Esterification of Carboxylic Acids with Difluoromethyl Diazomethane and Interrupted Esterification with Trifluoromethyl Diazomethane: A Fluorine Effect

Insights into the Halogen Effects on Regioselectivities of Nucleophilic Haloalkylation of α,β-Enones

The halogen effects, specifically the replacement of chlorine with fluorine, on the shift in regioselectivity from 1,4- to 1,2-addition in the nucleophilic haloalkylation of α,β-enones have been investigated using density functional theory (DFT) calculations. Computational analysis revealed that the difluorocarbanion PhSO2CF2 -, being a hard nucleophile with lower energy of the highest occupied molecular orbital (HOMO), selectively undergoes 1,2-addition of chalcone, primarily dominated by Coulombic interactions. In contrast, the nucleophilic 1,4-addition of chalcone by a soft dichlorocarbanion PhSO2CCl2 - is controlled by frontier molecular orbital interactions. This work provides a deep understanding of the regioselective control in nucleophilic haloalkylation reactions.

BF3-Mediated C2-Amidation of Quinoline N-Oxides Employing Trifluorodiazoethane and Acetonitrile: Access to 2-N-(Trifluoroethyl)amidoquinolines

A Lewis acid-mediated C2-N-trifluoroethylamidation of quinolines, employing quinoline N-oxides, trifluorodiazoethane, and acetonitrile to forge a new class of N-(quinolin-2-yl)-N-(trifluoroethyl)acetamide is presented in this Letter. The reaction proceeds through a carbene generation/nitrile ylide formation/(3 + 2) cycloaddition/rearrangement cascade to furnish quinoline-2-N-(trifluoroethyl)acetamide derivatives in high yields.

2,2-Difluoroethylation of Heteroatom Nucleophiles via a Hypervalent Iodine Strategy

The 2,2-difluoroethyl group is an important lipophilic hydrogen bond donor in medicinal chemistry, but its incorporation into small molecules is often challenging. Herein, we demonstrate electrophilic 2,2-difluoroethylation of thiol, amine and alcohol nucleophiles with a hypervalent iodine reagent, (2,2-difluoro-ethyl)(aryl)iodonium triflate, via a proposed ligand coupling mechanism. This transformation offers a complementary strategy to existing 2,2-difluoroethylation methods and allows access to a wide range of 2,2-difluoroethylated nucleophiles, including the drugs Captopril, Normorphine and Mefloquine.

O-Trifluoromethylation of Carboxylic Acids via the Formation and Activation of Acyloxy(phenyl)trifluoromethyl-λ3-Iodanes

Here we report the challenging O-trifluoromethylation of carboxylic acids via the formation and activation of acyloxy(phenyl)trifluoromethyl-λ3-iodanes. The method provides an easy access to various potentially valuable and hitherto elusive trifluoromethyl carboxylic esters. A remarkably wide range of substrates with commonly encountered functional groups are compatible with this reaction, including aromatic and aliphatic carboxylic acids, as well as Food and Drug Administration (FDA) approved drugs and pharmaceutically relevant molecules. The reaction mechanism and the origins of the enhanced reactivity by zinc chloride (ZnCl2) were discussed from experimental evidence and density functional theory (DFT) calculation.

Copper-catalyzed multicomponent reaction of β-trifluoromethyl β-diazo esters enabling the synthesis of β-trifluoromethyl N,N-diacyl-β-amino esters

An efficient multicomponent reaction of newly designed β-trifluoromethyl β-diazo esters, acetonitrile, and carboxylic acids via an interrupted esterification process under copper-catalyzed conditions has been developed, which affords various unsymmetrical β-trifluoromethyl N,N-diacyl-β-amino esters in good to excellent yields. The reaction features mild conditions, a wide scope of β-amino esters and carboxylic acids, and also applicability to large-scale synthesis, thus providing an efficient way for the synthesis of β-trifluoromethyl β-diacylamino esters. Furthermore, this reaction represents the first example of a Mumm rearrangement of β-trifluoromethyl β-diazo esters.

Theoretical Study on the Origin of Abnormal Regioselectivity in Ring-Opening Reaction of Hexafluoropropylene Oxide

That nucleophiles preferentially attack at the less sterically hindered carbon of epoxides under neutral and basic conditions has been generally accepted as a fundamental rule for predicting the regioselectivity of this type of reaction. However, this rule does not hold for perfluorinated epoxides, such as hexafluoropropylene oxide (HFPO), in which nucleophiles were found to attack at the more hindered CF₃ substituted β-C rather than the fluorine substituted α-C. In this contribution, we aim to shed light on the nature of this intriguing regioselectivity by density functional theory methods. Our calculations well reproduced the observed abnormal regioselectivities and revealed that the unusual regiochemical preference for the sterically hindered β-C of HFPO mainly arises from the lower destabilizing distortion energy needed to reach the corresponding ring-opening transition state. The higher distortion energy required for the attack of the less sterically hindered α-C results from a significant strengthening of the C(α)-O bond by the negative hyperconjugation between the lone pair of epoxide O atom and the antibonding C-F orbital.

Visible-Light-Induced Imide Synthesis through a Nitrile Ylide Formation/Trapping Cascade

A visible-light-promoted three component reaction of diazo compounds, nitriles, and carboxylic acids is reported. The reaction utilizes acceptor-only diazo compounds as carbene precursors and nitriles as carbene-trapping reagents to form the key nitrile ylides. Under the optimal reaction conditions, a wide range of imide products were obtained in good to excellent yields. The gram-scale synthesis and synthetic application of the imide products to form isoquinoline-1,3(2H,4H)-dione derivatives further proved the value of this method.

O-Methylation of carboxylic acids with streptozotocin

The clinically used DNA-alkylating drug streptozotocin (STZ) was investigated using a simple work-up as an O-methylating agent to transform various carboxylic acids, sulfonic acids and phosphorous acids into corresponding methyl esters, and did so with yields of up to 97% in 4 h at room temperature. Good substrate tolerance was observed, and benefited from the mild conditions and compatibility of the reaction with water.

Difluorodiazoethane as a masked acetylene equivalent in formal [3 + 2] cycloadditions with ketones to access 2,3-functionalized furans

A transition-metal-free [3+2] cycloaddition of CF2HCHN2 with β-ketones is reported, which enables the synthesis of 2,3-functionalized furans. Sequential defluorination, nucleophilic addition, and cyclization are key elemental steps of the reaction.

Silver-Catalyzed Direct Synthesis of Trifluoromethylated Enaminopyridines and Isoquinolinones Employing Trifluorodiazoethane

This Letter reports a Ag-catalyzed three-component approach for the N-alkenylation of 2-aminopyridines employing aldehydes and trifluorodiazoethane. Unlike the known reactions of trifluorodiazoethane with imines, which generate Mannich adducts, aziridines, or triazolines depending on the substrates and conditions, this reaction, after Mannich addition, proceeds via a carbene formation and 1,2-aryl migration sequence to afford (E)-enaminopyridines. This surprising selectivity, which is effective for a wide range of aldehydes and 2-aminopyridines, has been subsequently explored to access trifluoromethylated isoquinolinones.

Late-Stage Transformation of Carboxylic Acids to N-Trifluoroethyl­imides with Trifluoromethyl Diazomethane

We report the first systematic evaluation of the reaction of trifluoromethyl diazomethane (2,2,2-trifluorodiazoethane, CF3CHN2) with drug-like molecules. We found our previous copper-catalyzed transformation of carboxylic acids to the corresponding N-trifluoro­ethylimides with CF3CHN2 and acetonitrile is well-suited for the late-stage modification of drug and drug-like acids. A procedure that enables the use of solid nitriles and nitriles with high boiling points as viable substrates is also disclosed.

Synthesis of tri(di)fluoroethylanilines via copper-catalyzed coupling reaction of tri(di)fluoroethylamine with (hetero)aromatic bromides

We have realized the first Ullmann type coupling reaction of tri(di)fluoroethylamine with (hetero)aromatic bromides, employing 5-20 mol% Cu₂O and an oxalamide ligand [N-(2,4,6-trimethoxyphenyl)acetamide]. This efficient and practical method has the following features: (i) avoids the use of an expensive catalyst; (ii) does not require anhydrous solvent and strict air extrusion; (iii) uses bench stable and inexpensive (hetero)aromatic bromides; (iv) is suitable for the synthesis of fluoroalkylated hetero-aromatic substrates; (v) is suitable for gram-scale synthesis. This work also shows the "negative fluorine effect" for the alkylamines in the copper catalysed coupling reactions.

Bilayer thickness determines the alignment of model polyproline helices in lipid membranes

Our understanding of protein folds relies fundamentally on the set of secondary structures found in the proteomes. Yet, there also exist intriguing structures and motifs that are underrepresented in natural biopolymeric systems. One example is the polyproline II helix, which is usually considered to have a polar character and therefore does not form membrane spanning sections of membrane proteins. In our work, we have introduced specially designed polyproline II helices into the hydrophobic membrane milieu and used 19F NMR to monitor the helix alignment in oriented lipid bilayers. Our results show that these artificial hydrophobic peptides can adopt several different alignment states. If the helix is shorter than the thickness of the hydrophobic core of the membrane, it is submerged into the bilayer with its long axis parallel to the membrane plane. The polyproline helix adopts a transmembrane alignment when its length exceeds the bilayer thickness. If the peptide length roughly matches the lipid thickness, a coexistence of both states is observed. We thus show that the lipid thickness plays a determining role in the occurrence of a transmembrane polyproline II helix. We also found that the adaptation of polyproline II helices to hydrophobic mismatch is in some notable aspects different from α-helices. Finally, our results prove that the polyproline II helix is a competent structure for the construction of transmembrane peptide segments, despite the fact that no such motif has ever been reported in natural systems.

Promotion of the collagen triple helix in a hydrophobic environment

The collagen triple helix is better suited for octanol than for water.

Exploring hydrophobicity limits of polyproline helix with oligomeric octahydroindole-2-carboxylic acid

The polyproline-II helix is the most extended naturally occurring helical structure and is widely present in polar, exposed stretches and "unstructured" denatured regions of polypeptides. Can it be hydrophobic? In this study, we address this question using oligomeric peptides formed by a hydrophobic proline analogue, (2S,3aS,7aS)-octahydroindole-2-carboxylic acid (Oic). Previously, we found the molecular principles underlying the structural stability of the polyproline-II conformation in these oligomers, whereas the hydrophobicity of the peptide constructs remains to be examined. Therefore, we investigated the octan-1-ol/water partitioning and inclusion in detergent micelles of the oligo-Oic peptides. The results showed that the hydrophobicity is remarkably enhanced in longer oligomeric sequences, and the oligo-Oic peptides with 3 to 4 residues and higher are specific towards hydrophobic environments. This contrasts significantly to the parent oligoproline peptides, which were moderately hydrophilic. With these findings, we have demonstrated that the polyproline-II structure is compatible with nonpolar media, whereas additional manipulations of the terminal functionalities feature solubility in extremely nonpolar solvents such as hexane.

Investigations into the flexibility of the 3D structure and rigid backbone of quinoline by fluorine addition to enhance its blue emission

Addition of fluorine to the quinoline structure was found to decrease its intermolecular interactions and influence its 3D structure.

Cu-Catalyzed/mediated synthesis of N-fluoroalkylanilines from arylboronic acids: fluorine effect on the reactivity of fluoroalkylamines

A copper catalyzed/mediated oxidative coupling of arylboronic acids and fluoroalkylated amines with an interesting reactivity trend is disclosed.

Hydrolysis, polarity, and conformational impact of C-terminal partially fluorinated ethyl esters in peptide models

Fluorinated moieties are highly valuable to chemists due to the sensitive NMR detectability of the 19F nucleus. Fluorination of molecular scaffolds can also selectively influence a molecule's polarity, conformational preferences and chemical reactivity, properties that can be exploited for various chemical applications. A powerful route for incorporating fluorine atoms in biomolecules is last-stage fluorination of peptide scaffolds. One of these methods involves esterification of the C-terminus of peptides using a diazomethane species. Here, we provide an investigation of the physicochemical consequences of peptide esterification with partially fluorinated ethyl groups. Derivatives of N-acetylproline are used to model the effects of fluorination on the lipophilicity, hydrolytic stability and on conformational properties. The conformational impact of the 2,2-difluoromethyl ester on several neutral and charged oligopeptides was also investigated. Our results demonstrate that partially fluorinated esters undergo variable hydrolysis in biologically relevant buffers. The hydrolytic stability can be tailored over a broad pH range by varying the number of fluorine atoms in the ester moiety or by introducing adjacent charges in the peptide sequence.