Chemoenzymatic Synthesis of Building Blocks for Statin Side Chains

Abstract

Biocatalysis: Enzymatic transformations have become competitive methods for the synthesis of complex organic compounds. The wide range of possibilities offered by biocatalysis for the synthesis of target molecules are highlighted based on the example of the statins. This class of compounds, which includes rosuvastatin (see picture), are good inhibitors of cholesterol synthesis and dominate the market for cholesterol-lowering drugs. Statins1 inhibit the enzyme HMG-CoA reductase (HMG=3-hydroxy-3-methylglutaryl), which catalyzes the reductive conversion of HMG-CoA into mevalonate, an early and rate-limiting step in cholesterol biosynthesis. This inhibition leads to decreased levels of LDL cholesterol (LDL=low-density lipoprotein). Large-scale clinical trials with the statin class of cholesterol-lowering drugs have shown the relationship between diminished levels of LDL cholesterol and decreased morbidity and mortality from coronary heart disease.2 Moreover, statins also decrease levels of triglycerides and increase levels of HDL cholesterol (HDL=high-density lipoprotein). The market for cholesterol-lowering drugs, valued at more than US$ 20 billion and thus the largest in the pharmaceutical sector, is dominated by the statins. In 2003, revenues of US$ 9.2 billion (2002: US$ 8.0 billion) and US$ 6.1 billion (2002: US$ 6.2 billion) were recorded for atorvastatin and simvastatin, respectively, which means that these pharmaceuticals are the two top-selling drugs in the world. Relative to simvastatin, pravastatin (2003: US$ 2.8 billion), and lovastatin (the first statin marketed in 1987), which are all derived from microorganisms through partial synthesis, the synthetic HMG-CoA reductase inhibitors atorvastatin,3 fluvastatin, and rosuvastatin (approved by the US Food and Drug Administration in August 2003)4 are increasing in value.5 With pitavastatin (NK 104) the next candidate is in phase III clinical trials.2b All statins have in common a homochiral side chain pharmacophore in the form of a 3,5-dihydroxy acid6 and a (hetero)aromatic or cyclic residue (Scheme 1). HMG-CoA reductase inhibitors. As a result of their extremely high market value and the requirement for high chemical and stereochemical purity (>99.5 % ee, >99 % de), immense effort has been invested in the production of synthetic statins by competing research groups. Pharmaceutical, chemical, and biotechnological companies have recently developed various chemoenzymatic strategies in which different biocatalysts are used for the stereoselective synthesis of the 3,5-dihydroxy acid side chain (Scheme 2). Several remarkable achievements have been made in the optimization of the biocatalytic step for application on an industrial scale. These examples nicely demonstrate that for the synthesis of a specific target molecule many different enzymatic transformations are applicable. Biocatalytic transformations for the synthesis of (3R,5S)-dihydroxyhexanoates. Bn=benzyl. The research group of Patel (Bristol-Myers Squibb) used cell suspensions of Acinetobacter calcoaceticus for the stereoselective direduction of ethyl 6-benzyloxy-3,5-dioxohexanoate. The corresponding diol was isolated in 85 % yield with 97 % ee (Scheme 2 B).11 Recently, scientists at Codexis together with Roger Sheldon described the combination of a ketoreductase- and a dehalogenase-catalyzed one-pot transformation (Scheme 2 E).19 Although this is an interesting approach, the published data are not yet adequate for the application of this two-step transformation on a larger scale. Another highly attractive approach has been developed by scientists at DSM20 based on a transformation introduced by Wong and co-workers.21 The 2-deoxyribose-5-phosphate aldolase (DERA) catalyzed formation of the pyran 17 proceeds through asymmetric CC bond formation starting from the cheap bulk chemicals acetaldehyde and chloroacetaldehyde. Although the reaction characteristics published in the initial article were not very promising (low substrate concentration, high catalyst loading, 7-day reaction time), this biotransformation is now in operation on an industrial scale.20c The optimized enzymatic transformation, which is performed at low temperature (2–4 °C), is characterized by a high final product concentration (>100 g L−1).20a These examples demonstrate that state-of-the-art enzymatic transformations have reached an extraordinary level, making them valuable and competitive methods for use in the chemical and pharmaceutical industry. Clearly, biocatalysis is “growing up” in terms of its acceptance by chemists. Biocatalytic strategies offer innovative solutions to problems such as crossed-aldol reactions, the regio- and stereoselective reduction of 1,3-diketones, the stereoselective formation of 1,3-diols, and the desymmetrization of prochiral compounds. These approaches will also have an impact on the development and elucidation of non-enzymatic transformations, as well as on the training of chemistry, pharmacy, and biology students. Dedicated to Heinz G. Floss on the occasion of his 70th birthday