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Enantioselective Protonation Justin T REVIEW ARTICLE PUBLISHED ONLINE: 24 JULy 2009 | DOI: 10.1038/NCHEM.297 Enantioselective protonation Justin T. Mohr, Allen Y. Hong and Brian M. Stoltz* Enantioselective protonation is a common process in biosynthetic sequences. The decarboxylase and esterase enzymes that effect this valuable transformation are able to control both the steric environment around the proton acceptor (typically an enolate) and the proton donor (typically a thiol). Recently, several chemical methods for achieving enantioselective protonation have been developed by exploiting various means of enantiocontrol in different mechanisms. These laboratory transformations have proved useful for the preparation of a number of valuable organic compounds. Here, we review recent reports of enanti- oselective protonations, classifying them according to mechanism, and discuss how a deeper understanding of the processes can lead to improved methods for effecting this most fundamental method of obtaining enantiopure compounds. fundamental method for generating a tertiary carbon the success of a protonation method. In other cases, however, the stereo centre is to deliver a proton to a carbanion intermedi­ two stereoisomers of enolate may in fact lead to the same enantio­ A ate. However, enantioselective transfer of a proton presents mer of product5. To obviate this concern many researchers choose unusual challenges — specifically, manipulating a very small atom to investigate cyclic substrates; in turn, this may lead to a limited and avoiding product racemization at a particularly labile stereo­ substrate scope for a particular system. The method of accessing the centre. As a result, the conditions for a successful enantioselective reactive proton acceptor is among the most important facets of each protonation protocol may be very specific to a certain substrate protonation system, and many strategies have been explored (for class. Tertiary carbon stereo centres are extremely common in val­ example, conjugate addition, addition to ketenes, and decarboxyl­ uable biologically active natural products, and thus the need for ation from β­ketoesters). synthetically useful enantio selective methods to form these stereo­ Finally, the fine mechanistic details of enantioselective proton­ centres is vital1. ations are often not well understood. As typical proton acceptors In this review, we discuss several strategic approaches to enantio­ are stabilized anions, there are multiple Lewis basic sites available selective protonation. Emphasis has been placed on recently devel­ for protonation. It is likely that these sites protonate at kineti­ oped methods and their accompanying mechanisms to update the cally different rates dependent on the specific reaction conditions. most recent prior reviews on this topic2–8. Each method relies on Potentially, enantioselective protonations may be achieved either particular stereo chemical control elements based on the mechanism by direct protonation to generate the desired stereocentre, or by of the protonation transformation. Appreciation of these control­ protonation at a different site followed by enantioselective tau­ ling elements may lead to improved methods for preparing valuable tomerization. In an important recent report, Fehr9 demonstrated chiral materials for a variety of synthetic applications. that isolated enol 1 (Fig. 1) could be transformed enantioselectively into ketone 4 (an immediate precursor to the rose­smelling fra­ Important factors in achieving enantioselective protonation grance compound (S)­(α)­damascone) via the proposed aggregate Several of the most important practical features of enantioselec­ complex 3, and this mechanistic course seemed to be operative in tive protonation were enumerated in Fehr’s 1996 review2. Principal the analagous proton ation of a lithium enolate with the conjugate among these is the fact that enantioselective protonations are nec­ acid of alkoxide 2. Based on these findings, perhaps some proton­ essarily kinetic processes, because under thermodynamic control ation protocols are more accurately described as enantioselective a racemate would be formed. Accordingly, it is often necessary tautomerization reactions. Although ultimately inconsequen­ to match the pKa of the proton donor and the product to prevent tial in terms of the products obtained, insights into the specific racemization before product isolation. It is unfortunate that the same anion stabilizing groups (for example, ketones) that make proton­ 3K ations relatively easy to achieve also impart a degree of instability 3K 1 1 2+ 2 in the product. This has led some researchers to explore hydrogen 2/L 2 + atom transfer reactions in lieu of Brønsted acid­mediated proton­ 2 + /L ations (see ‘Enantioselective hydrogen atom transfer’ below). 2 THF, PhCH3 In addition to the obvious challenges of product stability under –78 °C 4 the reaction conditions, the rapid rate of proton exchange in solu­ 2 equiv. 2, 97% e.e. tion often leads to significant levels of background reaction without 130.5 equiv. 2, 83% e.e. the intercession of the chiral control element. As a result, typical proton donors are relatively weak acids that react with the proton Figure 1 | Enantioselective tautomerization of an isolated enol. Fehr9 acceptor in a slower and more controlled fashion. demonstrated that enols in the presence of a chiral Lewis base may be As substrates for enantioselective protonation generally involve a transformed into enantioenriched ketones. This indicates a possible prochiral sp2­hybridized atom, the stereochemistry of the substrate alternative mechanism for enantioselective protonation and suggests is a concern. In some cases, the ability to generate stereodefined that sometimes these transformations may be better described as proton acceptors (for example, a pure E­ or Z­enolate) is critical to enantioselective tautomerization rather than protonation. The Arnold and Mabel Beckman Laboratories of Chemical Synthesis, Department of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard, Mail Code 164‑30, Pasadena, California 91125, USA. e‑mail: [email protected] NATURE CHEMISTRY | VOL 1 | AUGUST 2009 | www.nature.com/naturechemistry 359 © 2009 Macmillan Publishers Limited. All rights reserved. REVIEW ARTICLE NATURE CHEMISTRY DOI: 10.1038/NCHEM.297 a Enzymatic decarboxylative protonation with wild-type and mutant decarboxylases two popular classes of natural enzymes for the construction of α­stereocentres adjacent to ketones. Esterases release latent enolates &+ AMDase &+ Wild-type enzyme, 99% e.e. from prochiral substrates whereas decarboxylases generate enolates 6 Tris/HCl buffer 6 &2+ + G74C mutant, 0% e.e. &2 + &2 + in situ from malonic acid derivatives (Fig. 2). G74C/C188S mutant, –94% e.e. 10,11 H2O Ohta and co­workers isolated arylmalonate decarboxylase (S)-6 5 (AMDase) from the Gram­negative bacterium Alcaligenes bronchi­ septicus and found that it catalyses the decarboxylative enantio­ + &+ + 2 &\V &\V &\V selective protonation of α­aryl­α­methyl­malonates through the +& 6 6 &+ 6 $U + proposed mechanism in Fig. 2a. Yields and enantiomeric excesses + 2 $U 2 $U were excellent for substrates with various α­aryl substitutents (up to 2 2 2+ 2 2 *O\ *O\ + *O\ 99% yield and 99% e.e.). Experiments have shown that the Cys188 Wild-type enzyme Wild-type enzyme Wild-type enzyme residue is essential for activity, and this site is the putative proton 12 donor that stabilizes the enolate intermediate. A Hammett study &+ + &+ of the reaction found a value for the reaction constant, ρ, of +1.19, 6 &\V 6HU which is consistent with a negatively charged transition state10. $U 2 $U 2 6 + 6 + Recently, preliminary X­ray diffraction experiments and an X­ray 2 2 13,14 &\V + &\V + crystal structure of AMDase were reported . Accessing the opposite enantiomeric series of products required G74C mutant G74C/C188S mutant additional investigation15,16. Analysis of the enzyme amino acid sequence was carried out to check for homology with known enzymes. b Enzymatic oxidation/decarboxylation/protonation/oxidation cascade The Cys188 residue is conserved in several racemase enzymes from other microorganisms. Glutamate racemase, found in the bacte­ +& &2+ Rhodococcus sp. KU1314 +& &2+ 2+ Glycine/NaOH buffer 2 rium Lactobacillus fermenti, contains an active site similar to that of AMDase, but with cysteine residues (Cys188 and Cys74) on both sides H2O + +&2 (61% yield) +&2 of the substrate. Presumably, one of the cysteine residues acts as a base 7 8 and generates an enolate intermediate, which can then be protonated &+ Enantioselective non­selectively from either cysteine residue to give rise to a racemic –CO2 2 protonation mixture. When Ohta and co­workers prepared a G74C mutant of + AMDase to mimic these racemase enzymes, they found that racemic +&2 α­thienylpropionic acid (6, Fig. 2a) was formed in 37% yield from the 9 malonic acid substrate (5). Further explorations based on this homol­ &+ &+ ogy hypothesis led to the preparation of a double mutant of AMDase + [O] + (G74C/C188S) that removed the native cysteine residue while main­ 2 &2+ taining the mutant residue on the opposite face of the substrate. The + +&2 +&2 (R)-11 opposite enantiomer of product was indeed obtained with this new 10 74% e.e. enzyme in 94% enantiomeric excess, although yields decreased to c Enzymatic hydrolysis of enol acetates 60% and the activity of this mutant was several orders of magnitude lower than the wild type. Some activity was
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