Total synthesis of stereospecific sphingosine and ceramide

A small-scale synthesis of the four sphingosine stereoisomers (d-erythro, l-erythro, d-threo, and l-threo) and lignoceroyl d- and l-erythro-sphingosines, which is suitable for synthesis of tritium-labeled compounds, is described. Ethyl dl-erythro-2-acetamino-3-hydroxy-4t-octadecenoate was esterified with l(+)-acetylmandeloyl chloride and the two diastereomers obtained were separated from each other by thin-layer or column chromatography. Each diastereomer was subjected to ethanolysis to obtain ethyl d- or l-erythro-2-amino-3-hydroxy-4t-octadecenoate which was then reduced with LiAlH(4) or NaBH(4) to yield d- or l-erythro-sphingosine. d-erythro-[1-(3)H]Sphingosine with high specific activity was prepared by using LiAl(3)H(4) in the last step. d- and l-threo-sphingosines were synthesized from ethyl dl-threo-2-acetamino-3-hydroxy-4t-octadecenoate by using a similar procedure. Ceramide (lignoceroyl sphingosine) was prepared either by acylating sphingosine or by the following new method. Ethyl dl-erythro-2-amino-3-hydroxy-4t-octadecenoate was converted to the N-lignoceroyl derivative and esterified with l(+)-acetylmandeloyl chloride. The two diastereomers obtained were separated and each isomer was treated with a catalytic amount of sodium ethoxide. One of the products, ethyl d-erythro-2-lignoceroylamino-3-hydroxy-4t- octadecenoate, was reduced with NaBH(4) to yield ceramide. N-palmitoyl dl-erythro-sphingosine was also prepared using an identical procedure. N-lignoceroyl d-erythro-[1-(3)H]sphingosine was prepared by NaB(3)H(4) reduction of the corresponding amide ester. A doubly labeled ceramide, [1-(14)C]lignoceroyl [1-(3)H]sphingosine, containing high specific activity, was prepared by mixing the above N-lignoceroyl d-erythro-[1-(3)H]sphingosine and N-[1-(14)C]lignoceroyl d-erythro-sphingosine. The conversion of the doubly labeled ceramide to 3-keto derivative is also described.