Glycolipids from Candida bombicola: Polymerization of a 6-O-Acryloyl Sophorolipid Derivative
Glycolipids from Candida bombicola: Polymerization of a 6-O-Acryloyl Sophorolipid Derivative
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来自 Bombicola 假丝酵母的糖脂：6-O-丙烯酰槐糖脂衍生物的聚合
DOI：
10.1021/ma0001537
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
R. Gross
中科院分区：
文献类型：
--
作者：
K. Bisht;Wei Gao;R. Gross
Polymers with amphiphilic properties are of great interest since they are important components of a wide range of industrial and pharmaceutical products.1 The modification of naturally occurring polysaccharides, which has been practiced for over a century, is a viable route to amphiphilic glycolipid containing polymers. However, in view of the complex nature of polysaccharides, their regioselective modification to form welldefined products is tedious and is generally practiced only as an academic curiosity. Incorporation of sugars in synthetic polymers is a viable alternative for the generation of amphiphilic polymers.2 However, selectivity in such reactions often requires complex multistep synthetic pathways.3 Enzyme-catalyzed transformations can provide high selectivity. Furthermore, chemoenzymatic strategies have been developed for preparation of sugars that are linked to vinyl polymers.4 Vinyl monomers with sugar groups, such as glucosylethyl methacrylate, alkyl, or aryl 6-O-acryloyl-R-D-glucopyranosides have been prepared.5 A problem often encountered in such work is that polar aprotic solvents such as DMF and pyridine are required to dissolve carbohydrates. Unfortunately, these solvents do not support the catalytic activity of lipases, and therefore, slow reactions and low yields were reported.4,6 Moreover, the use of polar aprotic solvents restricts the practical adoption of such technology by industry. Sophorolipids are microbial extracellular glycolipids produced by resting cells of Candida bombicola.7 First described by Gorin et al.7 in 1961, sophorolipids occur as a mixture of macrolactones and free acid structures that are acetylated to various extents at the primary hydroxyl sophorose ring positions (Figure 1).7 Our laboratories are currently engaged in both the biosynthesis and selective modification of sophorolipids.8 Our hypothesis is that the unique structures of sophorolipids will find applications in many areas such as cleaning technology, bioremediation, and bioactive therapeutic agents. Considering that sophorolipids have complex structures, their modification to monovinyl compounds that will be polymerizable to linear polymers is challenging. In this paper we report an efficient chemoenzymatic route that led to an intriguing 6-O-acryloyl sophorolipid macrolactone analogue. The homopolymerization of this monomer as well as its copolymerization with acrylic acid and acrylamide is also reported. The strategy that was developed for the site-selective incorporation of an acryl group in the sophorolipid molecule is shown in Scheme 1. The methyl ester of the sophorolipid ([R]D -9.77; m/z 659.84 (M + Na+); 95%) was synthesized by refluxing the sophorolipid mixture with an alcoholic solution of sodium methoxide.8 The sophorolipid methyl ester was then subjected to acryloylation with vinyl acrylate (g2 equiv) in dry THF. The ability of the lipases PPL, CCL, PS-30, AK, MAP-10, Novozym-435, and Lipozyme IM to catalyze this transformation was studied.9 Of the lipases evaluated, Novozym-435 was found to be the preferred catalyst. Acryloylation with an excess of vinyl acrylate (vinyl acrylate:sophorolipid methyl ester g2:1) using Novozym-435 as the catalyst gave 6′,6′′-diacryloylate as the primary product.8 Surprisingly, when monoacryloylation was attempted with 1 mol equiv of vinyl acrylate, compound 1 was isolated by column chromatography ([R]D -4.25; m/z 627.95 (M + Na)+; 84%).10 The 1H NMR spectrum of compound 1 lacked signals corresponding to an acryloyl group, and there were significant differences relative to the sophorolipid methyl ester in the region 3.25-4.5 ppm. Also, the 1H NMR spectrum of compound 1 lacked resonances corresponding to the methyl ester group. These anomalous features in the 1H NMR of 1 suggested the formation of a lactone ring between the carboxylic acid end group of the fatty acid chain with one of the hydroxyl groups of the sophorose ring. The 1H NMR spectrum of 1 also showed a 0.5 ppm downfield shift relative to the sophorolipid methyl ester in the resonance position of C-6′′ protons, suggesting participation of a C-6′′ hydroxyl in the formation of the lactone ring. Its 13C NMR spectrum (edited by a DEPT135 pulse sequence) provided conclusive evidence toward exclusive participation of the C-6′′ hydroxyl in the lactone ring formation. The structure of the lactone 1 is very interesting, as it is an unnatural analogue of the microbially produced macrolactone. Specifically, in 1, unlike the natural sophorolipids, the fatty acid carboxyl carbon (C-1) is linked to the C-6′′ hydroxyl, not to the C-4′′ hydroxyl. The successful synthesis of 1 provided a sophorolipid analogue that had only one primary hydroxyl group. Hence, this compound was an excellent candidate for the regioselective conversion of 1 to the corresponding monoacryloyl derivative linked only to the one remaining primary site. The Novozym-435-catalyzed acryloylation of lactone 1 using vinyl acrylate in dry THF was conducted to give 2 ([R]D -2.81, m/z 681.90 (M + Na)+). The 1H NMR shifts at 5.92 (1H, dd, 10.1 and 2.0 Hz, COCHdCH2cis), 6.25 (1H, dd, 17.0 and 10.0 Hz, COCHd CH2), and 6.44 ppm (1H, dd, 17.0 and 2.0 Hz, COCHd CH2trans) confirmed monoacryloylation. In addition, the 1H NMR spectrum of 2 showed a 0.7 ppm downfield shift in the resonance position of the methylene on carbon 6′ * Communicating author. † Present address: Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, Tampa, FL 33620. Figure 1. Sophorolipids produced by Turolopsis bombicola when grown on a mixture of glucose and oleic acid. 6208 Macromolecules 2000, 33, 6208-6210
