Recent Advances in the Dimroth Rearrangement

Design, synthesis, and cytotoxicity screening of novel pyrazolopyrimidines over renal cell carcinoma (UO-31 cells) as p38α inhibitors, and apoptotic cells inducing activities

A series of novel molecules with pyrazolopyrimidine-4-amine core were designed and synthesized as potential cytotoxic agents over Renal Cell Carcinoma cells (UO-31). Results of cytotoxic activity against UO-31 cells showed that pyrazolopyrimidines 19 and 31 were found to be more cytotoxic than sorafenib (SOR). The cytotoxic activity of these compounds appeared to correlate with their ability to inhibit p38α MAPK which are 2.53- and 2.27- folds more potent than SOR. Moreover, results of the cell cycle analysis as well as the results of annexin-V on the (UO-31) cells showed that pyrazolopyrimidines 19 and 31 had a pro-apoptotic activity higher than SOR by 1.42- and 1.20- folds, respectively. Furthermore, compounds 19 and 31 were found to be effective in arresting the cell cycle throughout the accumulation of the cells at G2/M phase. Finally, the tested compounds decreased the TNF concentration as well as increased the expression of tumor suppressor gene p53, Bax/BCL-2 ratio and caspase 3/7.

Access to 4-((Pyridin-2-yl)amino)quinazolinones via Annulation of 2-Aminobenzonitriles with N'-(Pyridin-2-yl)-N,N-dimethyl Ureas

We have developed a convenient protocol for synthesizing N-(2-pyridyl)-substituted 4-(amino)quinazolin-2(1H)-ones by reacting N,N-dimethyl-N'-pyridylureas with 2-aminobenzonitriles. The method relies on the ability of N,N-dimethyl-N'-pyridyl/quinolinyl ureas to act as masked isocyanates under thermal activation, followed by a Dimroth rearrangement of 4-imino-3-(hetaryl)-3,4-dihydroquinazolin-2(1H)-ones. Conducted at 120 °C, either in DMF or under solvent-free conditions, this approach has produced 28 derivatives of 4-aminoquinazolinones, featuring pyridine or quinoline substituents, with yields of up to 92%.

Design and Synthesis of Bistetrazole-Based Energetic Salts Bearing the Nitrogen-Rich Fused Ring

A series of bistetrazole-based energetic salts bearing a nitrogen-rich fused ring were designed and synthesized. Among them, compounds 4-10 showed good detonation properties and excellent thermostability. By treating nitrogen-rich fused ring 3 with concentrated hydrochloric acid, a new type of Dimroth rearrangement was observed that afforded compound 12 efficiently. This new transformation herein constitutes a valuable addition to the Dimroth rearrangement.

Synthesis of 1,2,3-Triazine Derivatives by Deoxygenation of 1,2,3-Triazine 1-Oxides

A convenient, efficient, and inexpensive method has been developed for the synthesis of 1,2,3-triazine derivatives via deoxygenation of 1,2,3-triazine 1-oxide using trialkyl phosphites. Triethyl phosphite is more reactive than trimethyl phosphite, and both phosphites form their corresponding phosphates in these reactions. This procedure provides a range of aromatic and aliphatic substituted 1,2,3-triazine-4-carboxylate derivatives cleanly in high yields. Unexpected 1,2,4-triazine derivatives were also obtained as minor products during deoxygenation of 1,2,3-triazine-4-carboxylate 1-oxides having an aliphatic substituent at the 5-position.

Development of Mild Chemical Catalysis Conditions for m1A-to-m6A Rearrangement on RNA

The conversion of N¹-methyladenosine (m¹A) to N⁶-methyladenosine (m⁶A) on RNA is an important step for both allowing efficient reverse transcription read-though for sequencing analysis and mapping modifications in the transcriptome. Enzymatic transformation is often used, but the efficiency of the removal can depend on local sequence context. Chemical conversion through the application of the Dimroth rearrangement, in which m¹A rearranges into m⁶A under heat and alkaline conditions, is an alternative, but the required alkaline conditions result in significant RNA degradation by hydrolysis of the phosphodiester backbone. Here, we report novel, mild pH conditions that catalyze m¹A-to-m⁶A arrangement using 4-nitrothiophenol as a catalyst. We demonstrate the efficient rearrangement in mononucleosides, synthetic RNA oligonucleotides, and RNAs isolated from human cell lines, thereby validating a new approach for converting m¹A-to-m⁶A in RNA samples for sequencing analyses.

Dimroth-type N/S-interchange of N-aminothioglycolurils in the synthesis of 2-hydrazonoimidazo[4,5-d]thiazolones

An original method for the synthesis of 2-hydrazonoimidazo[4,5-d]thiazolone derivatives has been developed based on a one-pot acid-induced sequence of hydrazone formation from 3-thioxoperhydroimidazo[4,5-e]-1,2,4-triazinones and aromatic aldehydes, triazine ring contraction to imidazolidine one, and Dimroth-type N/S-interchange of N-aminothioglycolurils formed in situ into 2-hydrazonoimidazo[4,5-d]thiazolones. 3-Phenylacroleine derivatives are also suitable substrates for the reaction with thioxoperhydroimidazotriazinones.

An original method for the synthesis of 2-hydrazonoimidazo[4,5-d]thiazolone derivatives has been developed based on an acid-induced Dimroth-type N/S-interchange of N-aminothioglycolurils formed in situ into 2-hydrazonoimidazo[4,5-d]thiazolones.

Interrupted reactions in chemical synthesis

Interrupted reactions reroute established processes to new and often unanticipated end points. Of particular interest are the cases in which a known reactive intermediate takes on a new reaction pathway, either because this pathway is lower in energy or because the conventional pathway is no longer available. Through analysis of documented cases, we aim to dissect the known interrupted reactions and trace their mechanistic origins. As new chemical processes are being discovered at a seemingly ever-increasing pace, it is likely that new interrupted reactions will continue to emerge. Our hope is that the cases considered in this Review will help identify new classes of these fascinating transformations.

When a "Dimroth Rearrangement" Is Not a Dimroth Rearrangement

In the Dimroth rearrangement of heterocycles, often pyrimidines, an exocyclic and a ring substituent are interchanged. However, the term Dimroth rearrangement is frequently used even when there is no knowledge of the reaction mechanism and alternatives are likely. Here, we have employed density functional theory (DFT) calculations at the M06-2X/6-311+G(d,p) level to determine the most plausible rearrangement pathways of 3-aminothiocarbonylquinazoline 5, tetrahydrofuranylpyrimidine 21, and 5-allyltriazocine 30. For the rearrangement of quinazoline 5 to 9, the [1,3]-sigmatropic shift of the thioamido group with an activation barrier of 26.7 kcal/mol is much preferred over the Dimroth rearrangement (∼46 kcal/mol). An even lower barrier of 21.6 kcal/mol applies to a stepwise [1,3]-shift. The migration of the tetrahydrofuranyl unit in pyrimidines like 21 → 23 can take place by means of a [1,3]-sigmatropic shift with a low barrier (≤17.5 kcal/mol) rather than a Dimroth rearrangement under acidic conditions and most likely also under neutral conditions (∼30 kcal/mol). In the rearrangement of 5-allyl-6-iminotriazocine 30 to 32, the [3,3]-sigmatropic shift (aza-Cope rearrangement) is preferred over the Dimroth mechanism under neutral conditions, but in the presence of acid, the azonia-Cope rearrangement of an allyl group and the true Dimroth rearrangement have comparable activation energies.

The Dimroth Rearrangement in the Synthesis of Condensed Pyrimidines - Structural Analogs of Antiviral Compounds

The review discusses the use of the Dimroth rearrangement in the synthesis of condensed pyrimidines which are key structural fragments of antiviral agents. The main attention is given to publications over the past 10 years. The bibliography includes 107 references.

Controlling Pd-Catalyzed N-Arylation and Dimroth Rearrangement in the Synthesis of N,1-Diaryl-1H-tetrazol-5-amines

The Pd-catalyzed N-arylation method for the synthesis of eighteen N,1-diaryl-1H-tetrazol-5-amine derivatives is reported. By running the reactions at 35 °C, compounds were isolated as single isomers since the undesired Dimroth rearrangement was completely suppressed. Furthermore, the Dimroth rearrangement of N,1-diaryl-1H-tetrazol-5-amines was rationalized by conducting comprehensive experiments and NMR analysis as well as density functional theory (DFT) calculations of thermodynamic stability of the compounds. It was established that the Dimroth rearrangement is thermodynamically controlled, and the equilibrium of the reaction is determined by the stability of the corresponding isomers. The mechanism was investigated by additional DFT calculations, and the opening of the tetrazole ring was shown to be the rate-determining step. By maneuvering Pd-catalyzed N-arylation and the subsequent Dimroth rearrangement, two more N,1-diaryl-1H-tetrazol-5-amine derivatives were acquired, which otherwise cannot be synthesized by employing the C-N cross-coupling reaction.

Renaissance of Ring-Opening Chemistry of Benzotriazoles: New Wine in an Old Bottle

1,2,3-Benzotriazoles could undergo ring cleavage to form ortho-amino arenediazonium or α-diazo-imine species via a Dimroth-type equilibrium. Historically, the synthetic potential of this unique reactivity had remained underdeveloped. Recently, some new strategies have been developed to effect the ring-opening chemistry of benzotriazoles in more practical manners. A wide range of conceptually novel and synthetically useful reactions have been developed, which enable the access to diverse valuable heterocycles and ortho-amino arene derivatives. As one of the players in this field, our group has also contributed a series of intriguing transition-metal-catalyzed denitrogenative functionalizations of benzotriazoles. In this account, we aim to provide an overview of the ring-opening chemistry of benzotriazoles, with a focus on relevant works published in the past decade. In order to show a whole picture of the research field, some pioneering works in its developing history will also be discussed briefly.

Directed Search of Anti-inflammatory Agents Among (3HQuinazoline- 4-ylidene)hydrazides of N-protected Amino acids and their Heterocyclization Products

BACKGROUND

(Quinazoline-4-ylidene)hydrazides are valued intermediates in modern organic chemistry, as they are commonly used for the synthesis of substituted [1,2,4]triazolo[1,5-c]quinazolines.

OBJECTIVE

Unknown N-acyl-2-([1,2,4]triazolo[1,5-c]quinazoline-2-yl)-alkyl-(alkaryl-, aryl-) amines were synthesized and evaluated for anti-inflammatory potential.

METHODS

The peculiarities of the synthesized compounds structures were studied by IR-, NMR spectroscopy and chromatography-mass spectrometry and were discussed in detail. Probable molecular mechanisms of activity (inhibition of COX-1 and COX-2) were predicted due to molecular docking. Anti-inflammatory activity of synthesized compounds was determined by their ability to reduce the formalin-induced paw edema in rats. Diclofenac sodium was used as reference drug.

RESULTS

In this study, the synthesis of N-acetyl-(benzoyl)-2-([1,2,4]triazolo[1,5-c]quinazolinе- 2-yl)alkyl-(aralkyl-, aryl-)amines, using (3H-quinazoline-4-ylidene)hydrazides of Nprotected amino acids or 4-hydrazinoquinazoline and N-prorotected amino acids as starting compounds was developed. It was established that the reaction of (3H-quinazoline-4- ylidene)hydrazides of Boc-amino acids occurred with the formation of N-acetyl-substituted triazoloquinazolines. High anti-inflammatory activity was detected for unknown (3Hquinazoline- 4-ylidene)hydrazides Boc-amino acids (1.13-1.15) and N-acetyl-(benzoyl)-2- ([1,2,4]triazolo[1,5-c]quinazoline-2-yl-)aralkyl-(aryl-)amines (3.2, 3.3, 3.11, 3.12), using the experimental formalin test.

CONCLUSION

The conducted SAR-analysis allowed to detect critical fragments. Namely, the Boc-aminoaralkyl-(aryl-)acid residue in (3H-quinazoline-4-ylidene)hydrazides (1.13- 1.15), benzyl and phenyl linker groups in N-acetyl-(benzoyl)-2-([1,2,4]triazolo[1,5- c]quinazoline-2-yl-)aralkyl-(aryl-) amines (3.2, 3.3, 3.11, 3.12) are believed to be substantial for anti-inflammatory activity.

New One-Pot Synthesis of 1,3,5-Triazines: Three-Component Condensation, Dimroth Rearrangement, and Dehydrogenative Aromatization

A new, effective one-pot synthesis of the 6, N²-diaryl-1,3,5-triazine-2,4-diamines under microwave irradiation was developed. The method involved an initial three-component condensation of cyanoguanidine, aromatic aldehydes, and arylamines in the presence of hydrochloric acid. Without isolation, the resulting 1,6-diaryl-1,6-dihydro-1,3,5-triazine-2,4-diamines were treated with a base to initiate Dimroth rearrangement and spontaneous dehydrogenative aromatization, affording the desired compounds. The developed method was found to be sufficiently general in scope, tolerating various aromatic aldehydes and amines; by using their combinations in the first step, a representative library of 110 compounds was successfully prepared and screened for anticancer properties.

Palladium-catalyzed denitrogenative cycloadditions and alkenylations of benzotriazoles with alkynes

Palladium-catalyzed denitrogenative cycloadditions and alkenylations of benzotriazoles with alkynes have been achieved under different conditions, which enable the divergent syntheses of three types of valuable scaffolds.

Denitrogenative Suzuki and carbonylative Suzuki coupling reactions of benzotriazoles with boronic acids

Unprecedented palladium-catalyzed denitrogenative Suzuki and carbonylative Suzuki coupling reactions of benzotriazoles with boronic acids have been realized, which afforded structurally diverse ortho-amino-substituted biaryl and biaryl ketone derivatives. The key to this success is due to the development of a rationally designed strategy to achieve the ring opening of benzotriazoles with a synergistic activating-stabilizing effect, which enables the in situ generation of the corresponding ortho-amino-arenediazonium species. The present work opens up a new avenue to utilize benzotriazoles as synthetic equivalents of ortho-amino-arenediazoniums, which otherwise could not be directly accessed by existing synthetic methods.

Stereoselective Synthesis of 1-Tuberculosinyl Adenosine; a Virulence Factor of Mycobacterium tuberculosis

Despite its status as one of the world's most prevalent and deadly bacterial pathogens, Mycobacterium tuberculosis (Mtb) infection is not routinely diagnosed by rapid and highly reliable tests. A program to discover Mtb-specific biomarkers recently identified two natural compounds, 1-tuberculosinyl adenosine (1-TbAd) and N(6)-tuberculosinyl adenosine (N(6)-TbAd). Based on their association with virulence, the lack of similar compounds in nature, the presence of multiple stereocenters, and the need for abundant products to develop diagnostic tests, synthesis of these compounds was considered to be of high value but challenging. Here, a multigram-scale stereoselective synthesis of 1-TbAd and N(6)-TbAd is described. As a key-step, a chiral auxiliary-mediated Diels-Alder cycloaddition was developed, introducing the three stereocenters with a high exo endo ratio (10:1) and excellent enantioselectivity (>98% ee). This constitutes the first entry into the stereoselective synthesis of diterpenes with the halimane skeleton. Computational studies explain the observed stereochemical outcome.

In vivo biosynthesis of terpene nucleosides provides unique chemical markers of Mycobacterium tuberculosis infection

Although small molecules shed from pathogens are widely used to diagnose infection, such tests have not been widely implemented for tuberculosis. Here we show that the recently identified compound, 1-tuberculosinyladenosine (1-TbAd), accumulates to comprise >1% of all Mycobacterium tuberculosis lipid. In vitro and in vivo, two isomers of TbAd were detected that might serve as infection markers. Using mass spectrometry and nuclear magnetic resonance, we established the structure of the previously unknown molecule, N(6)-tuberculosinyladenosine (N(6)-TbAd). Its biosynthesis involves enzymatic production of 1-TbAd by Rv3378c followed by conversion to N(6)-TbAd via the Dimroth rearrangement. Intact biosynthetic genes are observed only within M. tuberculosis complex bacteria, and TbAd was not detected among other medically important pathogens, environmental bacteria, and vaccine strains. With no substantially similar known molecules in nature, the discovery and in vivo detection of two abundant terpene nucleosides support their development as specific diagnostic markers of tuberculosis.

Synthesis of [1,2,4]Triazolo[4″,3″:1′,6′]Pyrimido[4′,5′:3,4]Pyridazino[1,6-D] [1,2,4]Triazine; A Novel Tetracyclic System

The condensation reaction of 6-acetyl-3-amino-5-aryl-2-phenyl-2,5-dihydropyridazine-4-carbonitrile with triethyl orthoformate and then with hydrazine hydrate to obtain the new fused bicyclic 4-aryl-5-hydrazinyl-3-(1-hydrazonoethyl)-1-phenyl-1,4-dihydropyrimido[4,5-c]pyridazines via the Dimroth rearrangement is reported. Further cyclisation was achieved by the treatment of latter compounds with triethyl orthoformate to afford several derivatives of a novel tetracyclic fused system [1,2,4] triazolo[4″,3″:1′,6′]pyrimido[4′,5′:3,4]pyridazino[1,6-d][1,2,4]triazine.

Long-range 1H-15N J couplings providing a method for direct studies of the structure and azide-tetrazole equilibrium in a series of azido-1,2,4-triazines and azidopyrimidines

The selectively (15)N labeled azido-1,2,4-triazine 2*A and azidopyrimidine 4*A were synthesized by treating hydrazinoazines with (15)N-labeled nitrous acid. The synthesized compounds were studied by (1)H, (13)C, and (15)N NMR spectroscopy in DMSO, TFA, and DMSO/TFA solutions, where the azide-tetrazole equilibrium could lead to the formation of two tetrazoles (T, T') and one azide (A) isomer for each compound. The incorporation of the (15)N label led to the appearance of long-range (1)H-(15)N coupling constants (J(HN)), which can be measured easily by using amplitude-modulated 1D (1)H spin-echo experiments with selective inversion of the (15)N nuclei. The observed J(HN) patterns enable the unambiguous determination of the mode of fusion between the azole and azine rings in the two groups of tetrazole isomers (2*T', 4*T' and 2*T, 4*T), even for minor isoforms with a low concentration in solution. However, the azide isomers (2*A and 4*A) are characterized by the absence of detectable J(HN) coupling. The analysis of the J(HN) couplings in (15)N-labeled compounds provides a simple and efficient method for direct NMR studies of the azide-tetrazole equilibrium in solution.

Novel cytidine-based orotidine-5'-monophosphate decarboxylase inhibitors with an unusual twist

Orotidine-5'-monophosphate decarboxylase (ODCase) is an interesting enzyme with an unusual catalytic activity and a potential drug target in Plasmodium falciparum, which causes malaria. ODCase has been shown to exhibit unusual and interesting interactions with a variety of nucleotide ligands. Cytidine-5'-monophosphate (CMP) is a poor ligand of ODCase, and CMP binds to the active site of ODCase with an unusual orientation and conformation. We designed N3- and N4-modified CMP derivatives as novel ligands to ODCase. These novel CMP derivatives and their corresponding nucleosides were evaluated against Plasmodium falciparum ODCase and parasitic cultures, respectively. These derivatives exhibited improved inhibition of the enzyme catalytic activity, displayed interesting binding conformations and unusual molecular rearrangements of the ligands. These findings with the modified CMP nucleotides underscored the potential of transformation of poor ligands to ODCase into novel inhibitors of this drug target.

Synthesis of 2-arylamino substituted 5,6-dihydropyrido[2,3-d]pyrimidine-7(8H)-ones from arylguanidines

A practical protocol was developed for the synthesis of 2-arylamino substituted 4-amino-5,6-dihydropyrido[2,3-d]pyrimidin-7(8H)-ones from α,β-unsaturated esters, malononitrile, and an aryl substituted guanidine via the corresponding 3-aryl-3,4,5,6- tetrahydropyrido[2,3-d]pyrimidin-7(8H)-ones. Such compounds are formed upon treatment of 2-methoxy-6-oxo-1,4,5,6-tetrahydropyridine-3-carbonitriles with an aryl substituted guanidine in 1,4-dioxane and are converted to the desired 4-aminopyridopyrimidines with NaOMe/MeOH through a Dimroth rearrangement. The overall yields of this three-step protocol are, generally speaking, higher than the multicomponent reaction, previously developed by our group, between an α,β-unsaturated ester, malononitrile, and an aryl substituted guanidine.

Recyclization reactions of 1-alkylpyrimidinium salts

The reaction of 4-amino-2-benzyl-1-methyl-5-ethoxycarbonylpyrimidinium iodide (3) with alcoholic methylamine resulted in the formation of the methylimine of 2-amino-4-hydroxy-6-methylamino-5-phenylpyridine-3-carbaldehyde (5). Heating of the same pyrimidinium salt in benzylamine gave a mixture of products of two C-C recyclizations: 2-benzyl-4-benzylamino-5-carbamoylpyrimidine (7) and the benzylimine of 4-amino-2-benzyl-6-benzylaminopyrimidine-5-carbaldehyde (8). The reaction of 2-amino-1,4-dimethyl-5-ethoxycarbonylpyrimidinium iodide (10) with KOH ethanolic solution gave a single product of C-C-recyclization: 2-amino-5-acetyl-4-hydroxypyrimidine (11).