Sphingosine configuration

Structural studies of glycosphingolipids involves determination of the structure of the oligosaccharide chain and of the lipid moiety. For the oligosaccharide chain, it is necessary to determine the composition, molar ratio, and sequence of the monosaccharides, their pyranose or furanose nature, and the position of glycosidic bonds and their configuration for the lipid moiety, the composition of the fatty acids and sphingosine bases must be determined. Used for these purposes are the classical, chemical methods, conventionally accepted in the chemistry of carbohydrates and lipids and based on the degradation of compounds, enzymic, and physicochemical methods, primarily mass spectrometry and n.m.r. spectroscopy. 

On the basis of the comparison of the infrared spectrum of synthetic compounds with sphingosine from naturally occurring sources, a trans configuration was assigned to the C-4 double-bond system. 

The syntheses of sphingosines just described have a substantial advantage in comparison with those previously reported, because carbohydrates offer a variety of starting materials having known absolute configuration and, therefore, the necessity of stereoisomer separation does not arise. 

Figure 6 Structures of the four stereoisomers of sphingosine. Sphingosine has two chiral carbon atoms (C-2 and C-3). The Fischer projection formula of each structure is also shown, with C-1 at the top, to illustrate the D/L and erythro/threo stereochemical nomenclature. C-3 has an erythro orthreo configuration as it relates to C-2, depending on whether the similar groups (amino and hydroxy) are on the same or opposite side of the Fischer projection. D versus L refers to the configuration at C-2 relative to the configuration of D-glyceraldehyde versus L-glyceraldehyde.

In 2006, Rezanka etal44 isolated an antifeedant, syriacin (59), from the freshwater sponge Ephydatia syriaca in the Jordan river. It is an unusual sulfated ceramide glycoside with branched-chain sphingosine and also a branched-chain fatty acid. From the viewpoint of absolute configuration, 59 is very unusual, because it contains both (if)- and (.V)-configured. rcc-butyl terminals in its alkyl chains. It seems to be biosynthesized from precursors with opposite absolute configuration. 

As to the stereochemistry of 163, it almost certainly possesses (2S,3/ ,4 )-conflguration, since all the known mammalian sphingosines possess that configuration. However, the absolute configuration at C-6 of 163 remained unknown. We became interested in solving this problem by synthesizing both 6R- and 6S-isomers of ceramide B as shown in Figure 6.26.38,39... 

Configurational Correlation of C18-Sphingosine with D-Clucose (D-Glyceraldehyde) and L-Serine... 

The absolute configuration of C-2 in the sphingosine molecule was also proved, because oxidation with hydrogen perioxide (following the degradation with ozone) led to L-serine. 

The anomeric configuration of the D-galactopyranosyl group on the sphingosine moiety was determined by the degradation method of... 

The diastereoisomeric configuration of the sugar-free cerebrosides (ceramides) was determined by Carter and coworkers,80 and found to be erythro, in accordance with the structure of sphingosine, which is isolated after drastic hydrolysis of sphingolipid mixtures or purified cerebrosides with acid. 

Tosyl isocyanate (226) was allylated smoothly with the alkenyloxirane 225 using isopropyl phosphite as a ligand to give the oxazolidinone 227 in 75 % yield, which was converted to the syn amino alcohol 228 in high yield. Oxazolidinone formation proceeds with overall retention of configuration, and is used for the synthesis of regioisomers of sphingosine [82],... 

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