Friday, September 21, 2012

David Gin's Synthesis of the Investigational Immunostimulatory Adjuvant QS-21(Api)

The 21st fraction from the reverse-phase HPLC purification of the extract of the South American tree, Quillaja saponaria Molina, was shown to contain the complex triterpene QS-21-Apiose (1). The natural product 1 is comprised of a central quillaic acid triterpene core, flanked on both sides by complex oligosaccharides. As we discussed here previously, a family of immunostimulatory natural products structurally related to 1 have emerged as promising new adjuvants for immune response potentiation. Notably, scientists at Memorial Sloan-Kettering Cancer Center have synthesized several of these complex isomeric saponins, permitting, for the first time, access to pure samples of homogenous composition. In an effort to understand the structure-activity relationships (SARs) associated with this family of investigational adjuvants, the same group has also prepared non-natural analogues of 1. Synthetic derivatives such as 2 provide improved chemical stability and favorable toxicity profiles relative to QS-21-Apiose (1). Naturally-derived QS-21 has been used in more than 100 clinical trials to augment the human immune response to vaccine antigens targeting disorders including cancer, HIV and hepatitis. We will examine here the chemical synthesis of QS-21-Apiose conducted by the laboratory of the late David Gin.
The de novo synthesis of the complex fatty acid chain of QS-21 relies on the pseudosymmetry of this substructural fragment. The propionaldehyde derivative 3 first undergoes asymmetric diastereoselective crotylation under conditions developed by H. C. Brown. The enantioenriched crotylation product 5 is then elaborated in two steps to the aldehyde 6, which next undergoes a diastereoselective aldol reaction with a chiral enolate (shown in the scheme above) to afford the beta-hydroxy ester 7. Subsequent functional group interconversions lead to the secondary alcohol 9, the substrate for a dehydrative glycosylation reaction with an arabinofuranose-derived glycosyl donor, ultimately providing the glycoconjugate 10. Finally, barium hydroxide-mediated ester hydrolysis followed by a  Yamaguchi-type esterification/condensation were successfully implemented to assemble the intact fatty acyl moiety of QS-21 in protected form (11).
The construction of the requisite linear tetrasaccharide fragment involves another stereocontrolled dehydrative glycosylation reaction to give  14, which is immediately used as a glycosyl acceptor in a subsequent glycosylation event. This product (not shown) is then elaborated to the linear trisaccharide 15 in three additional synthetic operations. A final dehydrative glycosylation reaction then secures the fully protected tetrasaccharide 17 following a standard acetate methanolysis (of the acetate 16).
The advanced fragments 11 and 17 are condensed via the intermediacy of a mixed anhydride derived from 2,4,6-trichlorobenzoyl chloride. Subsequent cleavage of the anomeric TIPS silyl ether then reveals a hemiacetal group which is converted to a competent glycosyl donor, the alpa-trichloroacetimidate 18, by condensation with trichloracetonitrile under basic conditions. The triterpene-trisaccharide substructure 19, obtained by a lengthy semisynthetic protocol starting from natural semi-purified Quillaja extracts, efficiently couples with the acylated tetrasaccharide 18 upon exposure to a catalytic amount of boron trifluoride diethyl etherate. Finally, fully protected QS-21(Api) readily undergoes global deprotection under the conditions specified above to generate, in useful quantities, structurally homogenous samples of this clinically-relevant immunostimulatory natural product (1). The elegant work of the Gin laboratory expands the availability a precious bioactive triterpenoid natural product and provides access to nonnatural synthetic derivatives of 1 (e.g. 2) with improved drug properties. 

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