Homologation chemistry with nucleophilic α-substituted organometallic reagents: chemocontrol, new concepts and (solved) challenges

Base-mediated homologative rearrangement of nitrogen-oxygen bonds of N-methyl-N-oxyamides

Due to the well known reactivity of C(O)-N functionalities towards canonical C1-homologating agents (e.g. carbenoids, diazomethane, ylides), resulting in the extrusion of the N-centered fragment en route to carbonyl compounds, formal C1-insertions within N-O bonds still remain obscure. Herein, we document the homologative transformation of N-methyl-N-oxyamides - with high tolerance for diverse O-substituents - into N-acyl-N,O-acetals. Under controlled basic conditions, the N-methyl group of the same starting materials acts as a competent precursor of the methylene synthon required for the homologation. The logic is levered on the formation of an electrophilic iminium ion (via N-O heterolysis) susceptible to nucleophilic attack by the alkoxide previously expulsed. The procedure documents genuine chemocontrol and flexibility, as judged by the diversity of substituents placed on both amide and nitrogen linchpins. The mechanistic rationale was validated through experiments conducted on D-labeled materials which unambiguously attributed the origin of the methylene fragment to the N-methyl group of the starting compounds.

Highly chemoselective homologative assembly of the α-substituted methylsulfinamide motif from N-sulfinylamines

α-Substituted methylsulfinamide are prepared through the homologation of electrophilic N-sulfinylamines with Li-CHXY reagents. The transformation takes place under full chemocontrol and exhibits good flexibility for preparing both N-aryl and N-alkyl analogues. Various sensitive functionalities can be accommodated on the starting materials, thus documenting a wide reaction scope.

Programmable Amine Synthesis via Iterative Boron Homologation

The value of Matteson-type reactions has been increasingly recognized for developing automated organic synthesis. However, the typical Matteson reactions almost exclusively focus on homologation of carbon units. Here, we report the detailed development of sequential insertion of nitrogen and carbon atoms into boronate C-B bonds, which provides a modular and iterative approach to access functionalized tertiary amines. A new class of nitrenoid reagents is uncovered to allow direct formation of aminoboranes from aryl or alkyl boronates via N-insertion. The one-pot N-insertion followed by controlled mono- or double-carbenoid insertion has been realized with widely available aryl boronates. The resulting aminoalkyl boronate products can undergo further homologation and various other transformations. Preliminary success on homologation of N,N-dialkylaminoboranes and sequential N- and C-insertions with alkyl boronates have also been achieved. To broaden the synthetic utility, selective removal of a benzyl or aryl substituent permits access to secondary or primary amine products. The application of this method has been demonstrated in the modular synthesis of bioactive compounds and the programmable construction of diamines and aminoethers. A plausible reaction mechanism, supported by preliminary NMR (nuclear magnetic resonance) and computational studies, is also proposed.

External Flash Generation of Carbenoids Enables Monodeuteration of Dihalomethanes

In this study, incorporation of one deuterium atom was achieved by H-D exchange of one of the two identical methylene protons in various dihalomethanes (halogen=Cl, Br, and I) through a rapid-mixing microflow reaction of lithium diisopropylamide as a strong base and deuterated methanol as a deuteration reagent. Generation of highly unstable carbenoid intermediate and suppression of its decomposition were successfully controlled under high flow-rate conditions. Monofunctionalization of diiodomethane afforded various building blocks composed of boryl, stannyl, and silyl groups. The monodeuterated diiodomethane, which served as a deuterated C1 source, was subsequently subjected to diverted functionalization methods to afford various products including biologically important molecules bearing isotope labelling at specific positions and homologation products with monodeuteration.

Selective noncovalent proteasome inhibiting activity of trifluoromethyl-containing gem-quaternary aziridines

The ubiquitin-proteasome pathway (UPP) represents the principal proteolytic apparatus in the cytosol and nucleus of all eukaryotic cells. Nowadays, proteasome inhibitors (PIs) are well-known as anticancer agents. However, although three of them have been approved by the US Food and Drug Administration (FDA) for treating multiple myeloma and mantel cell lymphoma, they present several side effects and develop resistance. For these reasons, the development of new PIs with better pharmacological characteristics is needed. Recently, noncovalent inhibitors have gained much attention since they are less toxic as compared with covalent ones, providing an alternative mechanism for solid tumors. Herein, we describe a new class of bis-homologated chloromethyl(trifluoromethyl)aziridines as selective noncovalent PIs. In silico and in vitro studies were conducted to elucidate the mechanism of action of such compounds. Human gastrointestinal absorption (HIA) and blood-brain barrier (BBB) penetration were also considered together with absorption, distribution, metabolism, and excretion (ADMET) predictions.

Single-carbon atom transfer to α,β-unsaturated amides from N-heterocyclic carbenes

Single-carbon atom transfer reactions are lacking in organic synthesis, partly because of the absence of atomic carbon sources under standard solution-phase conditions. We report here that N-heterocyclic carbenes can serve as atomic carbon donors through the loss of a 1,2-diimine moiety. This strategy is applicable to single-carbon atom transfer to α,β-unsaturated amides, which can be converted into homologated γ-lactams through the formation of four single bonds to one carbon center in one operation.

Unlocking geminal fluorohaloalkanes in nucleophilic fluoroalkylation chemistry: generation and trapping of lithiumfluorocarbenoids enabled by flow microreactors

A direct nucleophilic monofluoroalkylation strategy leveraging on lithium fluorocarbenoids has been developed. Flow microreactor technology allows capitalization of the synthetic potential of these scarcely explored short-lived intermediates - namely 1-fluoro-2-phenylethyllithium, 1-fluoro-3-phenylpropyllithium, and 1-fluorononyllithium - generated through lithium/iodine exchange reaction. This robust protocol was employed to prepare new fluorinated products, adopting various classes of electrophiles. The inherent advantages of microreactor technology contribute to rendering this approach a new valuable tool for direct fluoroalkylation chemistry.

The synthetic versatility of fluoroiodomethane: recent applications as monofluoromethylation platform

In recent years, fluoroiodomethane (CH₂FI) has emerged as an easy-to-handle, non-ozone depleting agent and readily available platform for monofluoromethylation strategies. Recent applications in nucleophilic substitutions, lithiation reactions, transition-metal catalyzed transformations, radical processes, and ¹⁸F-radiolabelling chemistry showcase the potential of this reagent for the preparation of organofluorine compounds. In this minireview, we provide an update to the field covering the recent relevant literature on the use of CH₂FI.

Straightforward synthesis of bench-stable heteroatom-centered difluoromethylated entities via controlled nucleophilic transfer from activated TMSCHF2

The commercially available and experimentally convenient (bp 65 °C) difluoromethyltrimethylsilane (TMSCHF₂) is proposed as a valuable difluoromethylating transfer reagent for delivering the CHF₂ moiety to various heteroatom-based electrophiles. Upon activation with an alkoxide, a conceptually intuitive nucleophilic displacement directly furnishes in high yields the bench-stable analogues.

29th Annual GP2A Medicinal Chemistry Conference

The 29th Annual GP₂A (Group for the Promotion of Pharmaceutical chemistry in Academia) Conference was a virtual event this year due to the COVID-19 pandemic and spanned three days from Wednesday 25 to Friday 27 August 2021. The meeting brought together an international delegation of researchers with interests in medicinal chemistry and interfacing disciplines. Abstracts of keynote lectures given by the 10 invited speakers, along with those of the 8 young researcher talks and the 50 flash presentation posters, are included in this report. Like previous editions, the conference was a real success, with high-level scientific discussions on cutting-edge advances in the fields of pharmaceutical chemistry.

Consecutive and Selective Double Methylene Insertion of Lithium Carbenoids to Isothiocyanates: A Direct Assembly of Four-Membered Sulfur-Containing Cycles

A formal CH₂−CH₂ homologation conducted with C1 carbenoids on a carbon electrophile for the obtainment of a four‐membered cycle is reported. The logic proposes the consecutive delivery of two single nucleophilic CH₂ units to an isothiocyanate—as competent electrophilic partner—resulting in the assembling of a rare imino‐thietane cluster. The single synthetic operation procedure documents genuine chemocontrol, as indicated by the tolerance to various reactive elements decorating the starting materials. Significantly, the double homologation protocol is accomplished directly on a carbon electrophile, thus not requiring the installation of heteroatom‐centered manifolds (e.g. boron).

Broadening of horizons in the synthesis of CD3-labeled molecules

In the light of the recent potentials of deuterated molecules as pharmaceuticals or even in mechanistic understanding, efficient methods for their synthesis are continually desired. CD₃-containing molecules are prominent amongst these motifs due to the parallel of the "magic methyl effect": introducing a methyl group into pharmaceuticals could positively affect biological activities. The trideuteromethyl group is bound to molecules either by C, N, O, or S atom. For a long time, the preparation methods of such labeled compounds were underestimated and involved multi-step syntheses. More recently, specific approaches dealing with the direct incorporation of the CD₃ group have been developed. This Review gives an overview of the methods for the preparation of CD₃-labeled molecules from conventional functional group interconversion techniques to catalytic approaches and include radical strategy. Detailed reaction mechanisms are also discussed.

Direct and straightforward transfer of C1 functionalized synthons to phosphorous electrophiles for accessing gem-P-containing methanes

The direct transfer of different α-substituted methyllithium reagents to chlorinated phosphorous electrophiles of diverse oxidation state (phosphates, phosphine oxides and phosphines) is proposed as an effective strategy to synthesize geminal P-containing methanes. The methodology relies on the efficient nucleophilic substitution conducted on the P-chlorine linkage. Uniformly high yields are observed regardless the specific nature of the carbanion employed: once established the conditions for generating the competent nucleophile (LiCH₂Hal, LiCHHal₂, LiCH₂CN, LiCH₂SeR etc.) the homologated compounds are obtained via a single operation. Some P-containing formal carbanions have been evaluated in transferring processes, including the carbonyl-difluoromethylation of the opioid agent Hydrocodone.

Taking advantage of lithium monohalocarbenoid intrinsic α-elimination in 2-MeTHF: controlled epoxide ring-opening en route to halohydrins

The intrinsic degradative α-elimination of Li carbenoids somehow complicates their use in synthesis as C1-synthons. Nevertheless, we herein report how boosting such an α-elimination is a straightforward strategy for accomplishing controlled ring-opening of epoxides to furnish the corresponding β-halohydrins. Crucial for the development of the method is the use of the eco-friendly solvent 2-MeTHF, which forces the degradation of the incipient monohalolithium, due to the very limited stabilizing effect of this solvent on the chemical integrity of the carbenoid. With this approach, high yields of the targeted compounds are consistently obtained under very high regiocontrol and, despite the basic nature of the reagents, no racemization of enantiopure materials is observed.

Pseudo-Dipeptide Bearing α,α-Difluoromethyl Ketone Moiety as Electrophilic Warhead with Activity against Coronaviruses

The synthesis of α-fluorinated methyl ketones has always been challenging. New methods based on the homologation chemistry via nucleophilic halocarbenoid transfer, carried out recently in our labs, allowed us to design and synthesize a target-directed dipeptidyl α,α-difluoromethyl ketone (DFMK) 8 as a potential antiviral agent with activity against human coronaviruses. The ability of the newly synthesized compound to inhibit viral replication was evaluated by a viral cytopathic effect (CPE)-based assay performed on MCR5 cells infected with one of the four human coronaviruses associated with respiratory distress, i.e., hCoV-229E, showing antiproliferative activity in the micromolar range (EC₅₀ = 12.9 ± 1.22 µM), with a very low cytotoxicity profile (CC₅₀ = 170 ± 3.79 µM, 307 ± 11.63 µM, and 174 ± 7.6 µM for A549, human embryonic lung fibroblasts (HELFs), and MRC5 cells, respectively). Docking and molecular dynamics simulations studies indicated that 8 efficaciously binds to the intended target hCoV-229E main protease (M^pro). Moreover, due to the high similarity between hCoV-229E M^pro and SARS-CoV-2 M^pro, we also performed the in silico analysis towards the second target, which showed results comparable to those obtained for hCoV-229E M^pro and promising in terms of energy of binding and docking pose.

Chemoselective Homologation-Deoxygenation Strategy Enabling the Direct Conversion of Carbonyls into (n+1)-Halomethyl-Alkanes

The sequential installation of a carbenoid and a hydride into a carbonyl, furnishing halomethyl alkyl derivatives, is reported. Despite the employment of carbenoids as nucleophiles in reactions with carbon-centered electrophiles, sp³-type alkyl halides remain elusive materials for selective one-carbon homologations. Our tactic levers on using carbonyls as starting materials and enables uniformly high yields and chemocontrol. The tactic is flexible and is not limited to carbenoids. Also, diverse carbanion-like species can act as nucleophiles, thus making it of general applicability.

Enantioselective formal synthesis of the marine macrolide (-)-callyspongiolide

A short enantioselective synthesis of the macrocyclic core 19 of callyspongiolide, involving a homocrotylboration of aldehyde 4, a Still-Genari olefination, an esterification with alcohol 17, and a ring-closing metathesis, is reported.

Direct and Chemoselective Electrophilic Monofluoromethylation of Heteroatoms (O-, S-, N-, P-, Se-) with Fluoroiodomethane

The commercially available fluoroiodomethane represents a valuable and effective electrophilic source for transferring the CH₂F unit to a series of heteroatom-centered nucleophiles under mild basic conditions. The excellent manipulability offered by its liquid physical state (bp 53.4 °C) enables practical and straightforward one-step nucleophilic substitutions to retain the chiral information embodied, thus allowing it to overcome de facto the requirement for fluoromethylating agents with no immediate access. The high-yielding methodology was successfully applied to a variety of nucleophiles including a series of drugs currently in the market.

Straightforward chemoselective access to unsymmetrical dithioacetals through a thiosulfonate homologation-nucleophilic substitution sequence

A two-step electrophilic sulfur homologation strategy for building up unsymmetrical dithioacetals.

The synthetic versatility of the Tiffeneau–Demjanov chemistry in homologation tactics

Abstract

The Tiffeneau–Demjanov rearrangement can be regarded as an interesting alternative to the more common semi-pinacol transposition. It is usually employed for ring extension but, under specific conditions, it can also be used for ring contraction. Compared to other techniques, such as the Demjanov rearrangement or homologations with diazo compounds, the Tiffeneau–Demjanov pathway presents attractive features including high yielding and selective processes. Ring enlargements follow very strict and simple rules, such as the movement of the less substituted carbon and retention of the configuration. The rearrangement process is mainly affected by steric factors, due to presence of neighbouring groups, rather than electronic ones. The ring contraction may be achieved positioning the amine within the ring, thus achieving a high level of control. Unfortunately, applications of the reaction in modern homologation chemistry are rare; therefore, the aim of the review is re-proposing to the synthetic community the versatility of this venerable reaction and thus, spurring its employment for tackling challenging homologations processes.

Graphic abstract

Flow Technology for the Genesis and Use of (Highly) Reactive Organometallic Reagents

In the field of organic synthesis, the advent of flow chemistry and flow microreactor technology represented a tremendous novelty in the way of thinking and performing chemical reactions, opening the doors to poorly explored or even impossible transformations using batch methods. In this Concept article, we would like to highlight the impact of flow chemistry for exploiting highly reactive organometallic reagents, and how, alongside the well-known advantages concerning safety, scalability, and productivity, flow chemistry makes possible processes that are impossible to control by using the traditional batch approach.

Highly chemoselective difluoromethylative homologation of iso(thio)cyanates: expeditious access to unprecedented α,α-difluoro(thio)amides

The new motif - α,α-difluoromethyl thioamide - has been assembled starting from isothiocyanate (as thioamide precursor) and a formal difluoromethyl-carbanion generated from commercially available TMSCHF2. Upon proper activation of this reagent with potassium tert-amylate, the high-yielding transfer of the difluorinated nucleophile takes place under high chemocontrol. Various sensitive functionalities (e.g. ester, nitrile, nitro, azido groups) can be accommodated across the isothiocyanate core, thus allowing a wide scope. The methodology is highly flexible and adaptable to prepare analogous α,α-difluoromethyl oxoamides by conveniently using isocyanates as the electrophilic building-blocks.

Direct and Chemoselective Synthesis of Tertiary Difluoroketones via Weinreb Amide Homologation with a CHF2-Carbene Equivalent

The homologation of Weinreb amides into difluoromethylketones with a formal nucleophilic CHF₂ transfer agent is reported. Activating TMSCHF₂ with potassium tert-amylate enables a convenient access to the difluorinated homologation reagent, which adds to the acylating partners. The high chemoselectivity showcased in the presence of variously multifunctionalized Weinreb amides, jointly with uniformly high yields, enables the strategy of general applicability without requiring any stabilization element for the putative carbanion.

A Combination of Pharmacophore and Docking-based Virtual Screening to Discover new Tyrosinase Inhibitors

Melanogenesis controls the formation of melanin pigment whose overproduction is related to various hyperpigmentary disorders in humans. Tyrosinase is a type-3 copper enzyme involved in the rate limiting step of melanin synthesis, therefore its inhibition could represent an efficient way for the development of depigmenting agents. In this work, a combination of pharmacophore and docking-based studies has been employed to screen two in-house 3D compound databases containing about 2,000 molecules from natural and synthetic sources. As result we selected two "hit compounds" which proved to inhibit tyrosinase activity showing IC₅₀ values in the micromolar range.

Straightforward chemo- and stereoselective fluorocyclopropanation of allylic alcohols: exploiting the electrophilic nature of the not so elusive fluoroiodomethyllithium

An unprecedented direct fluorocyclopropanation of allylic alcohols is reported. This simple method involves the not so elusive fluoroiodomethyllithium, a carbenoidic intermediate that under the developed conditions discloses its electrophilic nature. Gratifyingly, the reaction turned out to be highly chemo- and stereoselective, and DFT calculations provided insights into the structure and nature of this new type of carbenoid.

Sustainable Asymmetric Organolithium Chemistry: Enantio- and Chemoselective Acylations through Recycling of Solvent, Sparteine, and Weinreb "Amine"

The well-established Hoppe-Beak chemistry, which involves enantioselective generation of organolithium compounds in the presence of (-)-sparteine, was revisited and applied to unprecedented acylations with Weinreb amides to access highly enantioenriched α-oxyketones and cyclic α-aminoketones. Recycling of the sustainable solvent cyclopentyl methyl ether, sparteine, and the released Weinreb "amine" [HNMe(OMe)] was possible through a simple work-up procedure that enabled full recovery of these precious materials. The methodology features a robust scope and flexibility, thus allowing the enantioselective preparation of scaffolds amenable of further derivatization.

Telescoped, Divergent, Chemoselective C1 and C1-C1 Homologation of Imine Surrogates: Access to Quaternary Chloro- and Halomethyl-Trifluoromethyl Aziridines

A conceptually novel, high‐yielding, mono‐ or bis‐homologation was realized with lithium halocarbenoids and enables the one‐step, fully chemocontrolled assembly of a new class of quaternary trifluoromethyl aziridines. Trifluoroacetimidoyl chlorides (TFAICs) act as convenient electrophilic platforms, enabling the addition of either one or two homologating elements by simply controlling the stoichiometry of the process. Mechanistic studies highlighted that the homologation event, carried out with two different carbenoids (LiCH₂Cl and LiCH₂F), leads to fluoromethyl analogues in which the first nucleophile is employed for constructing the cycle and the second for decorating the resulting molecular architecture.

Cyclopentyl Methyl Ether: An Elective Ecofriendly Ethereal Solvent in Classical and Modern Organic Chemistry

Solvents represent one of the major contributions to the environmental impact of fine-chemical synthesis. As a result, the use of environmentally friendly solvents in widely employed reactions is a challenge of vast real interest in contemporary organic chemistry. Within this Review, a great variety of examples showing how cyclopentyl methyl ether has been established as particularly useful for this purpose are reported. Indeed, its low toxicity, high boiling point, low melting point, hydrophobicity, chemical stability towards a wide range of conditions, exceptional stability towards the abstraction of hydrogen atoms, relatively low latent heat of vaporization, and the ease with which it can be recovered and recycled enable its successful employment as a solvent in a wide range of synthetic applications, including organometallic chemistry, catalysis, biphasic reactions, oxidations, and radical reactions.

Modular and Chemoselective Strategy for the Direct Access to α-Fluoroepoxides and Aziridines via the Addition of Fluoroiodomethyllithium to Carbonyl-Like Compounds

An expeditious, high-yielding synthesis of rare α-fluoroepoxides and α-fluoroaziridines through the addition of the unkown fluoroiodomethyllithium (LiCHIF)-formed via deprotonation the commercially available fluoroiodomethane with a lithium amide base-to carbonyl-like compounds is documented. The ring-closure reactions, leading to α-fluorinated three-membered heterocycles, rely on the diversely reactive C-I and C-F bonds. Excellent chemoselectivity was observed in the presence of highly sensitive functionalities-aldehyde, ketone, nitrile, alkene-which remained untouched during the homologation sequence.

Chemoselective reduction of isothiocyanates to thioformamides mediated by the Schwartz reagent

Thioformamides are easily prepared – under full chemocontrol – through the partial reduction of isothiocyanates with the in situ generated Schwartz reagent.

A practical guide for using lithium halocarbenoids in homologation reactions

ABSTRACT

Lithium halocarbenoids are versatile reagents for accomplishing homologation processes. The fast α-elimination they suffer has been considered an important limitation for their extensive use. Herein, we present a series of practical considerations for an effective employment in the homologation of selected carbon electrophiles.

GRAPHICAL ABSTRACT

Homologation of halostannanes with carbenoids: a convenient and straightforward one-step access to α-functionalized organotin reagents

A direct, single synthetic homologative transformation of halostannanes into mono- or di-substituted methyl analogues is documented.