Search

Breaking aromaticity by inserting additional atoms within the skeleton of heteroaromatic rings has gained significant attention over the years. As part of the emerging concept of 'skeletal editing', this short review retraces the recent progress made on dearomative enlargement reactions of both five- and six-membered heterocycles. 1 Introduction 2 Dearomative Enlargement of Five-Membered Rings 2.1 Pyrroles, Furans, Thiophenes and Their Fused Analogues 2.2 Pyrazoles, Isoxazoles, Isothiazoles and Their Fused Analogues 3 Dearomative Enlargement of Six-Membered Rings 4 Conclusion and Perspectives [ABSTRACT FROM AUTHOR]

This paper outlines a novel strategy for the preparation of seven-membered-ring lactams from simple nitroarenes. The approach is based on a photochemical dearomative ring expansion starting with the conversion of the nitro group into a singlet nitrene. This process is mediated by blue light, occurs at room temperature and overall enables the insertion of the nitro N -atom into the benzenoid framework. This step transforms the aromatic starting material into a seven-membered ring azepine that, following hydrogenation and hydrolysis, is converted into the desired caprolactams in just three steps. [ABSTRACT FROM AUTHOR]

This document provides information on the synthesis and characterization of various thiochromeno[2,3-c]pyrrole derivatives. The compounds were synthesized using specific reactions and their structures were confirmed using spectroscopic techniques. Crystallographic analysis was also performed on one of the compounds. The document includes detailed information on the synthesis, characterization, and analysis of each compound. [Extracted from the article]

This article discusses the total synthesis of complanadine A, a Lycopodium alkaloid with potential applications in the treatment of neurological disorders and pain management. The authors describe their novel pyrrole-to-pyridine molecular editing strategy, which enables the efficient synthesis of complanadine A. They outline the retrosynthetic analysis and the key steps involved in the synthesis, including the use of cross-coupling and C-H arylation reactions. The authors also highlight the challenges and optimizations encountered during the synthesis process. The document provides a detailed account of a chemical reaction and the resulting compounds, including information about reaction conditions, purification methods, and characterization data. It is intended for researchers in the field of organic chemistry. [Extracted from the article]

The crude product was purified by PLC (40% EtOAc in hexane) to afford compound B 26 b [yield: 10.7 mg (25%); pale-yellow solid] and compound B 27 b . [13] Methylation of compound B 11a b was performed using MeI in the presence of K SB 2 sb CO SB 3 sb or NaH in THF as a solvent to introduce a methyl carbon at the I endo i - or I exo i -secondary amide of the lactam moiety, as shown in Table 1. The percentage inhibition was calculated using the following equation: % Inhibition = {[(Vc - Vb) - (Vi - Vb)] / (Vc - Vb)} × 100 where Vc is the enzymatic reaction rate of the control (absence of a test compound, negative control), Vb is the enzymatic reaction rate of the blank (absence of enzyme) and Vi is the enzymatic reaction rate of the test compound. Keywords: Clausena alkaloids; -clausenamides; balasubramides; benzo[ d ]azocinones; eight-membered lactams; ring expansion EN Clausena alkaloids -clausenamides balasubramides benzo[ d ]azocinones eight-membered lactams ring expansion 2241 2252 12 07/03/23 20230718 NES 230718 Plants in the genus I Clausena i (Rutaceae family) have been used as traditional medicinal herbs in the tropical and subtropical parts of Asia, particularly in China, India and Thailand. [Extracted from the article]

Phosphorus-containing N-heterocycles are important molecular motifs due to their unique structural features and biological activities. In this study, we developed a silver-catalyzed reaction for the construction of phosphorus-containing azepine derivatives via a domino-type dearomatization procedure, followed by ring expansion. In addition, diazomethylphosphonates were employed for the first time as nucleophiles in the 1,4-dearomative addition of activated N-heteroarenes, furnishing cyclopropane-fused piperidine intermediates that were readily restructured into their corresponding azepine derivatives. The reactivities of the diazomethylphosphonates in the developed dearomatization strategy were found to be superior to those of other diazo compounds, thereby resulting in the generation of the desired seven-membered N-heterocycles within a very short reaction time. [ABSTRACT FROM AUTHOR]

Enantioselective methylene insertion into prochiral cyclobutanones is described providing access to chiral β-substituted cyclopentanones as important structural motif in synthesis and natural products. Commercially available trimethylsilyl as well as other silyl diazomethanes act as one-carbon synthon and scandium triflate is found to be a potent Lewis acid catalyst. By using bis(oxazoline) ligands, enantioinduction is achieved for a number of β-substituted cyclopentanones including examples bearing all-carbon quaternary stereocentres. [ABSTRACT FROM AUTHOR]

Studies on the enantioselective transformation of silacyclobutanes (SCBs) have become an emerging topic in the recent decade, due to the feature of high ring strain, and manageable Si–C bond cleavage and formation. This short review summarizes the remarkable achievements in the asymmetric transition-metal-mediated reaction of silacyclobutanes and benzosilacyclobutanes, resulting in carbon- or silicon-stereogenic organosilanes with functional substituents. 1 Introduction 2 Asymmetric Transition-Metal-Catalyzed Ring Expansion Reaction 3 Asymmetric Transition-Metal-Catalyzed Ring-Opening Reaction 4 Conclusion and Outlook [ABSTRACT FROM AUTHOR]

A general strategy for the synthesis of 2-substituted cyclobutanone sulfides via a tandem Brønsted acid-catalyzed nucleophile addition/ring contraction/C3-C4 ring expansion reaction sequence has been exploited. The procedure led to a wide panel of four-membered cyclic ketones in good to excellent yields and with broad substrate scope. Mechanistic aspects and kinetic parameters were investigated by quantum chemical DFT calculations, allowing us to rationalize the different reactivity of 2-aryl- and 2-alkyl-substituted 2-hydroxycyclobutanones towards thiol nucleophiles in reactions mediated by sulfonic acids. NMR and in situ Raman techniques were employed to better understand the reaction kinetics and parameters that affect the desired outcome. [ABSTRACT FROM AUTHOR]

Nitrenoid-mediated C–H insertion and cycloaddition reactions are gaining increasing importance in modern organic synthesis. However, the nitrene-mediated benzene ring expansion, which constitutes a potent tool for breaking the benzene ring, has sparely been explored for synthetic purposes. Herein we report that nitrene cycloaddition to the benzene ring can be enabled intramolecularly by iron catalysis. By using FeCl2 and N -heterocyclic carbene SIPr·HCl as the catalyst, α-azido- N -phenylamides can be converted into 1,3-dihydro-2 H -azepin-2-ones in good yields through a novel skeleton rearrangement. The rearrangement features a cascade of nitrene cycloaddition, C–N cleavage, water addition, and electrocyclic ring-opening. In the case that the N -phenyl ring is substituted with a methoxy group at the meta or para position, azepin-2-one-fused imidazolinones can also be obtained. The present reactions provide an effective method for benzene ring expansion as well as for the preparation of azepin-2-one compounds. [ABSTRACT FROM AUTHOR]

A mechanistically interesting and practical method for the synthesis of functionalized spiro[4.5]decanes is developed, featuring oxidative dearomatization-induced ring expansion of cyclobutanes as the key element. The new method enables facile access to a variety of spiro[4.5]cyclohexadienones with good efficiency and generality. Further elaboration of the resulting products into other valuable scaffolds is also explored, leading to the discovery of an interesting compound that displays a promising biological profile. Moreover, we have also conducted a comprehensive computational study that provides a deep insight into the mechanism of the reaction. [ABSTRACT FROM AUTHOR]

Silaazacycles (or azasilacycles), containing both nitrogen and silicon atoms, are appealing ring structures in the development of silicon-containing functional molecules. The development of general and efficient methods towards these motifs has therefore attracted considerable attention from synthetic chemists. This short review intends to highlight representative advances in the synthesis of five- and six-membered silaazacycles. 1 Introduction 2 Five-Membered Silaazacycles 2.1 Five-Membered Silaazacycles bearing a 1,2-N/Si Moiety 2.2 Five-Membered Silaazacycles bearing a 1,3-N/Si Moiety 3 Six-Membered Silaazacycles 3.1 Six-Membered Silaazacycles bearing a 1,2-N/Si Moiety 3.2 Six-Membered Silaazacycles bearing a 1,3-N/Si Moiety 3.3 Six-Membered Silaazacycles bearing a 1,4-N/Si Moiety 4 Conclusion [ABSTRACT FROM AUTHOR]

Cyclopropanone derivatives constitute highly strained cycloalkanones with promising applications as three-carbon building blocks in organic synthesis. Due to the presence of a ketone in such a small ring system, all C–C bonds and the carbonyl group are considered to be labile in suitable conditions, leading to a wide variety of synthetic disconnections, including nucleophilic addition, ring expansion, ring-opening, and (formal) cycloaddition. Despite their synthetic potential, the widespread adoption of cyclopropanones as substrates has been considerably hampered by the difficulties associated with the preparation and storage of such unstable compounds, prompting the development of cyclopropanone surrogates that can equilibrate to the parent ketone in situ via elimination. This review summarizes the syntheses and applications of cyclopropanone derivatives and their equivalents, and offers a perspective of the state of the field as well as its expected future directions. 1 Introduction 2 Preparation of Cyclopropanones and Their Equivalents 2.1 Carbenoid Chemistry 2.2 Allene Oxide Rearrangement 2.3 Ring Closure by Dehydrohalogenation or Dehalogenation 2.4 Photolytic Processes 2.5 Miscellaneous Formation of Cyclopropanones 2.6 Cyclopropanone Equivalents 3 Synthetic Applications of Cyclopropanones and Their Equivalents 3.1 Nucleophilic Addition to the Carbonyl Group 3.2 Ring Expansion 3.3 Ring-Opening 3.4 Cycloaddition and Formal Cycloaddition 4 Conclusion and Outlook [ABSTRACT FROM AUTHOR]

A facile route to polysubstituted 2-pyrones from readily available 2-acylcyclopropane-1-aryl-1-carboxylates mediated by TfOH is reported. The strongly donating 1-aryl group is important for directing the C–C bond cleavage of the donor-acceptor cyclopropane ring, which then leads to the formation of the 2-pyrone ring through lactonization. [ABSTRACT FROM AUTHOR]

The title compounds were synthesized from readily available quinolone and coumarin derivatives by a cascade reaction (12 examples, 90–98% yield). The cascade comprised a [2+2] photocycloaddition which occurred upon sensitized irradiation at λ = 420 nm (or direct UV irradiation at λ = 366 nm) and a subsequent acid-catalyzed ring-opening reaction. A variety of substituents are compatible with the conditions and a 3-alkyl group in the coumarin (or quinolone) is crucial to achieve a high chemoselectivity. Key to the success of the ring opening is the formation of a 4,5,5a-trihydrocyclobuta-2 H -furan containing a strained bridgehead double bond which stems from the allenyl group tethered to the 4-position of the starting materials. [ABSTRACT FROM AUTHOR]

Two methods are presented for the synthesis of the title compounds starting from cyclobutenediones: an alkoxide substitution approach and a Staudinger reaction. Unsaturated lithiumorganyls may be added to the remaining carbonyl group and on heating lead to ring enlargement in a cascading process. 4-Alkenyl or 4-aryl derivatives yield aminophenols or -naphthols; 4-alkynyl compounds give cyclopenta-annulated quinolines. © Georg Thieme Verlag Stuttgart.

A stereoselective synthetic route to embellished cis -hydrindanes from simple aromatic precursors is described. Oxidative dearomatization, ring expansion, and photochemical 1,3-acyl shift are the key features of our approach. Oxidative dearomatization of o -(hydroxymethyl)phenols followed by π4 s + π2 s cycloaddition furnishes bicyclo[2.2.2]octanes with contiguous keto epoxide groups, which upon ring expansion lead to bicyclo[3.2.2]nonanes endowed with a β,γ-enone chromophore. Unbridging of bicyclo[3.2.2]nonanes upon singlet excitation furnishes embellished cis -hydrindanes. [ABSTRACT FROM AUTHOR]

A novel EtO2CCF2-substituted semisquarate is synthesized from diisopropyl squarate by selective 1,2-addition of the Reformatsky reagent derived from BrCF2CO2Et and subsequent rhenium-catalyzed allylic alcohol rearrangement. The compatibility of the highly reactive EtO2CCF2 group in ring transformations of the obtained semisquarate is investigated. Various EtO2CCF2-substituted, highly functionalized compounds, such as quinones, tetronates, cyclopentenones and a bicyclo- [3.2.0]heptenone, are successfully synthesized by ring transformations of the EtO2CCF2-substituted semisquarate. Also, an allylO2CCF2-substituted semisquarate is prepared and converted into the corresponding tetronate. Subsequent palladium-catalyzed deallylation followed by condensation of the resulting carboxylic acid with several amines under mild conditions affords the corresponding a,a-difluoroamides in good yields. [ABSTRACT FROM AUTHOR]

This review covers recent advances in synthesis of azocinecontaining systems. The most approaches towards azocines are discussed.1 Introduction 2 Ring-Expansion Reaction 2.1 Ring-Expansion Reaction of Cyclopentane Containing the 1,4-Diketone Moiety with Primary Amines (from 5 to 8) 2.2 Ring-Expansion Reaction of Annulated Tetrahydropyridines under the Action of Activated Alkynes (from 6 to 8) 2.3 Reductive Ring-Expansion Reaction of Cyclic Oximes 2.4 Other Ring-Expansion Reactions 3 Heck Reaction 4 Cycloaddition 5 Ring-Closing Metathesis (RCM) 6 Cyclization 6.1 Metal-Catalyzed Cyclization 6.2 Radical Cyclization 6.3 Friedel-Crafts Cyclization 6.4 Other Examples of Cyclizations 7 Microwave- and Photo-Assisted Reactions 8 Other Methods 8.1 Cascade and Tandem Reactions 8.2 Aldol Condensation 8.3 Thermolysis 8.4 Ring Opening 8.5 Other Methods 9 Conclusion. [ABSTRACT FROM AUTHOR]

In this review, we present the recent advances in the synthesis and reactivity of vinylcyclopropanes (VCPs). Various important methodologies involving ylides, carbenoid chemistry, metal-mediated cycloaddition reactions, and allylic cyclization methodologies for the synthesis of VCPs are described. Based on the type of VCPs, their reactivity is dealt under electrophile-, radical-, and nucleophile-mediated reactions. Several examples from transition-metal-mediated ring-opening reactions are discussed. [ABSTRACT FROM AUTHOR]

α-Chloro(diazo)acetate can be readily used in an intermolecular ring expansion catalyzed by a boron Lewis acid to construct α-chlorinated medium-size cyclic ketones. This intermolecular Tiffeneau- Demjanov reaction proceeds in good yield from easily prepared or commercially available starting materials and allows the concomitant formation of a tetrasubstituted stereocenter. [ABSTRACT FROM AUTHOR]

Due to their high strain, unique bonding and relative ease of ring-cleavage, and because they are readily accessible, cyclopropanols have been employed in an increasing number of transition-metal-catalyzed C-C and C-X (X = heteroatom) bond-forming reactions. We review the recent literature and organize all the methods developed along mechanistic lines. 1 Introduction and Scope 2 C-C Bond Formation via Catalytic Generation and Coupling of Cyclopropanol-Derived Homoenolates 3 C-N Bond Formation via Catalytic Generation and Coupling of Cyclopropanol-Derived Homoenolates 4 C-C Bond Formation via Stoichiometric Generation and Coupling of Cyclopropanol-Derived Homoenolates 5 Ring Expansion of Cyclopropanols to Cyclobutanones via Transition-Metal-Catalyzed Wagner-Meerwein Shift 6 Transition-Metal-Catalyzed Rearrangement of Cyclopropanols to Cyclopentenones and Cyclohexenones 7 Synthesis of Medium-Size Rings via Multicomponent Reactions Involving Cyclopropanols 8 C-C and C-X Bond Formation via Free-Radical Fragmentation of Cyclopropanols 9 Summary and Outlook. [ABSTRACT FROM AUTHOR]

Over the years, the oxidative ring enlargement of furfuryl alcohols, known as the Achmatowicz reaction, has been developed into a powerful and versatile synthetic tool for the preparation of 6-hydroxypyranones. This review provides a comprehensive collection of the various ways to perform Achmatowicz rearrangement reactions and explores the role of this ring-expansion process in contemporary organic synthesis. 1 Introduction 2 Classical Methods and Variants 3 Single-Electron-Transfer Oxidations 4 Metal-Catalyzed Ring Expansions 5 Photolytic Oxygenations 6 Enzymatic Transformations 7 Conclusions. [ABSTRACT FROM AUTHOR]

Tribenzotropone was efficiently synthesized by a ring-expansion method from readily available phenanthrene-9,10-dione via a ringopened diketone as a key intermediate; the diketone was prepared by nucleophilic addition of allyl and vinyl groups, followed by an oxidative ring-opening reaction with lead(IV) acetate. Ring closure by an intramolecular Diels-Alder reaction and subsequent dehydrogenation produced tribenzotropone in 38% overall yield. Ring closure by a Morita- Baylis-Hillman reaction, on the other hand, produced a dibenzo-fused nonanedione in 22% overall yield. [ABSTRACT FROM AUTHOR]

A novel procedure for the direct synthesis of 2,5-diaryloxazoles starting from N-H ketoaziridines is described. The method proceeds via the in situ formation of N-bromoketoaziridines in the presence of N-bromosuccinimide followed by the generation of intermediate azomethine ylides. A plausible mechanism for this transformation is proposed. [ABSTRACT FROM AUTHOR]

The first total syntheses of two (±)-laingolide A diastereomers were achieved in 11 and 12 steps, respectively. The key steps include a tandem cross-dimerization/oxonia-Cope reaction and an intramolecular dehydrative cyclization for the formation of either the transor cis-enamide moieties. [ABSTRACT FROM AUTHOR]

Three regioisomeric thieno[c]azocine derivatives were prepared in six steps from bromothiophene carboxylic acids. The reaction sequence started with an esterification with isopropyl alcohol. The resulting esters were submitted to a Heck reaction with tert-butyl acrylate followed by catalytic hydrogenation. Subsequent Dieckmann condensation gave cyclopentathiophenes with a cyclic β-oxo ester motif, which were α-alkylated with phenacyl bromide to furnish 1,4-diketones. The latter were converted in the key step, a bismuth-catalyzed ring transformation with methylamine, yielding the racemic eightmembered ring lactams, that is, tetrahydrothieno[2,3-c]-, [3,2-c]-, and -[3,4-c]azocine derivatives in overall yields of 25%, 16% and 12%, respectively. [ABSTRACT FROM AUTHOR]

In this work, for the first time, a cationic polymer containing pendant pyridinium groups was synthesized via post-functionalization of the polymeric backbone based on a calix[4]resorcinarene, and characterized by AFM, TEM, analytical CHN, TGA, and DTG techniques. The functionalized polymer is conveniently loaded with desired anions, thereby providing novel and recyclable polymer-supported reagents for achieving synthetic goals. The thermal stability of this cationic polymer (up to 250 °C by TGA) allows thermally demanding ring transformation and expansion of 2,4,6-triarylthiopyrylium salts on its side chains and polymeric backbone without decomposition and leaching of active species in the reaction media. This green method furnished a straightforward route for the synthesis of valuable 2,4,6-triarylpyridines, along with novel 2,5,7-triaryl-1,3-thiazepine derivatives, which are widely present as motifs in an assortment of biologically active molecules. [ABSTRACT FROM AUTHOR]

Rearrangements of nitrile-stabilized ammonium ylides are useful reactions for the construction of alkylamines, nitrogen heterocycles, or carbonyl compounds. In spite of their attractive synthetic potential, they are much less frequently used than rearrangements of ammonium ylides either carrying charge-stabilizing carbonyl groups or being devoid of additional stabilization of the negative charge. This review focuses on different classes of rearrangements of nitrile-stabilized ammonium ylides as well as their application in synthetic organic chemistry. 1 Introduction 2 [1,2]-Stevens Rearrangements 3 [2,3]-Sommelet-Hauser Rearrangement 4 [2,3]-Stevens Rearrangements 5 Competitive [1,2]-, [2,3]-, and [1,4]-Rearrangements 6 [1,4]-Rearrangements Involving Ring Expansion 7 Summary [ABSTRACT FROM AUTHOR]

A concise synthesis of spiropyrrolidines and spiropiperidines has been developed. The approach employs a ruthenium- alkylidene-catalysed cross-metathesis reaction of enantiopure Nprotected allylglycine with methylenecycloalkanes. The resultant alkene intermediates can then undergo a tandem acid-catalysed cyclisation to form spiropyrrolidines. Ring expansion of the spiropyrrolidine system, via an aziridinium intermediate, grants access to the homologous spiropiperidine ring system with excellent stereoretention. [ABSTRACT FROM AUTHOR]

Benzocyclobutenones are an intriguing class of four membered-ring ketones that have been used extensively as powerful synthetic intermediates in organic synthesis. Their high reactivity is primarily attributed to the unique high electrophilicity of the carbonyl unit and the ability to generate o-quinone dimethides, allowing a myriad of different transformations. However, the synthesis of benzocyclobutenones still represents a great challenge. This review provides an overview of the preparation, use and impact of benzocyclobutenones in organic synthesis. Selected applications in the synthesis of natural products are also described, in order to illustrate the utility of these compounds. [ABSTRACT FROM AUTHOR]

Treatment of 1-chlorocyclopropyl p-tolyl sulfoxides and 1-chlorocyclobutyl p-tolyl sulfoxides with a Grignard reagent at low temperature afforded cyclopropylmagnesium carbenoids and cyclobutylmagnesium carbenoids, respectively, via a sulfoxide- magnesium exchange reaction. The reaction of these magnesium carbenoids with lithium phenolates afforded spiro[2.6]nona-6,8- dien-5-ones and spiro[3.6]deca-7,9-dien-6-ones, respectively; however, the yields were rather low. Reaction of the cyclopropylmagnesium carbenoids with lithium 1-naphtholates gave spiro[2.6]nona- 5,7-dien-4-ones in moderate to good yields; however, reaction of the cyclobutylmagnesium carbenoids with lithium 1-naphtholates did not give the desired products. Reaction of the cyclopropyl- and cyclobutylmagnesium carbenoids with lithium 2-naphtholates gave spiro[2.6]nona-5,7-dien-4-ones and spiro[3.6]deca-7,9-dien-6- ones, respectively, in moderate to good yields. These unprecedented reactions provide a procedure for the synthesis of spiro[2.6]nona- 5,7-dien-4-ones and spiro[3.6]deca-7,9-dien-6-ones from 1-naphthols and 2-naphthols with a one-carbon ring expansion of the phenol ring. [ABSTRACT FROM AUTHOR]

In this review, the regioselectivity of transition-metalcatalyzed ring cleavage of cyclopropanes and cyclobutanes by bcarbon elimination is analyzed using experimental results and rationalization models from the literature. Most models proposed are generally consistent with the following trends: Cleavage of the less substituted bond of the small cycloalkane is favored by a minimization of steric hindrance, whereas cleavage of the more substituted bond becomes possible if steric hindrance is limited and if the substituent is electron-withdrawing or if it can coordinate the metal. Importantly, steric congestion can easily offset electronic activation. Moreover, regioselectivity models appear to be more generally applicable to cyclopropanes than to cyclobutanes. 1 Introduction 2 Important Features of &bgr;-C Elimination in Small Cycloalkanes 2.1 Reversibility of Ring-Opening &bgr;-C Elimination 2.2 Syn-Coplanarity of Ring-Opening &bgr;-C Elimination 2.3 Oxidative Addition during Ring-Opening &bgr;-C Elimination 2.4 Comparison with Other Transition-Metal-Catalyzed Ring- Opening Reactions 3 Case Studies 3.1 &bgr;-C Elimination Involving a Carbon–Metal Bond 3.1.1 Methylenecyclopropanes 3.1.2 Alkylidenecyclopropanes 3.1.3 Vinylcyclopropanes 3.1.4 Alkynylcyclopropanes 3.1.5 Allenylcycloalkanes 3.1.6 Phenyl-Substituted Cyclopropanes 3.1.7 Regioselective Ring Opening of Cyclobutane Derivatives to Cyclooctenones 3.2 &bgr;-C Elimination Involving a Nitrogen–Metal Bond 3.3 &bgr;-C Elimination Involving an Oxygen–Metal Bond 4 Conclusion [ABSTRACT FROM AUTHOR]

Starting from a suitably functionalized aziridine, a highly efficient stereoselective synthesis of (1R,2R)-phenyl-2-decanoylamino- 3-morpholinopropan-1-ol (D-threo-PDMP) is described. The approach, based on the ring expansion of the aziridine ring to oxazoline, could be applicable to the synthesis of many analogues with different amide chains and sugar mimic portions. [ABSTRACT FROM AUTHOR]

Deprotonated 1,2-diazetines react with various ketones containing an acidic CH to yield pyridazine derivatives. The reaction allows the synthesis of dihydropyridazines containing a cycloamidine substructure via a sequence of ring-opening and recyclization. In this case, the in situ generated anion of 1,2-diazetines acts as a base as well as a chelator. The structures of the novel synthesized derivatives were confirmed by X-ray structural analysis and multidimensional NMR experiments. [ABSTRACT FROM AUTHOR]