butyrolactone I skeleton. Thus, the HMBC analysis was performed to locate the methylene group. In the HMBC spectrum, correlations from H-5 to C-2 00 , and from H-7 00 to C-3 00 , C-4 00 , and C-2 00 were given (Fig S1), which implied the C-3 00 of the ring A as the position where the oxygen-substituted methylene group located. Thus, compound 1 was established as 3 00 -hydroxymethyl-butyrolactone II. The absolute configuration of C-4 of 1 was determined by ECD calculation, compared with the experimental CD curves. As shown in figure S3, the ECD curves for (4R)-isomer of 1 perfectly match the experimental CD curves for 1. Thus, the absolute configuration for 1 was determined as (4R)-3 00 -hydroxymethyl-butyrolactone II with the IUPAC name of 2-(4-hydroxy-3-hydroxyethyl-benzyl)-3-(4-hydroxyphenyl)-4-hydroxy-5oxo-2,5-dihydrofuran-2-carboxylic acid methyl ester.
Antimicrobial activity testing of other butyrolactones were reported previously (An et al. 2017) against Staphylococcus aureus using the agar diffusion technique. However, compound 1 was inactive in the microbial test (40 lg/disk), including Bacillus subtilis, Staphylococcus aureus, Escherichia coli, and Candida albicans. It was also reported that butyrolactones possess anti-inflammation activity (Sun et al. 2015). But compound 1 was also inactive in terms of anti-inflammation activity. Furthermore, the DPPH and ABTS scavenging ability of 1 was examined, and 1 showed strong DPPH scavenging activity (IC 50 ¼30.88 ± 0.55 for DPPH assay and IC 50 ¼ 2.64 ± 0.11 for ABTS assay) compared to the positive drug vitamin C (IC 50 ¼25.08±0.85 for DPPH assay and IC 50 ¼ 12.54 ± 0.21 for ABTS assay). Although some the butyrolactones exhibited potent antimicrobial activity (Chen et al. 2015) or anti-inflammation activity, the in vivo evaluation so far is not available, which might be due to the small amount of butyrolactones produced naturally by microorganisms. Thus, more in vivo test need to be implemented to confirm the bio-function for butyrolactones.

Experimental
See supplementary data.

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