Enantioselective Organocatalytic Synthesis of 2-Oxopiperazines from Aldehydes: Identification of the Elusive Epoxy Lactone Intermediate

Computational and Spectroscopic Studies on the Formation of Halogen-Bonded Complexes Between Tertiary Amines and CBr4 and Application in the Light-Mediated Amino Acid Coupling

In recent years, halogen-bonded complexes (XBCs), in solution, have played a pivotal role in inducing photochemical organic reactions. In this work, we explore the ability of various tertiary amines to act as XB acceptors in the presence of the XB donor CBr4 by computational and spectroscopic studies. DFT studies clearly showcase the formation of XBCs between the studied tertiary amines and CBr4. Simultaneously, computational and experimental UV-Vis studies display intense red shifts that are consistent with charge transfer observed from tertiary amines to CBr4. A detailed NMR study revealed a clear chemical shift of the carbon carrying the bromine atoms upon mixing the XB acceptor with the donor, suggesting that this spectroscopic technique is indeed an experimental tool to identify the generation of XBCs. An application of the ability of such XBCs to activate a carboxylic acid under UVA irradiation or sunlight is presented for amino acid coupling. Among the various tertiary amines studied, the pair DABCO-CBr4 was found to work well for the photochemical amide bond formation. Direct infusion-HRMS studies allowed us to propose a general mechanism for the photochemical amino acid coupling in the presence of a tertiary amine and CBr4, initiated by the photoactivation of an XBC.

Facile access to C-substituted piperazin-2-ones and mianserin derivative enabled by chemoselective carbene insertion and cyclization cascade

The chemoselective N-H insertion of unsymmetrical diamines into carbene is a longstanding challenge. A simple copper-catalyzed strategy for synthesizing C-substituted piperazinones is described, employing easily accessible diazo compounds and 1,2-diamines. The reaction proceeded via chemo-selective carbene insertion at the comparatively less nucleophilic amine, followed by instantaneous cyclization. The protocol was further extended to access NH-free piperazinone, and the synthesis of a Mianserin derivative.

Synthesis of Polypeptides and Poly(α-hydroxy esters) from Aldehydes Using Strecker Synthesis

This report presents a versatile approach for the synthesis of new polypeptide and polyester-based biomaterials. The well-established Strecker reaction was utilized, with hexanal serving as the model aldehyde, to synthesize α-amino and α-hydroxy acids as monomer units for the polymer system. Following the formation of the corresponding amino and hydroxy acid monomers, they were subsequently converted to N-carboxy and O-carboxy-anhydrides. The resultant cyclic anhydride molecules were then polymerized via ring-opening polymerization to yield the corresponding polypeptides and polyesters. This report establishes a straightforward methodology for the synthesis of new polypeptide and poly(a-hydroxy acid)-based biomaterials, thereby expanding the existing library of polymers for various biomedical applications.

Direct organocatalytic transfer hydrogenation and C-H oxidation: high-yielding synthesis of 3-hydroxy-3-alkyloxindoles

Readily available 3-alkylideneoxindoles were effectively reduced to 3-alkyloxindoles through transfer hydrogenation using Hantzsch ester as a reducing agent at ambient temperature and the greenness/sustainability of this protocol was assessed by correlation with Pd/C-mediated hydrogenation with hydrogen gas. Furthermore, an organocatalytic method was developed to access drug-like 3-alkyl-3-hydroxyoxindoles by C-H oxidation of 3-alkyl-indolin-2-one, using a catalytic amount of 1,1,3,3-tetramethylguanidine (TMG) as an organic base and dissolved oxygen in THF as an oxidant at room temperature. Key reaction intermediates were observed by controlled on-line ESI-HRMS experiments and identified by their corresponding mass (m/z) analysis. This two-step high-yielding transfer hydrogenation/C-H oxidation protocol was used for the total synthesis of medicinally important 3-cyanomethyl-3-hydroxyoxindole and formal total synthesis of (±)-alline and (±)-CPC-I in very good overall yields compared to previous methods.

Epoxides: Small Rings to Play with under Asymmetric Organocatalysis

Optically pure epoxides are recognized as highly valuable products and key intermediates, useful in different areas from pharmaceutical and agrochemical industries to natural product synthesis and materials science. The predictable fate of the ring-opening process, in terms of stereoselectivity and often of regioselectivity, enables useful functional groups to be installed at vicinal carbon atoms in a desired manner. In this way, products of widespread utility either for synthetic applications or as final products can be obtained. The advent of asymmetric organocatalysis provided a new convenient tool, not only for their preparation but also for the elaboration of this class of heterocycles. In this review, we focus on recent developments of stereoselective organocatalytic ring-opening reactions of meso-epoxides, kinetic resolution of racemic epoxides, and Meinwald-type rearrangement. Examples of asymmetric organocatalytic processes toward specific synthetic targets, which include ring opening of an epoxide intermediate, are also illustrated.

Development of a synthetic equivalent of α,α-dicationic acetic acid leading to unnatural amino acid derivatives via tetrafunctionalized methanes

Diethyl mesoxalate (DEMO) exhibits high electrophilicity and accepts the nucleophilic addition of a less nucleophilic acid amide to afford N,O-hemiacetal. However, our research showed that elimination of the amide moiety proceeded more easily than dehydration upon treatment with a base. This problem was overcome by reacting DEMO with an acid amide in the presence of acetic anhydride to efficiently obtain N,O-acetal. Acetic acid was eliminated leading to the formation of N-acylimine in situ upon treatment with the base. N-Acylimine is also electrophilic, accepting the second nucleophilic addition by pyrrole or indole to form α,α-disubstituted malonates. Subsequent hydrolysis followed by decarboxylation resulted in (α-indolyl-α-acylamino)acetic acid formation; homologs of tryptophan. Through this process, DEMO serves as a synthetic equivalent of α,α-dicationic acetic acid to facilitate nucleophilic introduction of the two substituents.

Investigation of the Organocatalytic Chlorination of 2-Phenylpropanal

Results of an examination of the organocatalytic enantioselective α-chlorination of 2-phenylpropanal are described. Synthetic investigation including the screening of primary and secondary aminocatalysts, many different reaction conditions, and other α-branched aldehydes show that especially primary aminocatalysts can catalyze the formation of the α-chloro branched aldehydes in good yields, but only with moderate enantioselectivities. In order to try to understand the challenge in obtaining high enantioselectivity for the aminocatalytic α-chlorination of α-branched aldehydes a series of experimental investigations were performed employing 2-phenylpropanal as a model system. These investigations have been coupled with computational investigations, which provided important insight into the moderate enantioselectivity of this chlorination reaction. Analysis of the reaction showed, that the lack of control over the selectivity of formation of the (E)- and (Z)-enamine intermediate, and the clustering of reaction barriers of possible reaction pathways help to rationalize difficulties in producing high enantioselectivity.

Synthesis of chiral piperazin-2-ones through palladium-catalyzed asymmetric hydrogenation of pyrazin-2-ols

A palladium-catalyzed asymmetric hydrogenation of pyrazines containing a tautomeric hydroxyl group was developed, providing a facile access to chiral disubstituted piperazin-2-ones with excellent diastereoselectivities and enantioselectivities.

Phenylglyoxylic Acid: An Efficient Initiator for the Photochemical Hydrogen Atom Transfer C-H Functionalization of Heterocycles

C-H functionalization at the α-position of heterocycles has become a rapidly growing area of research. Herein, a cheap and efficient photochemical method was developed for the C-H functionalization of heterocycles. Phenylglyoxylic acid (PhCOCOOH) could behave as an alternative to metal-based catalysts and organic dyes and provided a very general and wide array of photochemical C-H alkylation, alkenylation, and alkynylation, as well as C-N bond forming reaction methodologies. This novel, mild, and metal-free protocol was successfully employed in the functionalization of a wide range of C-H bonds, utilizing not only O- or N-heterocycles, but also the less studied S-heterocycles.

Recent progress toward the asymmetric synthesis of carbon-substituted piperazine pharmacophores and oxidative related heterocycles

The piperazine drugs are mostly N-substituted compared to only a few C-substituted drugs. To explore this unknown chemical space, asymmetric syntheses of C-substituted piperazines is the subject of this review.

The important requirement for approval of a new drug, in case it happens to be chiral, is that both enantiomers of the drug should be studied in detail, which has led synthetic organic and medicinal chemists to focus their attention on the development of new methods for asymmetric synthesis especially of relevant saturated N-heterocycles. On the other hand, the piperazine ring, besides defining a major class of saturated N-heterocycles, has been classified as a privileged structure in medicinal chemistry, since it is more than frequently found in biologically active compounds including several marketed blockbuster drugs such as Glivec (imatinib) and Viagra (sildenafil). Indeed, 13 of the 200 best-selling small molecule drugs in 2012 contained a piperazine ring. Nevertheless, analysis of the piperazine substitution pattern reveals a lack of structural diversity, with almost every single drug in this category (83%) containing a substituent at both the N1- and N4-positions compared to a few drugs having a substituent at any other position (C2, C3, C5, and C6). Significant chemical space that is closely related to that known to be biologically relevant, therefore, remains unexplored. In order to explore this chemical space, efficient and asymmetric syntheses of carbon-substituted piperazines and related heterocycles must be designed and developed. Initial, recent efforts toward the implementation of this particular target are in fact the subject of this review.

Catalytic Enantioselective Total Synthesis of (+)-Lycoperdic Acid

A concise enantio- and stereocontrolled synthesis of (+)-lycoperdic acid is presented. The stereochemical control is based on iminium-catalyzed Mukaiyama-Michael reaction and enamine-catalyzed organocatalytic α-chlorination steps. The amino group was introduced by azide displacement, affording the final stereochemistry of (+)-lycoperdic acid. Penultimate hydrogenation and hydrolysis afforded pure (+)-lycoperdic acid in seven steps from a known silyloxyfuran.

Direct access to vicinally functionalized and N-trifluoroacetylated (bicyclic)ketopiperazines using a readily affordable N-heterocyclic anhydride

Functionalized 2-piperazinones play essential roles as conformationally-constrained peptidomimetics by mimicking inverse γ-turns in peptides. Here, we describe an efficient, scalable, stereoselective, modular, and chemoselective annulation protocol between a novel N-trifluoroacetyl anhydride and several reactive partners, including lactim ethers, imidoyl chlorides, aryl aldimines, and 1,3-azadienes, leading to vicinally functionalized (bicyclic) 2-piperazinones.

Copper-catalyzed alkenylation of novel N-iodoarylated allylic ketopiperazinonates with unactivated alkenes

Novel N-iodoarylated allylic ketopiperazines have been engaged in a CuBr-catalyzed alkenylation protocol with simple alkenes, leading to epimerization-free, efficient, and regioselective synthesis of N-stilbeno-ketopiperazines

Enantioselective Organocatalysis-Based Synthesis of 3-Hydroxy Fatty Acids and Fatty γ-Lactones

3-Hydroxy fatty acids have attracted the interest of researchers, since some of them may interact with free fatty acid receptors more effectively than their non-hydroxylated counterparts and their determination in plasma provides diagnostic information regarding mitochondrial deficiency. We present here the development of a convenient and general methodology for the asymmetric synthesis of 3-hydroxy fatty acids. The enantioselective organocatalytic synthesis of terminal epoxides, starting from long chain aldehydes, is the key-step of our methodology, followed by ring opening with vinylmagnesium bromide. Ozonolysis and subsequent oxidation leads to the target products. MacMillan’s third generation imidazolidinone organocatalyst has been employed for the epoxide formation, ensuring products in high enantiomeric purity. Furthermore, a route for the incorporation of deuterium on the carbon atom carrying the hydroxy group was developed allowing the synthesis of deuterated derivatives, which may be useful in biological studies and in mass spectrometry studies. In addition, the synthesis of fatty γ-lactones, corresponding to 4-hydroxy fatty acids, was also explored.

Catalytic enantioselective one-pot approach to cis- and trans-2,3-diaryl substituted 1,5-benzothiazepines

The first enantioselective catalytic approach to cis- and trans-2,3-diaryl substituted 1,5-benzothiazepines has been conveniently developed in a one-pot fashion, starting from α,β-unsaturated acyl pyrazoles and 2-aminothiophenol. The organocatalytic two-step sulfa-Michael/lactamization sequence is promoted by a readily available bifunctional thiourea and p-toluenesulfonic acid, respectively. The protocol enables access to both N-unprotected cis- and trans-diastereoisomers in moderate to satisfactory overall yields (up to 84%) and good to excellent ee values (up to 99%). Mechanistic investigations helped to shed light on the regio- and stereoselective outcome of the process.

Mechanistic Studies on the Organocatalytic α-Chlorination of Aldehydes: The Role and Nature of Off-Cycle Intermediates

Herein we report the isolation and characterization of aminal intermediates in the organocatalytic α-chlorination of aldehydes. These species are stable covalent ternary adducts of the substrate, the catalyst and the chlorinating reagent. NMR-assisted kinetic studies and isotopic labeling experiments with the isolated intermediate did not support its involvement in downstream stereoselective processes as proposed by Blackmond. By tuning the reactivity of the chlorinating reagent, we were able to suppress the accumulation of rate-limiting off-cycle intermediates. As a result, an efficient and highly enantioselective catalytic system with a broad functional group tolerance was developed.

Photocatalytic Synthesis of γ-Lactones from Alkenes: High-Resolution Mass Spectrometry as a Tool To Study Photoredox Reactions

A mild photocatalytic manifold for the synthesis of γ-lactones has been developed. Utilizing Ru(bpy)₃Cl₂ as the photocatalyst, a cheap and reproducible synthetic protocol for γ-lactones has been introduced. Mechanistic studies revealed the successful monitoring of photocatalytic reactions and radical intermediates via high-resolution mass spectrometry.