Resolution, classical enzymic

Abstract Various approaches to the preparation of enantiomerically pure (2/i,27i)-(+)-Z/irco-mclhyl-phenidate hydrochloride (1) are reviewed. These approaches include synthesis using enantiomerically pure precursors obtained hy resolution, classical and enzyme-hased resolution approaches, enan-tioselective synthesis approaches, and approaches based on enantioselective synthesis of (2S,2 R)-ery-Zftro-methylphenidate followed by epimerization at the 2-position. 

After the first preparation of enantiomerically pure (27 ,2 7 )-t/zreo-methylphenidate hydrochloride (1) in 1958, it is only recently that a great deal of interest has been demonstrated in the synthesis of this molecule. Various approaches to the preparation of enantiomerically pure (2R,2>R)-(-i-)-t/zreo-methylpheni-date hydrochloride (1) are reviewed. These approaches include synthesis using enantiomerically pure precursors obtained by resolution, classical and enzyme-based resolution approaches, enantioselective synthesis approaches, and approaches based on enantioselective synthesis of 2S, 2 Ryerythro-methylphenidate followed by epimerization at the 2-position. Classical resolution approaches have been successfully upscaled to produce 1 on a multi-kilo-gram scale due to the ready availability of racemic ( )-t/zreo-methylphenidate hydrochloride (10). VVfiiIt some enantioselective approaches are short, they do not provide 1 of the desired enantiomeric purity necessary for drug development. Enantioselective synthesis approaches to produce 1, however, will be-... 

Enantiomerically pure propeneoxide can be obtained in three steps from lactic acid. Its reaction with prenyl cuprate directly yields sulcatol [9, 10]. Even with such a convincing route at hand, one should not fail to evaluate routes via a racemate. A classical resolution via the formation of a hemiphthalate and crystallization of its brucine salt appears circumstantial. Yet kinetic resolution using enzymes, e.g.. Upases, appears more attractive (Scheme 10.7) [11]. 

The balance of this chapter will be devoted to several classic and representative enzyme mechanisms. These particular cases are well understood, because the three-dimensional structures of the enzymes and the bound substrates are known at atomic resolution, and because great efforts have been devoted to kinetic and mechanistic studies. They are important because they represent reaction types that appear again and again in living systems, and because they demonstrate many of the catalytic principles cited above. Enzymes are the catalytic machines that sustain life, and what follows is an intimate look at the inner workings of the machinery. 

The identification of a novel BVMO from Mycobacterium tuberculosis (BVMOMtbs) complements this toolbox, as this particular biocatalyst performs a classical kinetic resolution instead of a regiodivergent oxidation vith complete consumption of substrate [140]. Notably, this enzyme accepts only one ketone enantiomer and converts it selectively to the abnormal lactone while the antipodal substrate remains unchanged (Scheme 9.24) [141]. 

Enzymes as chiral catalysts play a role in all three methods. In nature enzymes catalyse all production of chiral compounds. In the laboratory enzymes can catalyse asymmetric synthesis, as well as resolve racemates. Which of the three methods is chosen in different cases depends on several factors, like price of starting materials, number of synthetic steps, available production technology and know-how etc. There is at present a constant ongoing development of synthetic methods and biotransformation is one field. Utilization of method i) requires knowledge of classical organic synthesis, enzymes have already played their role. Enzymes may play a part both in asymmetric synthesis and resolution. 

Classic resolntion has been performed by formation of diastereomeiic salts which could be separated. In a series of synthetic steps and when resolution is one step, it is of utmost importance that the correct chirality is introduced at an early stage. When a racemate is subject to enzyme catalysis, one enantiomer reacts faster than the other and this leads to kinetic resolution (Figure 2.2c). Results of hydrolysis using lipase B from Candida antarctica (CALB) and a range of C-3 secondary butanoates are shown in Table 2.1. 

Pathway B utilized conditions from the classical resolution route [7] which used 4 as the starting material instead of 13. Less exploratory work was done on this pathway, mainly because of the higher risk of epimerization of the C3 center under highly alkaline conditions as well as the difficult isolation of 18, which imphed carrying enzyme (as well as enzyme by-products) to the API isolation step. The poisoning of the Raney nickel by the enzyme found in pathway A also made this an undesirable option. 

A high-resolution structure of a native enzyme is an admirable basis for any mechanistic study relating activity to precise details of structure. It is even better when structures of complexes with substrates and intermediates are available, as is the case with the tyrosyl-tRNA synthetase and tyrosyl adenylate (Figure 15.1). The E Tyr-AMP complex has two remarkable features. The first is the absence of groups that are candidates for roles in classical catalysis. The second is the... 

Resolution of cheap racemic mixtures with enzymes is a common route to enantiomerically pure chemicals on an industrial scale. However, the yield with a classical resolution is limited to 50%. An in situ racemization of the undesired enantiomer, combined with the enzymatic kinetic resolution, gives rise to a dynamic kinetic resolution (DKR) that should in principle lead to a 100% yield in the desired isomer. In spite of several Ru and Pd homogeneous systems successfully combined with enzymes and successfully applied on industrial scale in DKR [71, 72], few metal-based heterogeneous catalysts active for alcohol racemization have been reported [19, 73, 74]. 

Our biotransformation group (Drs. David Dodds, Alex Zaks, and Brian Morgan) contributed to most of our chiral synthesis projects, although in most cases enzyme-based routes were not selected over chiral induction or classical resolution processes for the short-term needs in API synthesis. This area, however, remains one of huge promise with the prospect of working in water being one of its most appealing attractions. 

This example of a classical cyclic resolution-racemization methodology indicates the limitations of a multistep enzyme-catalyzed reaction when the reaction medium, the reagents, or the reaction conditions are not mutually compatible. 

The most powerful approaches, which can be used with several different enzyme systems, lead to a single enantiomer as the product in high yield and do not rely on a classic resolution approach in which the unwanted enantiomer is discarded. These approaches include dynamic kinetic resolutions, der-acemizations, and asymmetric and desymmetrization reactions (49, 50). In some cases, a chemical catalyst may be available to recycle the unwanted isomer in the same reactor vide infra). It is sometimes possible to racemize the unwanted isomer of the substrate and then to perform the reaction again (51). 

In addition to stereoselective metalation, other methods have been applied for the synthesis of enantiomerically pure planar chiral compounds. Many racemic planar chiral amines and acids can be resolved by both classical and chromatographic techniques (see Sect. 4.3.1.1 for references on resolution procedures). Some enzymes have the remarkable ability to differentiate planar chiral compounds. For example, horse liver alcohol dehydrogenase (HLADH) catalyzes the oxidation of achiral ferrocene-1,2-dimethanol by NAD to (S)-2-hydroxymethyl-ferrocenealdehyde with 86% ee (Fig. 4-2la) and the reduction of ferrocene-1,2-dialdehyde by NADH to (I )-2-hydroxymethyl-ferrocenealdehyde with 94% ee (Fig. 4-2lb) [14]. Fermenting baker s yeast also reduces ferrocene-1,2-dialdehyde to (I )-2-hydroxymethyl-ferro-cenealdehyde [17]. HLADH has been used for a kinetic resolution of 2-methyl-ferrocenemethanol, giving 64% ee in the product, (S)-2-methyl-ferrocenealdehyde... 

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