An adjuvant is an immunological agent that can be included in a vaccine to enhance the recipient's immune response to a supplied antigen. Adjuvants are often added to vaccines to enhance the immune system's response to the target antigen, but do not, in and of themselves, confer immunity. The adjuvant component enables dose-sparing of precious antigens and extends immunotherapeutic benefits to poor responders. Aluminum salts (Alum) are common adjuvants in vaccines sold in the United States and so it is not surprising that new small molecule entities, devoid of neurotoxicity or related side effects, are sought after as novel immunostimulatory agents for use in vaccine therapies. The various triterpenoidal molecular constituents of the extracts from the bark of the Quillaja saponaria tree, found in the desert regions of Chile, Bolivia and southern Peru, are considered, in aggregate, one such promising adjuvant which is currently undergoing clinical investigation. The immunoactive components of this material contain a central triterpene (quillaic acid), a branched trisaccharide at the C3 position of the triterpene and a complex polysaccharide attached at C28 of the quillaic acid. The laboratory of the late David Gin (Memorial Sloan-Kettering Cancer Center) accomplished a phenomenal chemical synthesis of QS-7-Apiose (1) in 2008. This tour de force synthetic campaign will be subject of the current post.
It was shown by Gin and co-workers that a commercially available semipurified extract from Quillaja saponaria could be converted into the polysilylated monodesmoside saponin 2 in only 3 steps with useful synthetic efficiency (257 mg of 2 obtained from 1.15 grams of extract). This triterpene-trisaccharide conjugate would serve as a glycosyl acceptor in a subsequent C28-carboxylate glycosylation reaction with a complex hexasaccharide donor component (vida infra).
To begin, monosaccharides 3 and 4 were subjected to dehydrative glycosylation conditions developed by the Gin laboratory to afford the beta-disaccharide 5 in excellent yield. Direct glycosylation of 5 with an apiose-derived donor (shown above) gave the trisaccharide 6. Subsequent functional group interconversions then led to 7, which underwent Schmidt glycosylation with a glucosyl imidate to provide the tetrasaccharide intermediate. A benzoate ester (required for neighboring group participation in the previous operation) was next converted to a silyl ether (in 2 steps) and then the anomeric TIPS group was transformed into the requisite trichloroacetimidate of the intact glycosyl donor fragment. The glycosyl acceptor coupling partner (shown above in the bottom-right box) was synthesized independently in an unremarkable fashion.
The aforementioned donor and acceptor were exposed to TMSOTf to generate a hexasaccharide advanced intermediate whose TIPS-acetal was subsequently cleaved with a fluoride source and then transformed into a trichloroacetimidate (8). Ultimately, the triterpene-trisaccharide conjugate (2) served as an effective glycosyl acceptor in the critical C28-carboxylate glycosylation reaction with the complex glycosyl donor 8. Finally, global deprotection of the resultant adduct afforded the natural adjuvant QS-7-Api (1) in a synthetic campaign that can only be considered a heartbreaking work of staggering genius. Subsequently, the design and synthesis of simplified semisynthetic congeners of the immunostimulatory natural product 1 was accomplished. These efforts resulted in the development of an adjuvant candidate molecule with enhanced chemical stability and aqueous solubility along with diminished levels of toxicity. These relatively recent medicinal chemistry studies (JACS, 2012) will be the topic of a forthcoming post at Modern Steroid Science.
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