Taxol, a substance originally isolated from the Pacific yew tree (Taxus brevifolia) more than two decades ago, has recently been approved for the clinical treatment of cancer patients. Hailed as having provided one of the most significant advances in cancer therapy, this molecule exerts its anticancer activity by inhibiting mitosis through enhancement of the polymerization of tubulin and consequent stabilization of microtubules. The scarcity of taxol and the ecological impact of harvesting it have prompted extension searches for alternative sources including semisynthesis, cellular culture production and chemical synthesis. The latter has been attempted for almost two decades, but these attempts have been thwarted by the magnitude of the synthetic challenge. Here we report the total synthesis of taxol by a convergent strategy, which opens a chemical pathway for the production of both the natural product itself and a variety of designed taxoids.
A method for the formation of Taxol's ABC ring system has been developed. General methods for the synthesis of versatile synthons for Taxol's A ring (8) and C ring (55) are presented. A model study using a simplified C ring synthon (17) confiied the viability of the sequential Shapiro-McMurry strategy for formation of Taxol's B ring. Careful exploration of the chemistry of various A-B ring conjugates allowed the development of a successful method for formation of the B ring in a more functionalized system.
A successful strategy for the enantioselective synthesis of the natural stereoisomer of Taxol (1) has been developed. This strategy utilized the convergent assembly of Taxol's central eight-membered B ring from preformed synthons for rings A (10) and C (9) followed by late introduction of the D ring and side chain. Degradative studies confiied the viability of certain crucial manipulations including oxidation of the C13 position (35 -3) and regioselective introduction of the C I-hydroxyl, CZbenzoyloxy moiety (29 -31). Additionally, a convenient method for the large-scale production of 29, a derivative useful for C2 analog production, was developed.
The total synthesis of (-)-Tax01 has been achieved. Functional group manipulation of diol 2 provided the ABC ring system with the correct C9-keto, C10-acetyloxy functionality. Careful optimization allowed the oxidation of the C5-C6 alkene in 4 at C5 via a hydroboration reaction. Functional group manipulation of this product, 29, provided, through two routes, the oxetane D ring as 36. Following the method developed by degradative studies provided the natural enantiomer of Taxol (1).
The synthesis of Taxol's ABC ring system has been achieved. The Shapiro coupling of an aldehydic C ring synthon (8) with an anionic A ring synthon derived from hydrazone 9 gave, diastereoselectively, A-B conjugate 10. Functional group manipulations and McMuny ring closure produced the highly functionalized ABC ring system 17. Extensive attempts to optimize the McMuny reaction revealed a single predominant side reaction leading to byproducts 19 and 20. Resolution of the C9,ClO-diol (f)-17 via its camphanyl esters provided the ABC ring system as its natural isomer (+)17.
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