Kinetic resolution of secondary alcohols using amidine-based catalysts

Isothiourea-Catalysed Acylative Dynamic Kinetic Resolution of Tetra-substituted Morpholinone and Benzoxazinone Lactols

The development of methods to allow the selective acylative dynamic kinetic resolution (DKR) of tetra-substituted lactols is a recognised synthetic challenge. In this manuscript, a highly enantioselective isothiourea-catalysed acylative DKR of tetra-substituted morpholinone and benzoxazinone-derived lactols is reported. The scope and limitations of this methodology have been developed, with high enantioselectivity and good to excellent yields (up to 89 %, 99 : 1 er) observed across a broad range of substrate derivatives incorporating substitution at N(4) and C(2), di- and spirocyclic substitution at C(5) and C(6), as well as benzannulation (>35 examples in total). The DKR process is amenable to scale-up on a 1 g laboratory scale. The factors leading to high selectivity in this DKR process have been probed through computation, with an N-C=O⋅⋅⋅isothiouronium interaction identified as key to producing ester products in highly enantioenriched form.

Synthesis of Tetra-Substituted 3-Hydroxyphthalide Esters by Isothiourea-Catalysed Acylative Dynamic Kinetic Resolution

A general and highly enantioselective method for the preparation of tetra-substituted 3-hydroxyphthalide esters via isothiourea-catalysed acylative dynamic kinetic resolution (DKR) is reported. Using (2S,3R)-HyperBTM (5 mol %) as the catalyst, the scope and limitations of this methodology have been extensively probed, with high enantioselectivity and good to excellent yields observed (>40 examples, up to 99 %, 99 : 1 er). Substitution of the aromatic core within the 3-hydroxyphthalide skeleton, as well as aliphatic and aromatic substitution at C(3), is readily tolerated. A diverse range of anhydrides, including those from bioactive and pharmaceutically relevant acids, can also be used. The high enantioselectivity observed in this DKR process has been probed computationally, with a key substrate heteroatom donor O⋅⋅⋅acyl-isothiouronium interaction identified through DFT analysis as necessary for enantiodiscrimination.

Isothiourea-Catalysed Acylative Kinetic Resolution of Tertiary Pyrazolone Alcohols

The development of methods for the selective acylative kinetic resolution (KR) of tertiary alcohols is a recognised synthetic challenge with relatively few successful substrate classes reported to date. In this manuscript, a highly enantioselective isothiourea-catalysed acylative KR of tertiary pyrazolone alcohols is reported. The scope and limitations of this methodology have been developed, with high selectivity observed across a broad range of substrate derivatives incorporating varying substitution at N(2)-, C(4)- and C(5)-, as well as bicyclic constraints within the pyrazolone scaffold (30 examples, selectivity factors (s) typically >100) at generally low catalyst loadings (1 mol %). The application of this KR method to tertiary alcohols derived directly from a natural product (geraniol), alongside pharmaceutically relevant drug compounds (indomethacin, gemfibrozil and probenecid), with high efficiency (s >100) is also described. The KR process is readily amenable to scale up using bench grade solvents and reagents, with effective resolution on a 50 g (0.22 mol) scale demonstrated. The key structural motif leading to excellent selectivity in this KR process has been probed through computation, with an NC=O⋅⋅⋅isothiouronium interaction from substrate to acylated catalyst observed within the favoured transition state. Similarly, the effect of C(5)-aryl substitution that leads to reduced experimental selectivity is probed, with a competitive π-isothiouronium interaction identified as leading to reduced selectivity.

Total Synthesis of Psammaplysins A, M, O, and Q and Ceratinamide A

The psammaplysins are a unique class of brominated marine alkaloids bearing a signature 5/7-spiroisoxazoline-oxepine core linked to a variable tyramine-derived unit. Here, we report the total synthesis of several members of this family via a dipolar cycloaddition between an in situ generated nitrile oxide and an unusual seven-membered enediol diether dipolarophile. Carefully orchestrated oxidative transformation toward the fully functionalized spirocycle and direct coupling with tyramine-derived amines provides access to five representative family members, psammaplysins A, M, O, and Q and ceratinamide A, the latter four for the first time. Additionally, kinetic resolution of a late-stage intermediate enables the first asymmetric synthesis of (-)-psammaplysin A, thereby confirming its absolute configuration.

Investigating Substituent Interactions with Cationic Catalysts

Rates of isothiourea catalyzed silylation and acylation reactions were measured for substrates with various electronic substituents at the aryl group. Through these measurements, the intermolecular interactions between cationic catalyst intermediates and different aryl groups were explored. These studies were performed to understand how changes in the catalyst structure affected electrostatic intermolecular interactions. Three different catalysts (N-methylimidazole and two isothioureas) were employed that varied in their ability to delocalize their cationic nature. The results show that more delocalization on the catalyst reduces the sensitivity to the electronics on the aryl group. Surprisingly, the isothiourea with a fused benzene ring provided additional points of interaction with groups that contained lone-pairs, significantly affecting the overall rate. This work helps explore the interactions that dominate in these types of catalytic systems, to aid in future organocatalysis development. Density functional theory (DFT) studies further confirmed isothiourea/aryl ring interaction with the alcohol substrate in the acylation process, which confirmed these hypotheses. Electron rich or lone-pair bearing functional groups stabilize the cationic catalyst core, thereby stabilizing the transition states and accelerating the reaction. It was also discovered that in one case, the formation of a stable substrate dimer was responsible for its lower reactivity.

Kinetics and Mechanism of Azole n-π*-Catalyzed Amine Acylation

Azole anions are highly competent in the activation of weak acyl donors, but, unlike neutral (aprotic) Lewis bases, are not yet widely applied as acylation catalysts. Using a combination of in situ and stopped-flow 1H/19F NMR spectroscopy, kinetics, isotopic labeling, 1H DOSY, and electronic structure calculations, we have investigated azole-catalyzed aminolysis of p-fluorophenyl acetate. The global kinetics have been elucidated under four sets of conditions, and the key elementary steps underpinning catalysis deconvoluted using a range of intermediates and transition state probes. While all evidence points to an overarching mechanism involving n-π* catalysis via N-acylated azole intermediates, a diverse array of kinetic regimes emerges from this framework. Even seemingly minor changes to the solvent, auxiliary base, or azole catalyst can elicit profound changes in the temporal evolution, thermal sensitivity, and progressive inhibition of catalysis. These observations can only be rationalized by taking a holistic view of the mechanism and a set of limiting regimes for the kinetics. Overall, the analysis of 18 azole catalysts spanning nearly 10 orders of magnitude in acidity highlights the pitfall of pursuing ever more nucleophilic catalysts without regard for catalyst speciation.

Cooperative Palladium/Isothiourea Catalyzed Enantioselective Formal (3+2) Cycloaddition of Vinylcyclopropanes and α,β-Unsaturated Esters

A protocol for the enantioselective synthesis of substituted vinylcyclopentanes has been realised using cooperative palladium and isothiourea catalysis. Treatment of vinylcyclopropanes with Pd(PPh3 )4 generates a zwitterionic π-allyl palladium intermediate that intercepts a catalytically generated α,β-unsaturated acyl ammonium species prepared from the corresponding α,β-unsaturated para-nitrophenyl ester and the isothiourea (R)-BTM. Intermolecular formal (3+2) cycloaddition between these reactive intermediates generates functionalised cyclopentanes in generally good yields and excellent diastereo- and enantiocontrol (up to >95 : 5 dr, 97 : 3 er), with the use of LiCl as an additive proving essential for optimal stereocontrol. To the best of our knowledge a dual transition metal/organocatalytic process involving α,β-unsaturated acyl ammonium intermediates has not been demonstrated previously.

Acylative Kinetic Resolution of Cyclic Hydroxamic Acids

Racemic cyclic hydroxamic acids bearing an aryl substituent adjacent to the hydroxyl group undergo effective acylative kinetic resolution promoted by benzotetramisole (BTM).

Total Synthesis of Leiodermatolide A via Transfer Hydrogenative Allylation, Crotylation, and Propargylation: Polyketide Construction beyond Discrete Allyl- or Allenylmetal Reagents

The total synthesis of leiodermatolide A was accomplished in 13 steps (LLS). Transfer hydrogenative variants of three carbonyl additions that traditionally rely on premetalated reagents (allylation, crotylation, and propargylation) are deployed together in one total synthesis.

Acylative Dynamic Kinetic Resolution of Secondary Alcohols: Tandem Catalysis by HyperBTM and Bäckvall's Ruthenium Complex

Non-enzymatic dynamic kinetic resolution (DKR) of secondary alcohols by enantioselective acylation using an isothiourea-derived HyperBTM catalyst and racemization of slowly reacting alcohol by Bäckvall's ruthenium complex is reported. The DKR approach features high enantioselectivities (up to 99:1), employs easy-to-handle crystalline 4-nitrophenyl isobutyrate as the acylating reagent, and proceeds at room temperature and under an ambient atmosphere. The stereoinduction model featuring cation-π system interactions between the acylated HyperBTM catalyst and π electrons of an alcohol aryl subunit has been elaborated by DFT calculations.

Direct enantioseparation of axially chiral 1,1'-biaryl-2,2'-diols using amidine-based resolving agents

Amidine-based optically active resolving agents for enantiomer separation of axially chiral 1,1'-biaryl-2,2'-diols have been developed. A strongly basic amidine bearing no substituents on its nitrogen atoms enables the formation of their diastereomeric salts upon being mixed with weakly acidic phenol derivatives. Enantiopure 1,1'-biaryl-2,2'-diols can be obtained in high yields after only one crystallization of their salts with the chiral amidine derived from dehydroabietic acid. X-ray crystallography revealed that the amidine moiety forms a salt with the phenol group and additional intermolecular NH/π interactions contribute to the efficient chiral recognition process.

Horeau amplification in the sequential acylative kinetic resolution of (±)-1,2-diols and (±)-1,3-diols in flow

The sequential acylative kinetic resolution (KR) of C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diols using a packed bed microreactor loaded with the polystyrene-supported isothiourea, HyperBTM, is demonstrated in flow. The sequential KRs of C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diols exploits Horeau amplification, with each composed of two successive KR processes, with each substrate class significantly differing in the relative rate constants for each KR process. Optimisation of the continuous flow set-up for both C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diol substrate classes allowed isolation of reaction products in both high enantiopurity and yield. In addition to the successful KR of C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diols, the application of this process to the more conceptually-complex scenario involving the sequential KR of C1-symmetric (±)-1,3-anti-diols was demonstrated, which involves eight independent rate constants.

Chiral isothiourea-catalyzed kinetic resolution of 4-hydroxy[2.2]paracyclophane

We herein report a method for the kinetic resolution of racemic 4-hydroxy[2.2]paracyclophane by means of a chiral isothiourea-catalyzed acylation with isobutyric anhydride. This protocol allows for a reasonable synthetically useful s-factor of 20 and provides a novel entry to obtain this interesting planar chiral motive in an enantioenriched manner.

Insight into the Mechanism of the Acylation of Alcohols with Acid Anhydrides Catalyzed by Phosphoric Acid Derivatives

Insight into the mechanism of a safe, simple, and inexpensive phosphoric acid (H₃PO₄)-catalyzed acylation of alcohols with acid anhydrides is described. The corresponding in situ-generated diacylated mixed anhydrides, unlike traditionally proposed monoacylated mixed anhydrides, are proposed as the active species. In particular, the diacylated mixed anhydrides act as efficient catalytic acyl transfer reagents rather than as Brønsted acid catalysts simply activating acid anhydrides. Remarkably, highly efficient phosphoric acid (1-3 mol %)-catalyzed acylation of alcohols with acid anhydrides was achieved and a 23 g scale synthesis of an ester was demonstrated. Also, phosphoric acid catalyst was effective for synthetically useful esterification from carboxylic acids, alcohols, and acid anhydride. Moreover, with regard to recent developments in chiral 1,1'-bi-2-naphthol (BINOL)-derived phosphoric acid diester catalysts toward asymmetric kinetic resolution of alcohols by acylation, some phosphate diesters were examined. As a result, a ³¹P NMR study and a kinetics study strongly supported not only the acid-base cooperative mechanism as previously proposed by other researchers but also the mixed anhydride mechanism as presently proposed by us.

Enantiopure isothiourea@carbon-based support: stacking interactions for recycling a lewis base in asymmetric catalysis

An enantiopure isothiourea (hyperBTM) was functionalized by a pyrene moiety via click chemistry; immobilized on reduced Graphene Oxide, this recyclable chiral organocatalyst promotes formal [3+2] cycloaddition of ammonium enolates with oxaziridines.

Isothiourea-Catalyzed Acylative Kinetic Resolution of Tertiary α-Hydroxy Esters

A highly enantioselective isothiourea-catalyzed acylative kinetic resolution (KR) of acyclic tertiary alcohols has been developed. Selectivity factors of up to 200 were achieved for the KR of tertiary alcohols bearing an adjacent ester substituent, with both reaction conversion and enantioselectivity found to be sensitive to the steric and electronic environment at the stereogenic tertiary carbinol centre. For more sterically congested alcohols, the use of a recently-developed isoselenourea catalyst was optimal, with equivalent enantioselectivity but higher conversion achieved in comparison to the isothiourea HyperBTM. Diastereomeric acylation transition state models are proposed to rationalize the origins of enantiodiscrimination in this process. This KR procedure was also translated to a continuous-flow process using a polymer-supported variant of the catalyst.

Synthesis of β-Glucosides with 3-O-Picoloyl-Protected Glycosyl Donors in the Presence of Excess Triflic Acid: A Mechanistic Study

Our previous study showed that picoloylated donors are capable of providing excellent facial stereoselectivity through the H-bond-mediated aglycone delivery (HAD) pathway. Presented herein is a detailed mechanistic study of stereoselective glycosylation with 3-O-picoloylated glucosyl donors. While reactions of glycosyl donors equipped with the 3-O-benzoyl group are typically non-stereoselective because these reactions proceed via the oxacarbenium intermediate, 3-O-picoloylated donors are capable of providing enhanced, but somewhat relaxed, β-stereoselectivity by the HAD pathway. In an attempt to refine this reaction, we noticed that glycosylations are highly β-stereoselective in the presence of NIS and stoichiometric TfOH. The HAD pathway is highly unlikely because the picoloyl nitrogen is protonated under these reaction conditions. The protonation and glycosylation were studied by low-temperature NMR, and the intermediacy of the glycosyl triflate has been observed. This article is dedicated to broadening the scope of this reaction in application to a variety of substrates and targets.

Recent advances in continuous-flow organocatalysis for process intensification

The progresses on continuous-flow organocatalysis from 2016 to early 2020 are reviewed with focus on transition from batch to flow.

Origin of stereoselectivity in an isothiourea catalyzed Michael addition reaction of aryl ester with vinyl disulfone

The origin of stereoselectivity in an isothiourea-catalyzed addition reaction of aryl ester with vinyl disulfone was explored for the first time.

Highly efficient asymmetric bioreduction of 1-aryl-2-(azaaryl)ethanones. Chemoenzymatic synthesis of lanicemine

Different ketoreductases (KREDs) have been used to promote a highly selective reduction of several 1-aryl-2-(azaaryl)ethanones (azaaryl = pyridinyl, quinolin-2-yl), the corresponding secondary alcohols being obtained with very high yields and enantiomeric excesses (ee > 99%). The absolute configuration of each optically active alcohol has been assigned by means of modified Mosher and Kelly methods, two shielding effects being evaluated: (1) the Mosher phenyl ring effect on the azaaryl protons and (2) the one of the azaaryl ring on the Mosher methoxy group. In addition, the biologically active amine lanicemine has been synthesized from (R)-1-phenyl-2-(pyridin-2-yl)ethanol, thus proving the utility of the secondary alcohols here prepared.

Isothiourea-Catalysed Regioselective Acylative Kinetic Resolution of Axially Chiral Biaryl Diols

An operationally simple isothiourea-catalysed acylative kinetic resolution of unprotected 1,1'-biaryl-2,2'-diol derivatives has been developed to allow access to axially chiral compounds in highly enantioenriched form (s values up to 190). Investigation of the reaction scope and limitations provided three key observations: i) the diol motif of the substrate was essential for good conversion and high s values; ii) the use of an α,α-disubstituted mixed anhydride (2,2-diphenylacetic pivalic anhydride) was critical to minimize diacylation and give high selectivity; iii) the presence of substituents in the 3,3'-positions of the diol hindered effective acylation. This final observation was exploited for the highly regioselective acylative kinetic resolution of unsymmetrical biaryl diol substrates bearing a single 3-substituent. Based on the key observations identified, acylation transition state models have been proposed to explain the atropselectivity of this kinetic resolution.

Chiral AuI - and AuIII -Isothiourea Complexes: Synthesis, Characterization and Application

During an investigation into the potential union of Lewis basic isothiourea organocatalysis and gold catalysis, the formation of gold-isothiourea complexes was observed. These novel gold complexes were formed in high yield and were found to be air- and moisture stable. A series of neutral and cationic chiral gold(I) and gold(III) complexes bearing enantiopure isothiourea ligands was therefore synthesized and fully characterized. The steric and electronic properties of the isothiourea ligands was assessed through calculation of their percent buried volume and the synthesis and analysis of novel iridium(I)-isothiourea carbonyl complexes. The novel gold(I)- and gold(III)-isothiourea complexes have been applied in preliminary catalytic and biological studies, and display promising preliminary levels of catalytic activity and potency towards cancerous cell lines and clinically relevant enzymes.

A Stereodynamic Redox-Interconversion Network of Vicinal Tertiary and Quaternary Carbon Stereocenters in Hydroquinone-Quinone Hybrid Dihydrobenzofurans

Reversible redox processes involving hydroquinones and quinones are ubiquitous in biological reaction networks, materials science, and catalysis. While extensively studied in intermolecular settings, less is known about intramolecular scenarios. Herein, we report hydroquinone-quinone hybrid molecules that form two-stereocenter dihydrobenzofurans via intramolecular cyclization under thermodynamic control. A π-methylhistidine peptide-catalyzed kinetic resolution allowed us to study the stereodynamic behavior of enantio- and diastereo-enriched dihydrofurans. In the course of this study, it was revealed that a reversible intramolecular redox-interconversion network connects all four possible stereoisomers via inversion of a quaternary carbon stereocenter without achiral intermediates. As a result, these findings on hydroquinone-quinone hybrid molecules provide insights into potential natural origin and synthetic access of the common dihydrobenzofuran motif.

Enantioselective Synthesis of Medium-Sized Lactams via Chiral α,β-Unsaturated Acylammonium Salts

Medium-sized lactams are important structural motifs found in a variety of bioactive compounds and natural products but are challenging to prepare, especially in optically active form. A Michael addition/proton transfer/lactamization organocascade process is described that delivers medium-sized lactams, including azepanones, benzazepinones, azocanones, and benzazocinones, in high enantiopurity through the intermediacy of chiral α,β-unsaturated acylammonium salts. An unexpected indoline synthesis was also uncovered, and the benzazocinone skeleton was transformed into other complex heterocyclic derivatives, including spiroglutarimides, isoquinolinones, and δ-lactones.

Heteroatom-Directed Acylation of Secondary Alcohols To Assign Absolute Configuration

Birman's HBTM catalyst is effective for the enantioselective acylation and kinetic resolution of benzylic secondary alcohols. The enantioselective acylation has now been extended to secondary alcohols bearing electron-withdrawing groups such as halides and other heteroatoms. The level of selectivity is modest to good and is sufficient for determining configuration using the competing enantioselective conversion method. A mathematical analysis identifies conditions for achieving maximum differences in conversion and, consequently, assigning configuration with greater confidence. The new method is effective for halohydrins and secondary-tertiary 1,2-diols and was used to confirm the configuration of two inoterpene natural products.

Catalytic enantioselective acyl transfer: the case for 4-PPY with a C-3 carboxamide peptide auxiliary based on synthesis and modelling studies

A series of 4-pyrrolidinopyridine (4-PPY) C-3 carboxamides containing peptide-based side chains have been synthesised and evaluated in the kinetic resolution of a small library of chiral benzylic secondary alcohols. A key design element was the incorporation of a tryptophan residue in the peptide side chain for promoting π-stacking between peptide side chain and the pyridinium ring of the N-acyl intermediate, in which modelling was used as a structure-based guiding tool. Together, a catalyst containing a LeuTrp-N-Boc side chain (catalyst 8) was identified that achieved s-values up to and in slight excess of 10. A transition-state model based on the modelling is proposed to explain the origin of enantioselectivity. This study establishes the usefulness of modelling as a structure-based guiding tool for enantioselectivity optimization as well as the potential for developing scalable peptide-based DMAP-type catalysts for large-scale resolution work.

Insights into the isothiourea-catalyzed asymmetric [4 + 2] annulation of phenylacetic acid with alkylidene pyrazolone

A computational study on the isothiourea-catalyzed asymmetric [4 + 2] annulation of phenylacetic acid with alkylidene pyrazolone was performed using the DFT method.