Total Syntheses of (+)-Geldanamycin, (-)-Ragaglitazar, and (+)-Kurasoin A and Phase-Transfer-Catalyzed Asymmetric Alkylation
Geldanamycin possesses various biological activities as seen in the NCI 60 cell line panel (13 nM avg., 70 nM SKBr-3 cells). The predominant mode of action providing these unique results arises from the ability of geldanamycin (GA) to bind to the chaperone heat shock protein 90 (Hsp90). Despite its complicated functionality, the first total synthesis of GA was accomplished, which included two new reactions developed specifically to address the stereochemical features. The final step in the synthesis of GA was a demethylation-oxidation sequence to generate the desired para-quinone. This step could only be accomplished with HNO3/AcOH, producing GA in 5% yield. A GA model study, which closely resembled the aromatic core, was extensively investigated to solve this critical oxidation issue. A protected hydroquinone model compound was determined to be the optimum choice. Using Pd in the presence of air with a 1,4-hydroquinone provided the desired para-quinone quickly and nearly quantitatively in 98% yield. This study formulated the recipe of success for para-quinone formation of GA and future synthetic analogs. Asymmetric glycolate alkylation has been developed using phase-transfer-catalysis (PTC). Diphenylmethoxy-2,5-dimethoxyacetophenone with trifluorobenzyl cinchonidinium catalyst and cesium hydroxide provided alkylation products at —35 °C in high yield (80-99%) and with excellent enantioselectivity (up to 90% ee). Useful α-hydroxy products were obtained using bis-TMS peroxide Baeyer—Villiger conditions and selective transesterification. The intermediate aryl esters can be obtained with >99% ee after a single recrystallization. The newly developed PTC glycolate alkylation was applied to the asymmetric syntheses of ragaglitazar and kurasoin A. Ragaglitazar is a potent antihyperglycemic and lipid modulator, currently in phase II clinical trials. Kurasoin A is a relatively potent protein farnesyltransferase (PFTase) inhibitor with an IC50 value of 59.0 micromolar. PTC glycolate alkylation was optimized to provide 4-benzyloxy glycolate intermediates in excellent overall yield and with 96% ee after recrystallization. Ragaglitazar was then synthesized after considerable experimentation to provide the potent lipid modulator with yields and enantiopurity rivaling the best-known routes produced by industry standards. Kurasoin A was produced through an α-triethylsiloxy Weinreb amide to provide the highest overall yielding route to this PFTase inhibitor currently disclosed.
geldanamycin, ragaglitazar, kurasoin, phase transfer catalysis, PTC, asymmetric alkylation, cinchonidine, natural product, organic synthesis, Biochemistry, Chemistry
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