TETRAHEDRON LETTERS Tetrahedron Letters 42 (2001) 7747–7750 Pergamon Mechanistic considerations pertaining to the solvolysis of paclitaxel analogs bearing ester groups at the C2 position Wieslaw A. Klis,* Jeffrey G. Sarver and Paul W. Erhardt Center for Drug Design and Development, The University of Toledo College of Pharmacy, Toledo, OH 43606 -3390, USA Received 19 June 2001; accepted 21 August 2001 Abstract—Dilute solutions of paclitaxel-related derivatives having chloroacetyl esters in the C2 position undergo ready methanolysis according to pseudo first-order kinetics while more concentrated solutions appear to be stabilized, possibly by the formation of hydrophobic aggregates that tend to bury this reaction center. Methanolysis is also attenuated in the presence of weak acid, suggesting that paclitaxel’s neighboring benzamide nitrogen may be participating in the reaction by serving as an assisting nucleophile. © 2001 Elsevier Science Ltd. All rights reserved. Selective manipulation of the hydroxyl groups present in paclitaxel (PAC) and in 10-deacetylpaclitaxel (DAP) (Fig. 1) to produce stable analogs and water-soluble prodrugs, has received considerable attention over the course of the last twenty years of PAC-related research. 1–11 Deutsch et al. 3 have shown that among all of the hydroxyl groups present in this family of com- pounds, the C2 OH is the most reactive toward acylation. When PAC is treated with carbonyldiimida- zole, the resulting C2 acylated intermediate can addi- tionally form an oxazolone derivative, an interesting side-reaction more recently encountered by de Groot et al. 4 as well. Amino acid derivatives connected via ester linkages at the C2 position are considerably less stable than when connected at C7 such that the C2 arrange- ment has been extensively pursued during prodrug strategies. Likewise, Mathew et al. 5 has exploited the instability of C2 esters to produce C7-amino acid esters of PAC by partial, selective hydrolysis of the 2,7-bis- substituted PAC analogs at pH 7.4. Harada et al. 6 have speculated that the instability of the C2 amino acid esters is due to steric repulsion of the bulky groups attached to C2 and C3 as well as to an electronic effect from these types of esters amino groups. The latter has also been previously implicated by Zhao et al. 7 and more recently by Pendri et al. 8 who suggested more specifically that protonation of the amino group could serve to assist attack of the C2 acyl functionality by external nucleophiles due to a simple inductive effect. Other investigators have further postulated that C2 esters of PAC are particularly susceptible to cleavage by various hydrolytic enzymes present in vivo. 9,10 In an attempt to better understand the interaction of PAC and related compounds with various components within cancer cells, 12,13 we have recently had occasion to deploy monochloroacetyl (CAC) protection of both PAC and DAP. During development of an HPLC method to purify 2,7-bis-monochloroacetyl-10- deacetylpaclitaxel (2,7-bis-CAC-DAP), we observed that the compound was unstable in methanol solution at a concentration of 1 mg/ml. After confirming this observation and characterizing the breakdown product Figure 1. Structures of paclitaxel (PAC), 10-deacetylpaclitaxel (DAP) and selected derivatives. For PAC, R=COCH 3 ; For DAP, R=H; For the monochloroacetyl (CAC) derivatives, a ClCH 2 CO-adduct replaces one or more of the hydroxy group hydrogens located at positions C2, C7 and C10. Keywords : paclitaxel-C2 esters; methanolysis; hydrophobic aggre- gates; neighboring group catalysis. * Corresponding author. 0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII:S0040-4039(01)01643-4