A New Natural Lactone from Dimocarpuslongan Lour. Seeds

A new natural product named longanlactone was isolated from Dimocarpuslongan Lour. seeds. Its structure was determined as 3-(2-acetyl-1H-pyrrol-1-yl)-5-(prop-2-yn-1-yl)dihydrofuran-2(3H)-one by spectroscopic methods and HRESIMS.


Introduction
Longan [Dimocarpus longan Lour. (syn. Euphoria longana Lam.)] is an evergreen tree of the Sapindaceae family, which is widely cultivated in Southern China, India, and Southeast Asia [1]. Longan fruit is one of the most favoured tropical fruits in China [2]. Longan seeds have long been used as a folk medicine in China for treatment of acariasis, hernia, wound hemorrhages, eczema and scrofula [3], which have recently also been proven to possess free radical-scavenging activity [4], antifatigue properties [5], growth inhibition of colorectal carcinoma cells [6], and hypoglycemic effects [7]. Longan seeds have been found to be a rich source of antioxidant phenolic compounds which are promising as functional food ingredients or natural preservatives. Soong and Barlow reported that longan seeds contained thirteen polyphenols, such as gallic acid, corilagin and ellagic acid [8]; recently Sudjaroen et al., identified eleven polyphenolic compounds from longan seed, including chebulagic acid, ellagic acid 4-O-α-L-arabinofuranoside, isomallotinic acid and geraniin, etc. [9]. We have investigated the chemical constituents of longan seeds and succeeded in the isolation of eight polyphenols and twelve other compounds [10,11]. This paper deals with the isolation and structure elucidation of a new natural lactone product named longanlactone from longan seeds.

Results and Discussion
The new compound (Figure 1) was obtained from the chloroform extracts of longan seeds as white acicular crystals with the melting point of 198-200 °C. The HR-ESI-MS spectrum revealed a quasi-molecular ion peak at 232.0971 [M+H] + (calculated 232.0973) corresponding to the molecular formula of C 13 H 14 NO 3 .   The 1 H-1 H COSY and 1 H-13 C COSY spectra revealed the partial structures shown by bold lines in Figure 2. Further connectivities were deduced from the HMBC and NOESY spectra ( Table 1). The HMBC correlations from H-7' to C-2', C-3', C-6' indicated the acetyl group was attached on C-2'. The HMBC correlations from H-3 to C-1 and C-2, indicated connectivity of C-2 to C-1 and C-3. The correlations from H-7 to C-5 and C-4, from H-4 to C-6, indicated connectivity of the propargyl to the lactone ring on C-4. Key HMBC correlations (arrows) of this compound are shown in Figure 2.

Figure 2. 1 H-1 H COSY (bold line) and key HMBC correlation (arrow) in longanlactone.
In the NOESY spectrum (Table 1 and Figure 3), the presence of mutual NOE correlations between H-5' and H-2 indicated the pyrrole ring and the lactone ring are connected through C-2 and the N atom. The mutual NOE correlations between H-2, H-3a and H-4 indicated that H-2 and H-4 are located on the same side of the lactone ring, so the relative configuration of this compound was established as depicted in Figure 3.

Plant Material
Longan seeds were collected from a commercial longan orchard located in Maoming, Guangdong, China, in September 2005. The seeds were sun dried, and ground to powder.

Extraction and Isolation
The longan seed powder (10.5 kg), after defatting with petroleum ether, was extracted with 95% EtOH (150 L) three times at room temperature (24 h per time). The EtOH solutions were combined and concentrated under vacuum. The residue was suspended in H 2 O and partitioned successively with petroleum ether and CHCl 3 (10 L, room temperature, 6 h) to obtain CHCl 3 -soluble extracts (39.0 g). The CHCl 3 -soluble extract was subjected to silica gel column chromatography (CC) and eluted with CHCl 3 -MeOH mixtures with increasing polarities (10:0-4:1) to obtain sixteen fractions A-P. Fraction D (31 mg), obtained on elution with 99:1 CHCl 3 -MeOH was further subjected to silica gel CC, eluted with petroleum ether-acetone mixtures with increasing polarities (99:1-9:1), to afford five subfractions D1-D5. Subfraction D4 was separated by CC on Sephadex LH-20 with MeOH as eluant to yield the title compound (3 mg).