Introduction

The literature is now replete with articles covering the use of biocatalysis, with two commercially available databases (1,2), each listing over 20,000 reports of reactions using biologically derived catalysts, and several texts which extensively cover the subject (3-7). Even the more specific subtopic, the utility of biocatalysis in the pharmaceutical industry, is the subject of recent reviews listing hundreds of references (8,9). With this presence in the literature, it is timely to describe how biocatalysis has been applied by a Process Development Group to the synthesis of a single pharmaceutical entity. Using the antifungal compound SCH56592 (Fig. 1, compound 1) as an example, this chapter will describe several biocatalytic approaches that were explored for the synthesis of key intermediates of this molecule. In keeping with the subject of this volume, the description will concentrate on the use of commercially available enzymes in organic solvents, although hydrolytic reactions and microbial reductions will also be briefly addressed for comparison and to complete the narrative.

Fungal infections are a particularly serious class of disease, and the number of antifungal drugs available is considerably less than antibacterial drugs. At this time, the most widely used class of antifungal drugs are the "azoles," so named because of the five-member nitrogen-containing ring, usually a triazole ring, common in all of their structures (Fig. 1). The triazole ring binds the P450 enzyme responsible for the 14a-demethylation of lanosterol and inhibits this enzyme's activity. This prevents the synthesis of ergosterol, which performs

From: Methods in Biotechnology, Vol. 15: Enzymes in Nonaqueous Solvents: Methods and Protocols Edited by: E. N. Vulfson, P. J. Halling, and H. L. Holland © Humana Press Inc., Totowa, NJ

Fig. 1. Antifungal compound SCH56592 and 1,3-dioxolane azole antifungals.

the same essential structural function in fungal cells as cholesterol does in mammalian cells. These drugs also decrease the ratio of saturated to unsaturated fatty acids in the fungal cell, in addition to their blockade of ergos-terol synthesis. The result is growth inhibition, membrane dysfunction, and ultimately death of the fungal cell, with low toxicity to the cells of the infected mammalian host (10).

SCH56592 (1) is an azole antifungal, currently in Phase II clinical trials, with superior activity against systemic Candida infections and pulmonary Aspergillus infections, and is useful for treating both normal and immunocompromised patients. Like other azole antifungals, SCH56592 possesses the triazole ring, a dihalophenyl ring, and a rigid side chain, all distributed around a central five-member ring. Unlike the other azole antifungals, the central ring in SCH56592 is a tetrahydrofuran rather than a 1,3-dioxolane ring (Fig. 1). This hydrolytically stable tetrahydrofuran ring is believed to be responsible for the increased antifungal activity (11), the absolute configuration of the two stereochemical centers in this ring being essential. In addition, another pair of stereochemical centers exists at the other end of the molecule at the extreme end of the side chain. The configuration of this pair of stereocenters is also essential to the antifungal activity of SCH56592.

Construction of each of the pairs of stereochemical centers was a significant synthetic challenge. A convergent synthesis of the drug was planned. The left-hand 2,2,5-trisubstituted tetrahydrofuran (THF) portion of the molecule would be condensed with the linear tetracyclic fragment, which would already contain the vicinal stereocenters on the five-carbon side chain. The two chiral key intermediates 5 and 6, which are the result of this strategy, are shown in Fig. 2.

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