J. Med. Chem. 2001, 44, 1467-1470
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Brief Articles Efficient Preparations of the β-Glucuronides of Dihydroartemisinin and Structural Confirmation of the Human Glucuronide Metabolite Paul M. O’Neill,† Feodor Scheinmann,‡ Andrew V. Stachulski,*,‡ James L. Maggs,§ and B. Kevin Park§ Department of Chemistry, The Robert Robinson Laboratories, University of Liverpool, Liverpool L69 3BX, U.K., Ultrafine UFC Ltd., Synergy House, Guildhall Close, Manchester Science Park, Manchester M15 6SY, U.K., and Department of Pharmacology and Therapeutics, University of Liverpool, Liverpool L69 3GE, U.K. Received August 24, 2000
New and greatly improved preparations of the 12R,1′β- (5) and 12β,1′β- (6) glucuronides of dihydroartemisinin (DHA, 2) are reported using anomeric hydroxy and imidate glucuronate intermediates. Comparison of the synthetic and natural materials shows that the human metabolite of DHA is the 12R-epimer 5. Introduction
Chart 1
The trioxane derivative artemisinin (1), active principle of the medicinal herb Artemisia annua, has long been known to be an effective antimalarial agent.1 Many semisynthetic analogues of 1 based on the derived lactol dihydroartemisinin (DHA, 2) have appeared, notably the ether derivatives β-artemether (3)2 and β-arteether (4).3 A major drawback of such derivatives is that they undergo rapid metabolism in vivo, yielding initially DHA (2) via cytochrome P450-mediated dealkylation. Subsequently 2 is eliminated as a 12-glucuronide.4 Since DHA (2) exhibits neurotoxicity and a short half-life, an important goal in this therapeutic area is the synthesis of derivatives of 2 having enhanced metabolic stability. We have reported5,6 a series of 12-aryl ethers which satisfy this condition. It is important to have the human glucuronide metabolite of 2 available as a standard. Low-yielding syntheses of the 12R,1′β- and 12β,1′β-glucuronides 5 and 6 have been reported,7 without identification of the human metabolite. We therefore sought to establish practical syntheses of both 5 and 6 and to characterize the human metabolite. A valuable bonus was the identification of an ester of 2 which has also proved useful in the preparation of a 12β-carba analogue of DHA. Discussion and Chemistry The 12R,1′β-ester 7 had been made before7 in very low yield (2.5%) by a Koenigs-Knorr reaction between 2 and 3 equiv of bromo sugar 8; hydrolysis of 7 gave 5. The 12β,1′β-isomer 9 had been obtained in 17% yield by an acid-mediated condensation between 2 and a large * To whom correspondence should be addressed. Current address: Department of Chemistry, The Robert Robinson Laboratories, University of Liverpool, L69 3BX, U.K. Tel: +44(0)-151-794-3542. Fax: +44(0)-151-794-3588. E-mail:
[email protected]. † Department of Chemistry, University of Liverpool. ‡ Ultrafine UFC Ltd. § Department of Pharmacology and Therapeutics, University of Liverpool.
excess (6 equiv) of the 1-hydroxy sugar 10; hydrolysis of 9 led to 6.
10.1021/jm001061a CCC: $20.00 © 2001 American Chemical Society Published on Web 04/19/2001
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Journal of Medicinal Chemistry, 2001, Vol. 44, No. 9
Scheme 1. Synthesis of DHA 12β,1′β-Glucuronide 6a
a
Brief Articles
Scheme 2. Synthesis of DHA 12R,1′β-Glucuronide 5a
Reagents: (a) ref 11; (b) ZnCl2; (c) Na2CO3, aq MeOH.
We first studied the synthesis of 6. In our hands, reaction between 2 and 10 catalyzed by BF3‚Et2O in benzene at 20 °C7 gave only traces (
