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: Since targets 4 and 5 differ only in their acyl chain substitution, we elected to pursue a synthetic strategy that would allow the introduction of this point of diversity in the final step. We therefore examined several routes to amine 11 from which both glucosyl ceramide targets would then be accessed through chemoselective acylation of the amino residue. The key challenge in a synthesis of amine 11 is to form the glycosidic linkage with high α-selectivity. To this end, we first opted to employ a stereospecific glucosylation method developed by Bols (Scheme 1).19 This method involves the use of a silyl tether to attach the acceptor temporarily to the 2-position of the glucosyl donor prior to the key glycosylation step. Glycosylation proceeds with 1,2-syn specificity, owing to the formation of a five-membered silylacetal intermediate, which in the case of glucosyl donors, ensures the formation of the α-glycoside product. Thioglucoside 7, synthesised in three steps from d-glucal 6,20 was reacted with a fivefold excess of dichlorodimethylsilane. This reaction afforded a silyl chloride intermediate, which, after removal of the excess dichlorosilane reagent under reduced pressure, reacted with known alcohol 821 to form mixed silyl acetal 9, our glucosylation precursor, in modest yield. Treatment of silyl acetal 9 with N-iodosuccinimide (NIS) furnished the desired glucoside 10 as a single diastereoisomer, albeit in modest yield. Hydrogenolysis of the benzyl groups and reduction of the azide in 10 using Pd(OH)2 as the catalyst,22 provided our acylation precursor, amine 11 in 57% yield (Scheme 1).
Affiliation: School of Biosciences, University of Birmingham, Edgbaston, Birmingham, UK.