The stereoselective syntheses of C(18)-4,8-sphingadienines 3 and 4 as analogues of sphingosine 1 are described. The key step in these syntheses involved a novel S(N)2'-type homoallylic coupling reaction between the corresponding thioether-stabilized allylic copper reagents and the allylic mesylate 7. The thioether-stabilized allylic copper reagents were easily prepared and retained the configuration of their double bond during the coupling reactions, thus overcoming the problem of isomerization which was normally associated with the use of allylic organometallic reagents in such applications.