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The literature review discusses and systematizes synthetic approaches to medium-sized cycles and macrocycles based on ring expansion reactions of bi- or polycyclic systems via C-N bond cleavage. Ring expansion reactions of bicyclic ammonium salts proceed via thermal decomposition or the action of strong bases. Bi- or polycyclic systems containing a common amine group can be reduced with strong reducing reagents, e.g. lithium aluminum hydride. Ammonium derivatives are much more prone to nucleophilic attack and quite often are used as starting materials for the synthesis of medium-sized cycles. Bicyclic systems containing a common aminal or amidine group are used for the synthesis of medium-sized rings and macrocycles via cleavage of the endocyclic C-N bond. Various methods of their activation and reduction are discussed in the review.

A one-carbon difluorocyclopropanation/ring-expansion has been developed for the chiral synthon Cyrene, which is obtained via cellulose pyrolysis. The ring-enlargement was achieved by converting Cyrene (dihydrolevoglucosenone) into an enamine, reacting it with an in situ-generated difluorocarbene, and then heating it to ring-open the cyclopropane. Access to the product provides access to fluorinated analogues of this valuable chiral biomass derivative.

The one-carbon ring expansion of benzocyclobutenones is an efficient methodology for the assembly of cyclic ketones. However, hazardous reagents or noble metal catalysts are frequently required. Herein, we disclose a copper-catalyzed intermolecular formal [4 + 1] annulation of 1,5-diynes with benzocyclobutenones, allowing the practical and atom-economical construction of diverse pyrryl 1-indanones in generally good yields under mild reaction conditions. This reaction represents an important advancement in the ring expansion of benzocyclobutenones via vinyl cation pathway.

A series of imidazo[4,5-e][1,3]thiazino[2,3-c][1,2,4]triazines was synthesized via a cascade sequence of hydrolysis and skeletal rearrangement of imidazo[4,5-e]thiazolo[2,3-c][1,2,4]triazin-7(8H)-ylidene)acetic acid esters in methanol upon treatment with excess KOH. Imidazo[4,5-e]thiazolo[3,2-b][1,2,4]triazin-6(7H)-ylidene)acetic acid esters are also suitable substrates for the reaction. In this case hydrolysis and thiazole ring expansion were accompanied with the change of the thiazolotriazine junction type from thiazolo[3,2-b][1,2,4]triazine to thiazino[2,3-c][1,2,4]triazine.

Abstract The nitrogen‐heteroatom single bonds of 1,2‐azoles and isoxazolines underwent methylene insertion in the presence of CH2I2 (6 equiv.) and diethylzinc (3 equiv.) to produce a wide variety of the ring‐expanded six‐membered heterocycles. Density functional theory calculations suggest that the methylene insertion proceeds via cleavage of nitrogen‐heteroatom single bonds followed by ring closure.

Skeleton editing has rapidly advanced as a synthetic methodology in recent years, significantly streamlining the synthesis process and gaining widespread acceptance in drug synthesis and development. This field encompasses diverse ring reactions, many of which exhibit immense potential in skeleton editing, facilitating the generation of novel ring skeletons. Notably, reactions that involve the cleavage of two distinct rings followed by the reformation of new rings through ring insertion play a pivotal role in the construction of novel ring skeletons. This article aims to compile and systematize this category of reactions, emphasizing the two primary reaction types and offering a thorough exploration of their associated complexities and challenges. Our endeavor is to furnish readers with comprehensive reaction strategies, igniting research interest and injecting fresh impetus into the advancement of this domain.

A method has been developed for the preparation of 2-alkyl-6-aryl-, 2-aryl-6-aryl and 2,6-diaryl-5-aryl/hetaryl-substituted methyl 4-oxo-1,4-dihydropyridine-3-carboxylates by Mo(CO)6-mediated ring expansion of methyl 2-(isoxazol-5-yl)-3-oxopropanoates. The high reactivity of 4-oxo-1,4-dihydropyridine-3-carboxylates synthesized provide easy access to 2,4,6-triaryl-substituted and 1,2,5,6-tetrasubstituted nicotinates.

Alkyl 2-diazo-3-oxoalkanoates generate alkoxycarbonylketenes, which undergo an electrophilic ring expansion with aziridines to afford alkyl 2-(oxazolin-2-yl)alkanoates in good to excellent yields under microwave heating. The method is a convenient and clean reaction without any activators and catalysts and can be also applied in the synthesis of 2-(oxazolin-2-yl)alkanamides and 1-(oxazolin-2-yl)alkylphosphonates.

Expedient protocols for the synthesis of three types of highly functionalized azaheterocyclic scaffolds (dihydropyridazines, tetrahydropyridazines, and partially saturated tricyclic systems) from readily available hydroxypyrrolines and hydrazides are described. The directions of the transformation of a common initial intermediate, namely a Brønsted acid-activated hydroxypyrroline, depend on the reaction conditions and the structure of the hydrazides.

Thietanes are important aliphatic four-membered thiaheterocycles that are found in the pharmaceutical core and structural motifs of some biological compounds. They are also useful intermediates in organic synthesis. Various synthetic methods of thietanes have been developed, including inter- and intramolecular nucleophilic thioetherifications, photochemical [2 + 2] cycloadditions, ring expansions and contractions, nucleophilic cyclizations, and some miscellaneous methods. The recently developed methods provide some new strategies for the efficient preparation of thietanes and their derivatives. This review focuses on the synthetic methods to construct thietane backbones developed during 1966 to 2019.

A Lewis acid-promoted annulation of azadienes and cyclobutamines was developed. This reaction proceeded through Michael addition and ring-expansion cascade, affording the corresponding nitrogen-containing medium-sized rings with a broad scope in moderate to high yields. The catalytic asymmetric version of this reaction has also been explored using a chiral base.

This study presents the development of a mechanochemical protocol for a charge-accelerated aza-Claisen rearrangement. The protocol waives the use of commonly applied transition metals, ligands, or pyrophoric Lewis acids, e.g., AlMe3. Based on (heterocyclic) tertiary allylamines and acyl chlorides, the desired tertiary amides were prepared in yields ranging from 17% to 84%. Moreover, the same protocol was applied for a Belluš–Claisen-type rearrangement resulting in the synthesis of a 9-membered lactam without further optimization.

We present a short and efficient way of synthesizing two synthetically versatile 4-quinolone-3-carboxylate building blocks by cyclopropanation-ring expansion of 3-chloroindoles with α-halodiazoacetates as the key step. This novel transformation was applied towards the synthesis of the antibiotic drug norfloxacin.

The oxidative ring expansion of bio-derived furfuryl alcohols to densely functionalized six-membered O-heterocycles represents an attractive strategy in the growing network of valorization routes to synthetic building blocks out of the lignocellulosic biorefinery feed. In this study, two scenarios for the biocatalytic Achmatowicz-type rearrangement using methanol as terminal sacrificial reagent have been evaluated, comparing multienzymatic cascade designs with a photo-bio-coupled activation pathway.

Summary: Transition metal-catalyzed C–C bond functionalization has recently been identified as an increasingly versatile tool for organic syntheses, as otherwise unreactive functionalities can be used as efficient handles for synthetic manipulations. This work describes site-selective manganese(I)-catalyzed C–C hydroarylations of heteroarenes with substituted allenes. The C–C activations are efficiently catalyzed by MnBn(CO)5 and afford 1,2,3-tri-substituted arenes with notable diastereoselectivity. Moreover, the ring-expansion reaction proceeds via a C–C allylation/Michael addition cascade, providing challenging 8-membered carbocycles, thus highlighting the unique characteristics of the MnBn(CO)5 complex. Mechanistic studies reveal that the organometallic C–C activation occurs faster than typical C–H activations, thus providing a complementary approach to well-established methodologies for late-stage diversification.

In this letter, we report a novel synthesis of ethyl quinoline-3-carboxylates from reactions between a series of indoles and halodiazoacetates. The formation of the quinoline structure is probably the result of a cyclopropanation at the 2- and 3-positions of the indole followed by ring-opening of the cyclopropane and elimination of H–X.

A versatile and metal-free approach for the synthesis of carbocycles and of heterocycles bearing seven- and eight-membered rings is described. The strategy is based on ring expansion of 1-vinylcycloalkanols (or the corresponding silyl or methyl ether) mediated by the hypervalent iodine reagent HTIB (PhI(OH)OTs). Reaction conditions can be easily adjusted to give ring expansion products bearing different functional groups. A route to medium-ring lactones was also developed.

The reaction of nitropyridine derivatives and tributylphosphine (Bu3P) with the existence of nucleophilic solvent gives ring expansion product as diazepines in medium yield. Reaction mechanism subjected the formation of phenylnitrene, followed by intramolecular electrophilic insertion reaction to pyridine ring and subsequent ring enlargement. The intermediate in the reaction confirmed by computational calculation using B3LYP/6-31G* level. The intramolecular insertion reaction of pyridylnitrene is considered suppressed by the low HOMO (-9.932 eV) energy level of pyridine ring compared to that of benzene (-9.653 eV), hence 1,4-diazepine is obtained when employed 3-nitro-2,6-lutidine as starting material. The formation of diazepines was confirmed by the analysis of 1H NMR data. Separation of the product mixture using column chromatography on SiO2 was carried out and found to give expected diazepine along with the reduction product.

Recent syntheses of azetidines, pyrrolidines, piperidines and azepines through cycloaddition or sigmatropic rearrangements of vinylaziridines are described. Applications to natural product synthesis and mechanistic investigations are also summarized.

Synthesis of hydroazulene derivatives has been carried out through a ring-enlargement route by using carbene adduct intermediates. The protocol can be applied for the construction of functionalized hydroazulene skeletons as key components of many natural products as well as the core system of novel liquid-crystalline materials.

The development of new methodologies catalyzed by late transition metals involving cycloisomerizations of strained rings can open new venues for the synthesis of structurally complex molecules with interesting biological activities. Herein we summarize, from both a synthetic as well as a mechanistic point of view, the most recent developments in gold-catalyzed ring expansions.

A novel synthetic methodology has been developed for the synthesis of diethyl 5-alkyl/aryl/heteroaryl substituted 3,4-dihydro-2H-pyrrole-4,4-dicarboxylates (also called 2-substituted pyrroline-4,5-dihydro-3,3-dicarboxylic acid diethyl esters) by iodide ion induced ring expansion of 2-[(aziridin-1-yl)-1-alkyl/aryl/heteroaryl-methylene]malonic acid diethyl esters in very good to excellent yields under mild reaction conditions. The electronic and steric impact of the substituents on the kinetics of ring expansion of N-vinyl aziridines to pyrrolines has been studied. Various diversely substituted novel pyrroline derivatives have been synthesized by this methodology and the products can be used as key intermediates in the synthesis of substituted pyrrolines, pyrroles and pyrrolidines.

The synthesis of the tetracyclic molecule 2-(9,10-dihydro-9,10-propano-anthracen-9-yl)-N-methylethanamine(2)as a homologue of the antidepressant 1-(9,10-dihydro-9,10-ethanoanthracen-9-yl)-N-methylmethaneamine (1) was described. The key intermediate 9-(prop-2-en-1-yl)-9,10-dihydro-9,10-propanoanthracen-12-one (7)was successfully synthesized via a [4+2] cycloaddition of α-bromoacrolein and 9-allyl-anthracene, followed by ring expansion and samarium diiodide deoxygenation.

The conversion of α-benzocycloalkenones to homologous β-benzocyclo-alkenones containing six, seven and eight-membered rings is reported. This wasaccomplished via a Wittig olefination-oxidative rearrangement sequence using[hydroxy(tosyloxy)iodo]-benzene (HTIB) is the oxidant, that enables the synthesis ofregioisomeric pairs of methyl-substituted β-benzocycloalkenones. The incorporation ofcarbon-13 at C-1 of the β-tetralone nucleus was also demonstrated. The Wittig-HTIBapproach is a useful alternative to analogous sequences in which Tl(NO3)3·3H2O or thePrevost combination (AgNO3/I2) are employed in the oxidation step.

A comprehensive survey of pathways leading to the generation of α-trifluoromethyl monocyclic piperidinic derivatives is provided (65 references). These compounds have been synthesized either from 6-membered rings e.g., pipecolic acid or lactam derivatives by introduction a trifluoromethyl group, from pyridine or pyridinone derivatives by reduction, and from 5-membered rings e.g., prolinol derivatives by ring expansion, from linear amines by cyclization or from dienes/dienophiles by [4 + 2]-cycloaddition.

The catalytic activation of carbon–carbon single bonds represents a major challenge in organometallic chemistry. Strained ring substrates occupy in this respect a privileged role as their inherent ring strain facilitates the desired metal insertion. Employing symmetrically substituted tert-cyclobutanols, an enantioselective rhodium(i)-catalyzed ?-carbon elimination creates alkyl-rhodium species bearing all-carbon quaternary stereogenic centers. Downstream reactions enable access to a wide range of synthetically versatile products such as substituted cyclohexenones, lactones and indanols with excellent enantioselectivities of up to 99% ee.