Australifungin

In this study, we characterized ceramide synthase (CerS) of the protozoan parasite Trypanosoma cruzi at the molecular and functional levels. TcCerS activity was detected initially in a cell-free system using the microsomal fraction of epimastigote forms of T. cruzi, [(3)H]dihydrosphingosine or [(3)H]sphingosine, and fatty acids or acyl-CoA derivatives as acceptor or donor substrates, respectively. TcCerS utilizes both sphingoid long-chain bases, and its activity is exclusively dependent on acyl-CoAs, with palmitoyl-CoA being preferred. In addition, Fumonisin B(1), a broad and well-known acyl-CoA-dependent CerS inhibitor, blocked the parasite's CerS activity. However, unlike observations in fungi, the CerS inhibitors Australifungin and Fumonisin B(1) did not affect the proliferation of epimastigotes in culture, even after exposure to high concentrations or after extended periods of treatment. A search of the parasite genome with the conserved Lag1 motif from Lag1p, the yeast acyl-CoA-dependent CerS, identified a T. cruzi candidate gene (TcCERS1) that putatively encodes the parasite's CerS activity. The TcCERS1 gene was able to functionally complement the lethality of a lag1Δ lac1Δ double deletion yeast mutant in which the acyl-CoA-dependent CerS is not detectable. The complemented strain was capable of synthesizing normal inositol-containing sphingolipids and is 10 times more sensitive to Fumonisin B(1) than the parental strain.

Studies have advanced a stereocontrolled pathway for the synthesis of australifungin. Elaboration of the trans-fused IMDA product 4 led to the cis-diol 15, which produced the α-hydroxyenone 19 upon oxidation. Computational studies on model systems indicate that the keto-enol tautomer shown for 19 is higher in energy than the keto-enol tautomer represented by the natural product 1. The reactivity of 19 does not permit mild isomerization and subsequent deprotection to yield 1. These findings raise new questions regarding the synthesis and bioactivity of australifungin and related natural products.

Studies of australifungin illustrate an enantiocontrolled synthesis of the trans-decalin core 28 via an intramolecular [4π + 2π] cycloaddition. This strategy utilizes the nitroalkene dienophile of 27 as a surrogate ketene equivalent. Stereocontrol at C-2 is critically important for an effective intramolecular Diels-Alder (IMDA) process. Our studies report high asymmetric induction using a nonracemic Duthaler titanium enolate in the acetate aldol reaction with β,β'-branched aldehyde 13 to introduce the required C-2 chirality.