One-Pot Synthesis of Ureas from Boc-Protected Amines

Methyl Pyruvate Oxime as a Carbonyl Synthon: Synthesis of Ureas, Carbamates, Thiocarbamates, and Anilides

A new strategy for the synthesis of unsymmetrical ureas, carbamates, thiocarbamates, and anilides was developed with methyl pyruvate oxime as the carbonyl synthon. The intrinsic reactivity of the reagent enabled consecutive disubstitution involving direct amidation and one-pot deoximative substitution with various nucleophiles. The utility of the method was demonstrated with the synthesis of bioactive molecules.

Synthesis of Asymmetric Urea Derivatives from COS and Amines: Substrate Selection, Scope Studies, and Mechanism Investigation

A series of asymmetric ureas were synthesized by a one-pot reaction of amines and carbonyl sulfide (COS) under catalyst-free conditions. The highly selective synthesis of asymmetric urea was successfully achieved by the use of weakly nucleophilic aromatic amines and highly nucleophilic secondary aliphatic amines. Moreover, a reaction mechanism was proposed based on the detailed NMR and FTIR study. This efficient synthetic methodology provides a mild and selective method for synthesizing asymmetric urea derivatives.

A new synthetic approach to cyclic ureas through carbonylation using di-tert-butyl dicarbonate (boc anhydride) via one pot strategy

A new approach has been successfully employed to synthesize cyclic ureas via carbonylation, utilizing Boc anhydride and employing K2CO3 as a base along with N,N-dimethylformamide as the solvent. Remarkably high yields were achieved using K2CO3 in conjunction with (Boc)2O, enabling the streamlined preparation of benzimidazolones and 2-benzoxazolones within a single reaction vessel. Significantly, this approach obviates the necessity for using any dangerous reagents, rendering it environmentally friendly, and its key benefit lies in being a metal-free system. The method stands out for its efficiency, concise pathway, optimization from readily accessible starting materials, and ease of execution. The resulting benzimidazolones and 2-benzoxazolones were thoroughly characterized using techniques including LCMS, 1H NMR, and 13C NMR.

Hydroboration of isocyanates: cobalt-catalyzed vs. catalyst-free approaches

Hydroboration of isocyanates with HBPin was demonstrated using both catalytic and catalyst-free approaches. In arene solvents, the reactions employed the commercially available and bench-stable Co(acac)₂/dpephos (dpephos = bis[(2-diphenylphosphino)phenyl] ether) pre-catalyst and proved chemodivergent, showing the formation of either formamides or N-methylamines, depending on the concentration of HBPin and the reaction conditions used. Catalytic monohydroboration of isocyanates to formamides was found to be highly chemoselective, tolerating alkenes, alkynes, aryl halides, esters, carboxamides, nitriles, nitroarenes and heteroaromatic functionalities. The catalyst-free hydroboration reactions have been demonstrated in neat HBPin. Whereas monohydroboration proved less selective compared with Co(acac)₂/dpephos-catalyzed transformations, selective deoxygenative hydroboration of isocyanates to N-methylamines was observed under catalyst-free conditions.

Synthesis of unsymmetrical urea derivatives via Cu-catalysed reaction of acylazide and secondary amine

The synthesis of unsymmetrical urea generally requires toxic reagent, solvent and harsh reaction condition. Herein, we introduce Cu-catalyzed greener and safer unsymmetrical urea derivatives synthesis in ethyl acetate. This method minimized utilization of toxic reagent. A variety of indole, amines, and azides with bis-indole successfully employed leading to high yields and gram scale synthesis of isolated urea.

Chemo- and Site-Selective Replacement of N-Terminal Carbamates in Peptides

In peptide synthesis, it is important to distinguish the terminal amino group and carry out the selective transformation of only the N-terminal protecting group. We describe herein a reaction for the chemo- and site-selective replacement of carbamates with various other carbamates only at the N-terminus of peptides. We demonstrate the scope of carbamates and peptides and the introduction of fluorine into a peptide. This strategy is applicable to the late stage of peptide synthesis.

New efficient synthesis of tRNA related adenosines bearing the hydantoin ring (ct6A, ms2ct6A) by intramolecular cyclization of N6-(N-Boc-α-aminoacyl)-adenosine derivatives

A novel and efficient way for synthesis of N6-hydantoin modified adenosines, which utilizes readily available N6-(N-Boc-α-aminoacyl)-adenosine derivatives, was developed. The procedure is based on the epimerization free Tf2O-mediated conversion of the Boc group into an isocyanate moiety, followed by intramolecular cyclization. Using this method two recently discovered hydantoin modified tRNA adenosines, i.e. cyclic N6-threonylcarbamoyl-adenosine (ct6A) and 2-methylthio-N6-threonylcarbamoyladenosine (ms2ct6A) were prepared in good yields.

The Chosen Few: Parallel Library Reaction Methodologies for Drug Discovery

Parallel library synthesis is an important tool for drug discovery because it enables the synthesis of closely related analogues in parallel via robust and general synthetic transformations. In this perspective, we analyzed the synthetic methodologies used in >5000 parallel libraries representing 15 prevalent synthetic transformations. The library data set contains complex substrates and diverse arrays of building blocks used over the last 14 years at AbbVie. The library synthetic methodologies that have demonstrated robustness and generality with proven success are described along with their substrate scopes. The evolution of the synthetic methodologies for library synthesis over the past decade is discussed. We also highlight that the combination of parallel library synthesis with high-throughput experimentation will continue to facilitate the discovery of library-amenable synthetic methodologies in drug discovery.

Functionalized Polyhydroquinolines from Amino Acids Using a Key One-Pot Cyclization Cascade and Application to the Synthesis of (±)-Δ7-Mesembrenone

Substituted polyhydroquinolines are ubiquitous skeletal cores found in drugs and bioactive natural products. As a new route to access this motif, we successfully developed a one-pot cyclization cascade with high chemocontrol and diastereoselectivity. The sequence generates two cycles, three carbon-carbon bonds, and an all-carbon quaternary center in a highly convergent process. Functionalized polyhydroquinolines and congeners can be accessed from commercially available amino acids. This versatile and robust strategy was applied to the synthesis of (±)-Δ7-mesembrenone.

Practical one-pot amidation of N-Alloc-, N-Boc-, and N-Cbz protected amines under mild conditions

A facile one-pot synthesis of amides from N-Alloc-, N-Boc-, and N-Cbz-protected amines has been described. The reactions involve the use of isocyanate intermediates, which are generated in situ in the presence of 2-chloropyridine and trifluoromethanesulfonyl anhydride, to react with Grignard reagents to produce the corresponding amides. Using this reaction protocol, a variety of N-Alloc-, N-Boc-, and N-Cbz-protected aliphatic amines and aryl amines were efficiently converted to amides with high yields. This method is highly effective for the synthesis of amides and offers a promising approach for facile amidation.

One-pot efficient transformation of N-Alloc-, N-Boc-, and N-Cbz protected amines to amides was achieved by using 2-chloropyridine and trifluoromethanesulfonyl anhydride as well as Grignard reagent and MgCl₂.

Efficient access to 3'-O-phosphoramidite derivatives of tRNA related N 6-threonylcarbamoyladenosine (t6A) and 2-methylthio-N 6-threonylcarbamoyladenosine (ms2t6A)

An efficient method of ureido linkage formation during epimerization-free one-pot synthesis of protected hypermodified N 6-threonylcarbamoyladenosine (t6A) and its 2-SMe analog (ms2t6A) was developed. The method is based on a Tf2O-mediated direct conversion of the N-Boc-protecting group of N-Boc-threonine into the isocyanate derivative, followed by reaction with the N 6 exo-amine function of the sugar protected nucleoside (yield 86-94%). Starting from 2',3',5'-tri-O-acetyl protected adenosine or 2-methylthioadenosine, the corresponding 3'-O-phosphoramidite monomers were obtained in 48% and 42% overall yield (5 step synthesis). In an analogous synthesis, using the 2'-O-(tert-butyldimethylsilyl)-3',5'-O-(di-tert-butylsilylene) protection system at the adenosine ribose moiety, the t6A-phosphoramidite monomer was obtained in a less laborious manner and in a remarkably better yield of 74%.

A novel route to unsymmetrical disubstituted ureas and thioureas by HMPA catalyzed reductive alkylation with trichlorosilane

A HMPA catalyzed reductive alkylation of ureas and thioureas with trichlorosilane under mild reaction conditions has been developed.

Hydroamination of isocyanates and isothiocyanates by alkaline earth metal initiators supported by a bulky iminopyrrolyl ligand

Synthesis of heteroleptic and homoleptic alkaline earth metal complexes supported by bulky bis-iminopyrrolyl ligands are reported. The catalytic hydroamination of isocyanates and isothiocyanates with aryl amines using calcium complex is presented.

Simple and efficient synthesis of bicyclic enol-carbamates: access to brabantamides and their analogues

A simple and short synthesis of bicyclic enol-carbamates with high E/Z selectivity and the synthesis of brabantamide A analogue are presented.

Synthesis of unsymmetrical urea derivatives via one-pot sequential three-component reactions of cyclic 2-diazo-1,3-diketones, carbodiimides, and 1,2-dihaloethanes

An effective and operationally simple one-pot strategy has been developed for the synthesis of unsymmetrical urea derivatives via sequential three-component reactions of cyclic 2-dizao-1,3-diketones with carbodiimides and 1,2-dihaloethane.

Urea Derivatives in Modern Drug Discovery and Medicinal Chemistry

The urea functionality is inherent to numerous bioactive compounds, including a variety of clinically approved therapies. Urea containing compounds are increasingly used in medicinal chemistry and drug design in order to establish key drug-target interactions and fine-tune crucial drug-like properties. In this perspective, we highlight physicochemical and conformational properties of urea derivatives. We provide outlines of traditional reagents and chemical procedures for the preparation of ureas. Also, we discuss newly developed methodologies mainly aimed at overcoming safety issues associated with traditional synthesis. Finally, we provide a broad overview of urea-based medicinally relevant compounds, ranging from approved drugs to recent medicinal chemistry developments.

Organocatalytic, Enantioselective Reductive Bis-functionalization of Secondary Amides: One-Pot Construction of Chiral 2,2-Disubstituted 3-Iminoindoline

We report the first catalytic, enantioselective reductive bis-functionalization of common amides, which provides a facile access to a variety of 2,2-disubstituted 3-iminoindolines in good yields and with excellent enantioselectivities. The reaction conditions are quite mild and can be run on a gram scale. In this one-pot reaction, three C-C bonds, one ring, and one nitrogen-containing tetrasubstituted carbon stereocenter are created in a catalytic enantioselective manner.

Synthesis of Nitrogen-Containing Goniothalamin Analogues with Higher Cytotoxic Activity and Selectivity against Cancer Cells

Two series of racemic goniothalamin analogues displaying nitrogen-containing groups were designed and synthesized. A total of 19 novel analogues were evaluated against a panel of four different cancer cell lines, along with the normal prostate cell line PNT2 to determine their selectivity. Among them, goniothalamin chloroacrylamide 13 e displayed the lowest IC₅₀ values for both MCF-7 (0.5 μm) and PC3 (0.3 μm) cells, about 26-fold more potent than goniothalamin (1). Besides its higher potency, compound 13 e also displayed much higher selectivity than goniothalamin. In contrast, goniothalamin isobutyramide 13 c was the most potent analogue against Caco-2 cells (IC₅₀ =0.8 μm), about 10-fold more potent and 17-fold more selective than 1. These results reveal the potential of compounds 13 c and 13 e for further in vivo studies, representing the first goniothalamin analogues with IC₅₀ values in the low micromolar range and high selectivity against MCF-7, Caco-2, and PC3 cancer cell lines.

One stone two birds: cobalt-catalyzed in situ generation of isocyanates and benzyl alcohols for the synthesis of N-aryl carbamates

An efficient method for the synthesis of N-aryl carbamates from N-Boc-protected amines has been developed. The cobalt-catalyzed in situ generation of isocyanates from N-Boc-protected amines and benzyl alcohols from benzyl formates has been achieved for the first time, which in turn furnished the corresponding benzyl carbamates in moderate to high yields. The reaction was catalyzed by CoI₂ with tris-(4-dimethylaminophenyl)-phosphine as the ligand and zinc powder as the reductant. The developed reaction conditions were found to be compatible for aromatic amines with both electron-donating and -withdrawing substituents.

Studies with Guanidinium- and Amidinium-Based Inhibitors Suggest Minimal Stabilization of Allylic Carbocation Intermediates by Dehydrosqualene and Squalene Synthases

Dehydrosqualene and squalene synthases catalyze the redox neutral and the reductive, head-to-head dimerization of farnesyl diphosphate, respectively. In each case, the reaction is thought to proceed via an initial dissociation of farnesyl diphosphate to form an allylic carbocation-pyrophosphate ion pair. This work describes the synthesis and testing of inhibitors in which a guanidinium or amidinium moiety is flanked by a phosphonylphosphinate group and a hydrocarbon tail. These functional groups bear a planar, delocalized, positive charge and therefore should act as excellent mimics of an allylic carbocation. An inhibitor bearing a neutral urea moiety was also prepared as a control. The positively charged inhibitors acted as competitive inhibitors against Staphylococcus aureus dehydrosqualene synthase with K_i values in the low micromolar range. Surprisingly, the neutral urea inhibitor was the most potent of the three. Similar trends were seen with the first half reaction of human squalene synthase. One interpretation of these results is that the active sites of these enzymes do not directly stabilize the allylic carbocation via electrostatic or π-cation interactions. Instead, it is likely that the enzymes use tight binding to the pyrophosphate and lipid moieties to promote catalysis and that electrostatic stabilization of the carbocation is provided by the bound pyrophosphate product. An alternate possibility is that these inhibitors cannot bind to the "ionization FPP-binding site" of the enzyme and only bind to the "nonionizing FPP-binding site". In either case, all reported attempts to generate potent inhibitors with cationic FPP analogues have been unsuccessful to date.

Spontaneous and Direct Transformation of N,O-Diaryl Carbamates into N,N'-Diarylureas

We have discovered a spontaneous reaction of N,O-diaryl carbamates to afford symmetrical N,N'-diarylureas. Optimization of the conditions indicated that N,N-dimethylformamide (DMF) was the best solvent and triethylamine (Et₃N) was the best additive for this transformation. The reaction requires the presence of aryl groups on the nitrogen and oxygen atoms of carbamates. Substrates bearing an electron-donating methoxy group on either of the aryl groups reacted slowly under these conditions.

t-Butyl-Oxycarbonylated Diamines as Building Blocks for Isocyanate-Free Polyurethane/Urea Dispersions and Coatings

t-Butyl-oxycarbonylated diamines ("di-Boc-carbamates") are investigated as dicarbamate monomers for diamine/dicarbamate polymerizations. Polyureas (PUs) and polyurethanes (PURs) with high molecular weights are prepared from stoichiometric polymerizations of diamines or diols with N-N'-di-t-butyl-oxycarbonyl isophorone diamine (DiBoc-IPDC) using KOt-Bu as a catalyst, while gelation is observed when an excess of DiBoc-IPDC is used with respect to the diamines or diols. Stable dispersions are obtained from PUs and PURs with 3,3'-diamino-N-methyldipropylamine (DMDPA) as internal dispersing agent. The corresponding PU-based coatings exhibit superior mechanical properties and solvent resistances compared to the polyurethane urea coatings synthesized from diols, DiBoc-IPDC, and DMDPA.

Chemoselective isocyanide insertion into the N–H bond using iodine–DMSO: metal-free access to substituted ureas

A chemoselective metal-free method for isocyanide insertion into amines is developed to synthesize symmetrical and unsymmetrical ureas.

Alkyl Isocyanates via Manganese-Catalyzed C-H Activation for the Preparation of Substituted Ureas

Organic isocyanates are versatile intermediates that provide access to a wide range of functionalities. In this work, we have developed the first synthetic method for preparing aliphatic isocyanates via direct C-H activation. This method proceeds efficiently at room temperature and can be applied to functionalize secondary, tertiary, and benzylic C-H bonds with good yields and functional group compatibility. Moreover, the isocyanate products can be readily converted to substituted ureas without isolation, demonstrating the synthetic potential of the method. To study the reaction mechanism, we have synthesized and characterized a rare Mn^IV-NCO intermediate and demonstrated its ability to transfer the isocyanate moiety to alkyl radicals. Using EPR spectroscopy, we have directly observed a Mn^IV intermediate under catalytic conditions. Isocyanation of celestolide with a chiral manganese salen catalyst followed by trapping with aniline afforded the urea product in 51% enantiomeric excess. This represents the only example of an asymmetric synthesis of an organic urea via C-H activation. When combined with our DFT calculations, these results clearly demonstrate that the C-NCO bond was formed through capture of a substrate radical by a Mn^IV-NCO intermediate.

Scalable, One-Pot, Microwave-Accelerated Tandem Synthesis of Unsymmetrical Urea Derivatives

We report a facile, microwave-accelerated, one-pot tandem synthesis of unsymmetrical ureas via a Curtius rearrangement. In this method, one-pot microwave irradiation of commercially available (hetero)aromatic acids and amines in the presence of diphenylphosphoryl azide enabled extremely rapid (1-5 min) construction of an array of unsymmetrical ureas in good to excellent yields. We demonstrate the utility of our method in the efficient, gram-scale synthesis of key biologically active compounds targeting the cannabinoid 1 and α7 nicotinic acetylcholine receptors.

Hydroamination of carbodiimides, isocyanates, and isothiocyanates by a bis(phosphinoselenoic amide) supported titanium(iv) complex

The hydroamination of heterocumulenes such as carbodiimides, isocyanates, and isothiocyanates by a bis(phosphinoselenoic amide) supported titanium(iv) complex as a precatalyst is reported here. The titanium(iv) complex [{Ph₂P(Se)NCH₂CH₂NPPh₂(Se)}Ti(NMe₂)₂] (1) was synthesised by the reaction of tetrakis-(dimethylamido)titanium(iv) [Ti(NMe₂)₄] with [{Ph₂P(Se)NHCH₂CH₂NHPPh₂(Se)}] in toluene at ambient temperature. Titanium complex 1 proved to be a competent pre-catalyst for the addition of an amine N-H bond to carbodiimides, isocyanates, and isothiocyanates. The reaction scope was expanded to reactions of aliphatic and aromatic amines with phenylisocyanates and phenylisothiocyanates in toluene solvents proceeding rapidly at room temperature with 5 mol% catalyst loadings to yield the corresponding urea and thio-urea derivatives up to 99%. However, ambient temperature was needed for hydroamination of 1,3-dicyclohexylcarbodiimide. The amine addition reactions with isocyanates showed first order kinetics with respect to catalyst 1 as well as substrates. The most plausible mechanism for the hydroamination reaction was established by isolating 1,1-dimethylphenyl urea as a side product.

Chemoselective Schwartz Reagent Mediated Reduction of Isocyanates to Formamides

Addition of the in situ generated Schwartz reagent to widely available isocyanates constitutes a chemoselective, high-yielding, and versatile approach to the synthesis of variously functionalized formamides. Steric and electronic factors or the presence of sensitive functionalities (esters, nitro groups, nitriles, alkenes) do not compromise the potential of the method. Full preservation of the stereochemical information contained in the starting materials is observed. The use of formamides in the nucleophilic addition of organometallic reagents (Chida-Sato allylation, Charette-Huang addition to imidoyl triflate activated amides, Matteson homologation of boronic esters) is briefly investigated.

Tandem buildup of complexity of aromatic molecules through multiple successive electrophile generation in one pot, controlled by varying the reaction temperature

While some sequential electrophilic aromatic substitution reactions, known as tandem/domino/cascade reactions, have been reported for the construction of aromatic single skeletons, one of the most interesting and challenging possibilities remains the one-pot build-up of a complex aromatic molecule from multiple starting components, i.e., ultimately multi-component electrophilic aromatic substitution reactions. In this work, we show how tuning of the leaving group ability of phenolate derivatives from carbamates and esters provides a way to successively generate multiple unmasked electrophiles in a controlled manner in one pot, simply by varying the temperature. Here, we demonstrate the autonomous formation of up to three bonds in one pot and formation of two bonds arising from a three-component electrophilic aromatic substitution reaction. This result provides a proof-of-concept of our strategy applicable for the self-directed construction of complex aromatic structures from multiple simple molecules, which can be a potential avenue to realize multi-component electrophilic aromatic substitution reactions.