Synthetic and Biosynthetic Studies on Squalestatin S1.

Abstract

Abstract : Cardiovascular diseases (CVD) remains the largest cause of human death in industrialized nations. Since the discovery of the relationship between CVD and hypercholesterolemia, extensive efforts have been focused on finding drugs that reduce serum cholesterol level by controlling key enzymes in the biosynthetic pathway of cholesterol. Squalestatins are a family of novel fungal metabolites which possess potent inhibitory action against squalene synthase and hence cholesterol biosynthesis. Therefore, squalestatins could provide a method for treatment of hypercholesterolemia. The aim of the project is to elucidate the biosynthetic pathway to squalestatin S1. Efficient methods for the synthesis of 13C- and fluorine-labeled analogues (43), (144), (151) and (163) of the putative diketide precursor to squalestatins have been developed. The introduction of the fluorine alpha to the carbonyl was to try to prevent in vivo ẞ-oxidation of the diketide precursor which resulted in the degradation of (43). The fluorine was introduced stereoselectively using Evans' oxazolidinone chiral auxiliaries and an electrophilic fluorine source. Synthetic routes to the (E,E)-diasteriomers of putative hexaketide intermediates (200) and (235) were developed. The key step in the synthesis has been the palladium-catalysed coupling of the vinylstannane (222) or (233), and the allylic acetate (218). Studies on the incorporation of [1,2-13C2]acetate gave a level of incorporation up to 5.6% using pulse feeding to Phoma sp C2932 cultures. Incorporation studies were also undertaken with the fluorinated diketide analogues, however no incorporation was detected. [1,2-13C2] Acetate has been successfully incorporated into S1 using another S1 producer, strain ATCC 20986. On the other hand feeding (43) to either active fermentation or resting cell preparation of stain ATCC 20986 resulted in degradation of the precursor and random incorporation of the 13C label into S1.