SPHINGOSINE AND PHYTOSPHINGOSINE FROM D-THREOSE - SYNTHESIS OF A 4-KETO-CERAMIDE

Reaction of 2,4-O-benzylidene-D-threose 3 with tetradecyl magnesium br omide furnished D-arabino- and L-xylo-octadecane-1,2,3,4-tetrols 5a,x. Regioselective oxidation of the 4-OH group gave 4-keto-D-erythro-deri vative 6 which can be reduced with acetaldehyde in a SmI2-catalyzed Ti shtshenko reaction to afford exclusively 5a. Regioselective 2-O-mesyla tion of 5a (--> 7a) and then acid catalyzed debenzylation afforded exc lusively 2-O-mesyl-tetrol 9a. Reaction with NaN3 and ensuing azide red uction furnished D-ribo-C-18-phytosphingosine (2) in high overall yiel d. Treatment of 2-O-mesyl derivatives 7a,x with NaN3 and then with 4-n itrobenzenesulfonyl chloride in pyridine afforded 11r,l. Elimination w ith DBU or, alternatively, by treatment with phenylselenide and then w ith H2O2, gave known 1,3-O-benzylidene protected azidosphingosine 14, which can be readily converted into sphingosine (1). Transformation of 2 into ceramide 15, selective 1,3-O-silyl protection, oxidation of th e 4-OH group (--> 17) and then desilylation afforded the 4-keto cerami de 18 found in a marine sponge. Reduction of 17 offers a convenient po ssibility for radioactive labelling of ceramides with tritium.