Synthesis and biological activity of alpha-glucosyl C24:0 and C20:2 ceramides.
Bottom Line: Alpha-glucosyl ceramides 4 and 5 have been synthesised and evaluated for their ability to stimulate the activation and expansion of human iNKT cells.The key challenge in the synthesis of both target molecules was the stereoselective synthesis of the alpha-glycosidic linkage.Of the methods examined, glycosylation using per-TMS-protected glucosyl iodide 16 was completely alpha-selective and provided gram quantities of amine 11, from which alpha-glucosyl ceramides 4 and 5 were obtained by N-acylation. alpha-GlcCer 4, containing a C24 saturated acyl chain, stimulated a marked proliferation and expansion of human circulating iNKT cells in short-term cultures. alpha-GlcCer 5, which contains a C20 11,14-cis-diene acyl chain (C20:2), induced extremely similar levels of iNKT cell activation and expansion.
Affiliation: School of Biosciences, University of Birmingham, Edgbaston, Birmingham, UK.Show MeSH
Mentions: We therefore turned our attention to the use of glucosyl iodides,25 specifically per-TMS-protected glucosyl iodide 16, as an alternative donor. Du et al. have shown that the corresponding galactosyl iodide provides excellent levels of Î±-selectivity with a variety of alcohol acceptors.26 The reaction conditions for this glycosylation are also extremely mild and the silyl protecting groups are easily removed using an acid work-up. We reasoned that the use of phytosphingosine acceptor 17,27 in which the internal 1,2-diol is protected as an acetal, would deliver the completely O-deprotected glucoside 18 upon acid work-up. To this end, 1,2,3,4,6-penta-O-trimethylsilyl glucose 15, which is commercially available or can be readily synthesised on large scale by treating glucose with a mixture of TMSCl and hexamethyldisilazane (HMDS) in pyridine,28 was converted to glycosyl iodide 16 by treatment with TMSI in CH2Cl2 (Scheme 3). Adding a solution of crude 16 to a solution of alcohol 17, nBu4NI, HÃ¼nigâ€™s base and 4Â Ã… molecular sieves in CH2Cl2 successfully effected glycosylation. Treating the initially formed glycoside product with p-toluenesulfonic acid (pTSA) in methanol provided the fully O-deprotected glycoside 18 as a single anomer. Although the yield for this three-step process was a modest 45%, we now had very rapid access to our target molecules. A final Staudinger reduction of azide 18 delivered our requisite amine 11 in quantitative yield (Scheme 3).29 This reaction sequence is short and scalable and proved to be particularly effective for accessing multigram quantities of amine 11. The final acylation reactions were accomplished by adding either tetracosanoyl chloride or 11,14-eicosadienoyl chloride (formed from the corresponding carboxylic acids using oxalyl chloride) in THF to amine 11 in a vigorously stirred biphasic mixture of THF and 8Â M NaOAc solution. Both reactions provided the desired amide products 4 and 5 in good yields (Scheme 3).30,31
Affiliation: School of Biosciences, University of Birmingham, Edgbaston, Birmingham, UK.