Synthesis of sphingosine analogues by diastereospecific amination of enantiopure trans-gamma, delta-unsaturated-beta-hydroxyesters

Abstract

Sphingolipids play critical roles in signal transduction, intercellular membrane trafficking and cell growth. As bioactive sphingolipids, ceramide and sphingosine have been implicated in activating anti-proliferative and apoptotic responses in various cancer cells. Conversely, metabolic conversion of ceramide into sphingosine 1-phosphate, ceramide 1-phosphate and glucosylceramide regulates cell proliferation and suppresses ceramide programmed cell death. Many anticancer drugs and stress-induced agonists have been developed to increase endogenous ceramide levels. Sphingosine/ceramide analogues reportedly enhance antitumor activity and have been proposed as a potential new class of chemotherapeutic agents. Among these, sphingosines with aromatic substituents in the side chain often exhibit stronger biological activity compared to natural sphingosines. While a large number of synthetic pathways to sphingosine analogues have been described in the literature, very few pathways provide analogues with high stereospecificity. This dissertation describes a novel synthetic pathway to aromatic sphingosines which is highly stereospecific, provides good yields, uses commonly available reagents, and is versatile in terms of its potential to provide a large family of sphingosine analogues for future research. The entry into the synthetic pathway toward sphingosine analogues involves first the enantiospecific synthesis of trans-γ, δ-unsaturated β-hydroxyesters using biocatalytic reduction of trans-γ, δ-unsaturated-β-ketoesters with commercially available ketoreductases. With the advantages of compatibility with carbon-carbon double bonds, high enantioselectivity, broader substrate acceptance, mild environmentally-friendly reaction conditions and easy separation, the biocatalytic reduction was found to be superior compared to other more traditional chemical methods. Indeed, both (R) and (S)- enantiomers of trans-γ, δ-unsaturated β-hydroxyesters are synthesized by one or more ketoreductases in excellent stereochemical purity and high yield. The enantiopure trans-y, δ-unsaturated β-hydroxyesters were used to prepare erthyro-sphingosine analogues with aromatic substituents in the side chain. The strategy is based on the diastereospecific amination of trans-γ, δ-unsaturated β-hydroxyesters to introduce the amino group and establish anti N-Boc-α-hydrazine-β-liydroxyesters. Proper E1cB non-reductive elimination is essential for the successful cleavage of N-N bond of the hydrazine group. No racemization is detected during the entire course of the synthetic pathway. A total novel synthetic route of sphingosine analogues was thus accomplished with stereochemically pure intermediates and products.