General and efficient syntheses of C(18)-4,8-sphingadienines via S(N)2'-type homoallylic coupling reactions mediated by thioether-stabilized copper reagents

Practical multigram-scale synthesis of 4,6- and 4,8-sphingadienes, chemopreventive sphingoid bases

Sphingadienes are chemopreventive agents that act by blocking signaling pathways that are activated in cancer. A practical synthesis of 4,6- and 4,8-sphingadienes on a scale of gram quantities is reported here in order to allow evaluation of the biological properties of these sphingolipids. The key steps in the preparation of 4,6-sphingadiene (1a) are an intramolecular cyclization of N-Boc derivative 5a to oxazolidinone derivative 6a, followed by conversion to carbamate intermediate 7a and base-mediated hydrolysis to afford the product without further purification. 4,8-Sphingadiene (1b) was prepared in a similar fashion; the requisite trans-γ,δ-unsaturated aldehyde 15 was prepared by an ester enolate Ireland-Claisen rearrangement.

1,2-Thio Group Migration in Rh(II) Carbene Reactions

[reaction: see text] A series of beta-thio group substituted alpha-diazo carbonyl compounds have been prepared by nucleophilic substitution reactions of thiophenol, thionaphthol, or benzyl mercaptan with beta-acetoxy-alpha-diazo carbonyl compounds. The diazo decomposition of these diazo carbonyl compounds with various transition metal catalysts, including Rh(II) carboxylates and Cu(I) and Cu(II) complexes, has been investigated. It was found that the diazo decomposition of these compounds gave 1,2-thio group migration products. No 1,2-hydride or 1,2-aryl migration products were observed in all cases.