1,4-Dithianes: attractive C2-building blocks for the synthesis of complex molecular architectures

• Bram Ryckaert,
• Ellen Demeyere,
• Frederick Degroote,
• Hilde Janssens and
• Johan M. Winne

Beilstein J. Org. Chem. 2023, 19, 115–132, doi:10.3762/bjoc.19.12

• pioneered by Parham and co-workers to be the most suited (Scheme 4, 12 → 2) [28][29][30]. Parham described a ring expansion of 1,3-dithiolanes derived from α-halocarbonyls into the 1,4-dithianes which then dehydrohalogenate to afford dihydrodithiins. The ring expansion involves a 1,2-sulfur migration of a β

• -haloalkylsulfide via a cyclic sulfonium intermediate, which then ring opens and eliminates the halide to give the unsaturated 1,4-dithiane ring. We have found this Parham ring expansion to be the most practical preparatory procedure for 2 on large scale [30]. Thus, a simple condensation of ethane-1,2-dithiol (10

• then brominated with an excess of bromination reagent, which effects the 1,2-sulfur-migratory ring expansion, followed by bromination-induced dehydrogenation to the aromatic ring [31]. At the time of writing this review, the resulting benzoannelated dithiane 5 is also commercially available in small

Catalytic aza-Nazarov cyclization reactions to access α-methylene-γ-lactam heterocycles

• Bilge Banu Yagci,
• Selin Ezgi Donmez,
• Onur Şahin and
• Yunus Emre Türkmen

Beilstein J. Org. Chem. 2023, 19, 66–77, doi:10.3762/bjoc.19.6

• enantioselective organocatalytic aza-Nazarov cyclization affording six-membered heterocycles after a ring expansion of the cyclization products [22]. Rasapalli and co-workers recently developed an efficient aza-Nazarov cyclization of quinazolinonyl enones promoted by TfOH or MsOH (methanesulfonic acid) for the

Derivatives of benzo-1,4-thiazine-3-carboxylic acid and the corresponding amino acid conjugates

• Péter Kisszékelyi,
• Tibor Peňaška,
• Klára Stankovianska,
• Mária Mečiarová and
• Radovan Šebesta

Beilstein J. Org. Chem. 2022, 18, 1195–1202, doi:10.3762/bjoc.18.124

• -3(4H)-ones were also prepared by cyclization of 1,2-diaryldisulfanes with dialkyl but-2-ynedioates [24][25]. N-Substituted benzo-1,4-thiazine-2-carboxylates 4 (Figure 1) were prepared by m-CPBA-mediated oxidative ring expansion of substituted benzothiazoles [26], or via copper-catalyzed

An isoxazole strategy for the synthesis of 4-oxo-1,4-dihydropyridine-3-carboxylates

• Timur O. Zanakhov,
• Ekaterina E. Galenko,
• Mikhail S. Novikov and
• Alexander F. Khlebnikov

Beilstein J. Org. Chem. 2022, 18, 738–745, doi:10.3762/bjoc.18.74

• -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. Keywords: isoxazole; methyl nicotinate; molybdenum hexacarbonyl; 4-pyridone; ring expansion; Introduction Pyridine moieties are present in many natural products, drugs, pesticides and industrial materials. Pyridine fragments are used in drugs

Unusual highly diastereoselective Rh(II)-catalyzed dimerization of 3-diazo-2-arylidenesuccinimides provides access to a new dibenzazulene scaffold

• Anastasia Vepreva,
• Alexander S. Bunev,
• Andrey Yu. Kudinov,
• Grigory Kantin,
• Mikhail Krasavin and
• Dmitry Dar’in

Beilstein J. Org. Chem. 2022, 18, 533–538, doi:10.3762/bjoc.18.55

• intermediate D into cycloheptadiene E is an example of a relatively rare reaction of the cyclopropane-ring expansion through a 1,5-C‒C bond migration [15][16][17]. This concerted process is followed by yet another 1,5-migration of the hydrogen atom, leading to the final dimer 2a. The formation of a single

Efficient synthesis of ethyl 2-(oxazolin-2-yl)alkanoates via ethoxycarbonylketene-induced electrophilic ring expansion of aziridines

• Yelong Lei and
• Jiaxi Xu

Beilstein J. Org. Chem. 2022, 18, 70–76, doi:10.3762/bjoc.18.6

• 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. Keywords: aziridine; diazooxoester; diazo compound; ketene; oxazoline; ring expansion; Introduction Oxazoline derivatives are an important class of nitrogen and oxygen-containing five-membered unsaturated heterocycles [1] and widely exist in

• -diazo-3-oxoalkanoates first 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 current method is an activator- and catalyst-free, and clean synthetic strategy

α-Ketol and α-iminol rearrangements in synthetic organic and biosynthetic reactions

• Scott Benz and
• Andrew S. Murkin

Beilstein J. Org. Chem. 2021, 17, 2570–2584, doi:10.3762/bjoc.17.172

• use of aldehydes (R′ = H), which are usually less stable than their ketone counterparts; (2) ring expansion (Z, R = cyclic) or contraction (Z, R′ = cyclic) of strained cyclic α-ketols; (3) the use of α-dicarbonyl compounds (R′ = acyl, ester, amide, etc.), which lead to more stable β-dicarbonyl

• excellent yields (>85%), but the two most strongly electron-withdrawing, para-CF3 and meta-CH3, had lower yields (59% and 67%, respectively). Instead of a ring expansion driving the rearrangement, the authors also tested a series of α,α-diaryl-α-hydroxyaldehydes 12 and their corresponding aldimines 14

• concept by application to isolated reactions, while in the following section, examples of tandem reactions in total syntheses are given. Inspired by the direct ring expansion of 1-alkynylcyclobutanols to α-methylenecyclopentanones catalyzed by gold reported by Toste and co-workers [9], Kim et al

A comprehensive review of flow chemistry techniques tailored to the flavours and fragrances industries

• Guido Gambacorta,
• James S. Sharley and
• Ian R. Baxendale

Beilstein J. Org. Chem. 2021, 17, 1181–1312, doi:10.3762/bjoc.17.90

Synthesis of legonmycins A and B, C(7a)-hydroxylated bacterial pyrrolizidines

• Wilfred J. M. Lewis,
• David M. Shaw and
• Jeremy Robertson

Beilstein J. Org. Chem. 2021, 17, 334–342, doi:10.3762/bjoc.17.31

• Baeyer–Villiger-type ring expansion, hydrolysis and decarboxylation, cyclization and dehydration, and finally hydroxylation at C(7a). Just one month later, Bode reported the identification of an unknown gene cluster in the symbiotic bacterium Xenorhabdus stockiae [23]. Cloning and expression of this

Hydrazides in the reaction with hydroxypyrrolines: less nucleophilicity – more diversity

• Dmitrii A. Shabalin,
• Evgeniya E. Ivanova,
• Igor A. Ushakov,
• Elena Yu. Schmidt and
• Boris A. Trofimov

Beilstein J. Org. Chem. 2021, 17, 319–324, doi:10.3762/bjoc.17.29

• , namely a Brønsted acid-activated hydroxypyrroline, depend on the reaction conditions and the structure of the hydrazides. Keywords: hydrazides; pyridazines; pyrrolines; recyclization; ring expansion; Introduction Di- and tetrahydropyridazines are valuable heterocyclic motifs which are utilized as key

All-carbon [3 + 2] cycloaddition in natural product synthesis

• Zhuo Wang and
• Junyang Liu

Beilstein J. Org. Chem. 2020, 16, 3015–3031, doi:10.3762/bjoc.16.251

• % yield (Scheme 6B). This adduct 92a underwent decarboxylation to afford 92b in 72% yield [42]. Exposure of freshly prepared 92b to triazabicyclodecene [45] led to a ring-expansion/aromatization/aldol cascade producing 93, which was reduced with Et3SiH/TFA smoothly to give indane 94 in 68% yield over two

• -workers in 2014 [49] (Scheme 9A). The synthesis began with the conversion of ketone 112 into alcohol 113 in four steps, which involved a hypervalent iodine-mediated ring expansion [60]. A two-step synthesis from 113 gave epoxide 114. Epoxide 114 was converted to the corresponding β-ketoester and

Synthetic approaches to bowl-shaped π-conjugated sumanene and its congeners

• Shakeel Alvi and
• Rashid Ali

Beilstein J. Org. Chem. 2020, 16, 2212–2259, doi:10.3762/bjoc.16.186

Selective preparation of tetrasubstituted fluoroalkenes by fluorine-directed oxetane ring-opening reactions

• Clément Q. Fontenelle,
• Thibault Thierry,
• Romain Laporte,
• Emmanuel Pfund and
• Thierry Lequeux

Beilstein J. Org. Chem. 2020, 16, 1936–1946, doi:10.3762/bjoc.16.160

• ester function we expected the ring-opening reaction to proceed with the formation of additional products to alkenes E-13 and Z-13. In fact, two byproducts formed and were identified as β-hydroxymethyl-α-fluorolactone 14 and β-bromomethyl-α-fluorolactone 15 (Table 4) [30]. This ring expansion has been

An overview on disulfide-catalyzed and -cocatalyzed photoreactions

• Yeersen Patehebieke

Beilstein J. Org. Chem. 2020, 16, 1418–1435, doi:10.3762/bjoc.16.118

• , respectively, which have the same 5-electron 6-carbon cation radical character. Finally, the subsequent deprotonation and HAT by PhS− and PhSH yields the desired [4 + 2] cycloaddition products 22 and 23, respectively. In 1991, Kim and co-workers reported a disulfide-catalyzed ring expansion of cyclobutanone

• peptide-catalyzed enantioselective cycloaddition. Disulfide-catalyzed [3 + 2] methylenecyclopentane annulations. Disulfide as a HAT cocatalyst in the [4 + 2] cycloaddition reaction. Proposed mechanism of the [4 + 2] cycloaddition reaction using disulfide as a HAT cocatalyst. Disulfide-catalyzed ring

• expansion of vinyl spiro epoxides. Disulfide-catalyzed aerobic oxidation of diarylacetylene. Disulfide-catalyzed aerobic photooxidative cleavage of olefins. Disulfide-catalyzed aerobic oxidation of 1,3-dicarbonyl compounds. Proposed mechanism of the disulfide-catalyzed aerobic oxidation of 1,3-dicarbonyl

Recent synthesis of thietanes

• Jiaxi Xu

Beilstein J. Org. Chem. 2020, 16, 1357–1410, doi:10.3762/bjoc.16.116

• thietane backbones developed during 1966 to 2019. Keywords: cycloaddition; cyclization; ring contraction; ring expansion; thietane; thiotherification; Review 1. Introduction Thietanes are a class of important aliphatic four-membered thiaheterocycles. Some simple alkyl and dialkyl thietanes are components

• thietane-2-ylideneacetates 397 as byproducts [104] (Scheme 83). 4. Synthesis via the ring expansions and contractions 4.1 Synthesis via ring expansion The ring expansions of thiiranes are alternative ways to prepare thietane derivatives. The transformations included the nucleophilic ring expansion of (1

• -haloalkyl)thiiranes with various nucleophiles, nucleophilic ring expansion of thiiranes with sulfur ylides, and the electrophilic ring expansion of thiiranes with carbenes generated from sulfur ylides under the catalysis of transition-metal catalysts. 4.1.1 Synthesis via nucleophilic ring expansion of 2-(1

Synthesis of C₇₀-fragment buckybowls bearing alkoxy substituents

• Yumi Yakiyama,
• Shota Hishikawa and
• Hidehiro Sakurai

Beilstein J. Org. Chem. 2020, 16, 681–690, doi:10.3762/bjoc.16.66

• that with C60 fragment because most of them consists of acene and/or pyrene units, which might give unique photochemical and electrochemical properties. Recently, we synthesized a buckybowl C28H14 1, which is corresponding to a 40% fragment of C70, from C60-fragment sumanene (2) in three steps via ring

• expansion by Wagner–Meerwein rearrangement, followed by Pd-catalyzed annulation (Figure 1) [18]. An UV–vis spectroscopy study revealed that the electronic character of 1 rather resembled that of an indenopyrene moiety than that of benzopyrene. Our synthetic route allows to easily introduce substituents on

Regio- and stereoselective synthesis of new ensembles of diversely functionalized 1,3-thiaselenol-2-ylmethyl selenides by a double rearrangement reaction

• Svetlana V. Amosova,
• Andrey A. Filippov,
• Nataliya A. Makhaeva,
• Alexander I. Albanov and
• Vladimir A. Potapov

Beilstein J. Org. Chem. 2020, 16, 515–523, doi:10.3762/bjoc.16.47

• potassium selenocyanate proceeded via a rearrangement with ring expansion, leading to a six-membered 2,3-dihydro-1,4-thiaselenin-2-yl selenocyanate (kinetic product) which in turn underwent rearrangement with ring contraction to a 1,3-thiaselenol-2-ylmethyl selenocyanate (thermodynamic product). These

• -dihydro-1,4-thiaselenines (up to 96% yields) – products of rearrangement with ring expansion, which in turn underwent rearrangement with ring contraction, forming a new family of 1,3-thiaselenoles in up to 99% yield. The article was included in the RSC themed web collection "The chemistry of Selenium

• with various nucleophiles (Scheme 4) were accompanied by ring expansion leading to six-membered functionalized dihydrothiaselenines. Furthermore, the reaction of thiaselenole 1 with ammonium thiocyanate also gave the six-membered 2,3-dihydro-1,4-thiaselenin-2-yl thiocyanate (Scheme 4) [46]. It was

Cyclopropanation–ring expansion of 3-chloroindoles with α-halodiazoacetates: novel synthesis of 4-quinolone-3-carboxylic acid and norfloxacin

• Sara Peeters,
• Linn Neerbye Berntsen,
• Pål Rongved and
• Tore Bonge-Hansen

Beilstein J. Org. Chem. 2019, 15, 2156–2160, doi:10.3762/bjoc.15.212

• 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. Keywords

• : catalysis; cyclopropanation; indole; norfloxacin quinoline; quinolone; Rh(II); ring expansion; Introduction The development and use of metal carbenes occupy a central part in the field of the C–H functionalization [1]. Among the metal carbenes, the transient Rh carbenes, usually made by Rh-catalyzed

• substituted indoles with R-groups in positions 3–7. When R at position 8 = Cl, the reaction was sluggish, and when R at position 2 = Me there was no quinoline formed. The overall transformation is formally a cyclopropanation of the indole C2–C3 double bond followed by a spontaneous ring expansion of the

The cyclopropylcarbinyl route to γ-silyl carbocations

• Xavier Creary

Beilstein J. Org. Chem. 2019, 15, 1769–1780, doi:10.3762/bjoc.15.170

• bond c to the cationic center gives rearranged cation 44, the source of the rearranged acetate 39. Ring expansion via migration of bond b in 43 gives the β-trimethylsilyl-stabilized cyclobutyl cation 45, and subsequent desilylation provides cyclobutene (35). Alternatively, cyclobutyl to homoallylic

N-(1-Phenylethyl)aziridine-2-carboxylate esters in the synthesis of biologically relevant compounds

• Iwona E. Głowacka,
• Aleksandra Trocha,
• Andrzej E. Wróblewski and
• Dorota G. Piotrowska

Beilstein J. Org. Chem. 2019, 15, 1722–1757, doi:10.3762/bjoc.15.168

Synthesis of aryl cyclopropyl sulfides through copper-promoted S-cyclopropylation of thiophenols using cyclopropylboronic acid

• Emeline Benoit,
• Ahmed Fnaiche and
• Alexandre Gagnon

Beilstein J. Org. Chem. 2019, 15, 1162–1171, doi:10.3762/bjoc.15.113

• diastereoselective [3 + 2] cycloaddition reaction [14][15]. The ring expansion sequence 1 → 2 → 6 → 8 has been used as a key step in the synthesis of (±)-fragranol [16], (±)-grandisol [16], (±)-α-cuparenone [17] and (±)-herbertene [17]. Aryl cycloropyl sulfides 1 are most frequently prepared by cyclopropylation of

Multicomponent reactions (MCRs): a useful access to the synthesis of benzo-fused γ-lactams

• Edorta Martínez de Marigorta,
• Jesús M. de Los Santos,
• Ana M. Ochoa de Retana,
• Javier Vicario and
• Francisco Palacios

Beilstein J. Org. Chem. 2019, 15, 1065–1085, doi:10.3762/bjoc.15.104

• dialkyl ones (Scheme 26, path A) [104]. When cyclopropyl ketones 91 were used as substrates, a ring expansion and a new quaternary centre formation happened through the multicomponent reaction to produce pentacyclic derivatives 92 (Scheme 26, path B) [105]. These reactions, either with arenes 86 or

Oxidative radical ring-opening/cyclization of cyclopropane derivatives

• Yu Liu,
• Qiao-Lin Wang,
• Zan Chen,
• Cong-Shan Zhou,
• Bi-Quan Xiong,
• Pan-Liang Zhang,
• Chang-An Yang and
• Quan Zhou

Beilstein J. Org. Chem. 2019, 15, 256–278, doi:10.3762/bjoc.15.23

• presence of 2-indobenzoic acid anion. Recently, Shi’s group developed the first ring expansion of MCPs 63 with a nitrogen atom to furnish azetidines 64 (Scheme 14) [82]. The author proposed that Rh(II) had an effective impact on the reactions and could improve the reaction yields. Unfortunately, the MCPs

Ruthenium-based olefin metathesis catalysts with monodentate unsymmetrical NHC ligands

• Veronica Paradiso,
• Chiara Costabile and
• Fabia Grisi

Beilstein J. Org. Chem. 2018, 14, 3122–3149, doi:10.3762/bjoc.14.292

• ). Importantly, later on Grubbs and co-workers utilized this kind of catalysts, featuring a chelating N-to-Ru arm, for the preparation of cyclic polymers from cyclic monomers via a ring-expansion metathesis polymerization (REMP) process [38][39]. With the aim of developing catalysts suitable for covalent

Unprecedented nucleophile-promoted 1,7-S or Se shift reactions under Pummerer reaction conditions of 4-alkenyl-3-sulfinylmethylpyrroles

• Takashi Go,
• Akane Morimatsu,
• Hiroaki Wasada,
• Genzoh Tanabe,
• Osamu Muraoka,
• Yoshiharu Sawada and
• Mitsuhiro Yoshimatsu

Beilstein J. Org. Chem. 2018, 14, 2722–2729, doi:10.3762/bjoc.14.250

• radical cyclisations [28] and the 1,7-electrocyclisation of azomethine ylides or ring expansion sequences [29]), most of these approaches are ineffective for synthesising pyrrolo[3,2-c]azepines. Recently, Echavarren and Beller reported a novel Au- or Pt-catalysed cycloisomerisation reaction of pyrrole-2