“Cassette” In Situ Enzymatic Screening Identifies Complementary Chiral Scaffolds for Hydrolytic Kinetic Resolution Across a Range of Epoxides

Abstract

For combinatorial catalysis, [1,2] rapid and information-rich screening methods are very useful. Toward this end, we describe herein a new "cassette" in situ enzymatic screening (ISES) approach that allows the experimentalist to obtain a parallel readout on substrate specificity, as well as a sense and magnitude of enantioselectivity.[3] In its first iteration, cassette-ISES is used to "cherry pick" only those catalysts in the array that show high ISES ee-value readouts across both test substrates. Two such catalysts are then investigated further, yielding promising results.In our ISES approach, typically the reaction product [4] or by-product [5] diffuses from an organic layer into an aqueous layer containing the "reporting enzyme". [6][7][8][9] There, an enzyme-catalyzed reaction leads to a spectroscopic signal that is monitored in real time. The approach complements other emerging screens by using chiroptical techniques, [10] liquid crystalline arrays, [11] IR thermography, [12] mass, [13] NMR, [14] IR [15] and fluorescence spectroscopy.[16] The technique is sensitive (i.e. 10 nmol of product gives rise to DA 340 % 0.12 for a dehydrogenase reporting enzyme in a 500 mL aqueous volume), allowing one to get information on catalyst performance at relatively early conversions/short reaction times. Catalysts may be screened in parallel in a standard spectrophotometer with a multicell changer without the need to draw aliquots or work up the reaction. Moreover, the need to install a chromophore (adding steps and potentially altering substrate reactivity) is obviated.Herein, we describe a new pair of reporting enzymes (Scheme 1) that are capable of differentiating the 1,2-hexanediol antipodes (Lactobacillus kefir alcohol dehydrogenase (LKADH): highly S selective k S /k R % 20, and horseliver alcohol dehydrogenase (HLADH): modestly S selective k S /k R % 2.2). This allows one to obtain simultaneous enantioselectivity readouts on two distinct substrates for the Co IIIsalen-mediated (salen = (salicylidene)ethylenediamine) hydrolytic kinetic resolution (HKR) of epoxides, [17] presenting both "short" (propylene oxide; R = Me) [4] and long (hexene oxide: R = Bu) R groups. In this way, one can begin to address the question of substrate generality, which is so important in asymmetric catalysis today. [18,19] To demonstrate proof of principle for cassette-ISES, we employed a focused chiral salen array (Figure 1) that crosses chiral space variation in the constituent 1,2-diamines with considerable steric [20] and electronic variation in the "salicylaldehyde partners" (including the benzoylacetaldehyde (baen) precursor [21] ). Figure 2 illustrates the "four coordinate" structure-enantioselectivity relationship (SER) data that one obtains from such a cassette-ISES protocol. The x and y axes represent the two structural variables in the salen array. The directionality and length of the z vector provide the Scheme 1. A suitable pair of "reporting enzymes" for 1,2-hexanediol permits a cassette-ISES evaluation of HKR catalyst candidat...

Exploiting Enzymatic Dynamic Reductive Kinetic Resolution (DYRKR) in Stereocontrolled Synthesis

“…By utilizing two reporting ADH enzymes with complementary enantioselectivities, the experimentalist can glean information on relative rates and enantioselectivities of a series of catalysts or chiral ligands of interest, in a parallel screening format. [14] …”

“…To be clear, for classical kinetic resolutions, in the ideal case, using a lipase, say, one can obtain enantiopure acylated product in 50% yield, while also recovering 50% of the antipodal unreacted alcohol (Scheme 1, top). While it can be advantageous to access both enantiomeric forms of a building block, [14a] this is not always desirable. To address this limitation, dynamic kinetic resolution (DKR) may be employed, where the stereocenter in question can be racemized.…”

Halocarbocyclization Entry into the Oxabicyclo[4.3.1]decyl Exomethylene-δ-Lactone Cores of Linearifolin and Zaluzanin A: Exploiting Combinatorial Catalysis

“…3 This Letter describes the deployment of the latter two methods for stereocontrolled entry into bicyclic terpenoid natural product (NP) cores bearing a reactive exomethylene δ-lactone functionality.…”

“…Interestingly, these two terpenoid lactone cores have opposite handedness, therefore efficient hydrolytic kinetic resolution (HKR) 3, 13 should allow for assembly of both cores from a common racemic epoxide building block precursor (Fig. 2), one from the diol product (as the cyclic sulfate), the other from the remaining epoxide.…”

The In Situ Enzymatic Screening (ISES) Approach to Reaction Discovery and Catalyst Identification

“…Note that while the two reporting enzymes must have distinct enantioselectivities for the reaction product, these need not be opposite. Thus, while HLADH is modestly ( S )-selective and TBADH is ( R )-selective for the 1,2-propanediol product from the HKR of propylene oxide (Dey et al, 2005), Lactobacillus kefir ADH (LKADH) is highly ( S )-selective and HLADH modestly ( S )-selective for the 1,2-hexanediol product from the HKR of hexene oxide (Dey et al, 2007). The miniaturized ISES example described in Basic Protocol 4 also employs this technique across several wells of a micromulticell.…”