Exploring endoperoxides as a new entry for the synthesis of branched azasugars

• Svenja Domeyer,
• Mark Bjerregaard,
• Henrik Johansson and
• Daniel Sejer Pedersen

Beilstein J. Org. Chem. 2017, 13, 644–647, doi:10.3762/bjoc.13.63

• anticipated that the reaction conditions could be optimised to favour the desired diol product. Ring contraction of acetonide-protected endoperoxide 23 by treatment with triphenylphosphine provided ready access to tetrahydrofuran 24 in good yield [21]. Finally, treatment of endoperoxide 23 with Co(salen) gave

An effective one-pot access to polynuclear dispiroheterocyclic structures comprising pyrrolidinyloxindole and imidazothiazolotriazine moieties via a 1,3-dipolar cycloaddition strategy

• Alexei N. Izmest’ev,
• Galina A. Gazieva,
• Natalya V. Sigay,
• Sergei A. Serkov,
• Valentina A. Karnoukhova,
• Vadim V. Kachala,
• Alexander S. Shashkov,
• Igor E. Zanin,
• Angelina N. Kravchenko and
• Nina N. Makhova

Beilstein J. Org. Chem. 2016, 12, 2240–2249, doi:10.3762/bjoc.12.216

• aromatic aldehyde, similarly to the synthesis of compounds 1a–c. The reaction of aromatic aldehydes with imidazotriazinethiones without phenyl substituents in acidic media results in hydrazone formation and triazine-ring contraction [45]. The reaction of compounds 1d–f with sarcosine and isatins 3a,d,f

Biosynthesis of oxygen and nitrogen-containing heterocycles in polyketides

• Franziska Hemmerling and
• Frank Hahn

Beilstein J. Org. Chem. 2016, 12, 1512–1550, doi:10.3762/bjoc.12.148

• up the benzobisfuran motif in 106, the second rearranges the anthraquinone in 106 to the xanthone in 107 and the third is an oxidative ring contraction towards the cyclopentanone in 94 (Scheme 16). After several enzymatic post-PKS modifications, the oxoaverantin (OAVN) cyclase transforms 5

Synthesis of Xenia diterpenoids and related metabolites isolated from marine organisms

• Tatjana Huber,
• Lara Weisheit and
• Thomas Magauer

Beilstein J. Org. Chem. 2015, 11, 2521–2539, doi:10.3762/bjoc.11.273

• state, the substituents are oriented with minimal steric hindrance to give the E,E-configured nine-membered ring. Ring contraction reactions of 13-membered lactams afford cyclononenes via intramolecular acyl transfer reactions. The configuration of the double bond derives from precursor D and thus

Recent highlights in biosynthesis research using stable isotopes

• Jan Rinkel and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2015, 11, 2493–2508, doi:10.3762/bjoc.11.271

• shift to the tertiary cation 49. This step was elucidated using (10-2H)GGPP to follow the transannular movement of Hb. Ring contraction leads to the tetracyclic cation 50, which rearranges to 51 explaining the observed lost linkage between C-9 and C-10. Quenching of this cation with water leads to the

Nitro-Grela-type complexes containing iodides – robust and selective catalysts for olefin metathesis under challenging conditions

• Andrzej Tracz,
• Mateusz Matczak,
• Katarzyna Urbaniak and
• Krzysztof Skowerski

Beilstein J. Org. Chem. 2015, 11, 1823–1832, doi:10.3762/bjoc.11.198

• rather poor selectivity (70%) was observed for nG. Low selectivity of the reaction promoted by nG was the result of the formation of 13% of GC-observable byproducts (originated from double bond isomerization and ring contraction) as well as 10% of oligomeric/polymeric byproducts. The nG-SIPr-I2 was more

Selected synthetic strategies to cyclophanes

• Sambasivarao Kotha,
• Mukesh E. Shirbhate and
• Gopalkrushna T. Waghule

Beilstein J. Org. Chem. 2015, 11, 1274–1331, doi:10.3762/bjoc.11.142

Synthesis of 1,2-cis-2-C-branched aryl-C-glucosides via desulfurization of carbohydrate based hemithioacetals

• Henok H. Kinfe,
• Fanuel M. Mebrahtu,
• Mandlenkosi M. Manana,
• Kagiso Madumo and
• Mokela S. Sokamisa

Beilstein J. Org. Chem. 2015, 11, 583–588, doi:10.3762/bjoc.11.64

• -1 Raney Nickel in acetone at ambient temperature with no evidence on the formation of glucoside 4a and ring contraction by sulfur extrusion. Consideration of the conditions required for the formation of the mixture of carbaldehydes 5a and 5a’ suggests that hydrogenolytic desulfurization of the C(sp2

Hetero-Diels–Alder reactions of hetaryl and aryl thioketones with acetylenic dienophiles

• Grzegorz Mlostoń,
• Paulina Grzelak,
• Maciej Mikina,
• Anthony Linden and
• Heinz Heimgartner

Beilstein J. Org. Chem. 2015, 11, 576–582, doi:10.3762/bjoc.11.63

• ). The course of the oxidation reactions for these thiopyrans differs from a similar process reported for Se-containing systems. In these cases, ring contraction and elimination of an aryl group, but no formation of an oxidized product, was observed [21]. The same report describes the appearance of

Magnesium bis(monoperoxyphthalate) hexahydrate as mild and efficient oxidant for the synthesis of selenones

• Andrea Temperini,
• Massimo Curini,
• Ornelio Rosati and
• Lucio Minuti

Beilstein J. Org. Chem. 2014, 10, 1267–1271, doi:10.3762/bjoc.10.127

• fully characterized by classical spectroscopic analysis and high resolution mass spectrometry. However, it is important to note that the oxidation of selenide 1b with MCPBA in methanol did not give selenone 2b but the corresponding methyl cyclopentyl ketone deriving from ring contraction [20]. On the

Use of activated enol ethers in the synthesis of pyrazoles: reactions with hydrazine and a study of pyrazole tautomerism

• Denisa Tarabová,
• Stanislava Šoralová,
• Martin Breza,
• Marek Fronc,
• Wolfgang Holzer and
• Viktor Milata

Beilstein J. Org. Chem. 2014, 10, 752–760, doi:10.3762/bjoc.10.70

• species, which are possible for ester compounds 5b and 5c are shown in Figure 3, with the last character of the label being an E indicates enol, O denotes oxo, and (Z,E) – HO vs OH/=O Compound 5b was previously prepared by a ring contraction of pyrimidine-2,4-dione [19]. It is known from the literature

Isocyanide-based multicomponent reactions towards cyclic constrained peptidomimetics

• Gijs Koopmanschap,
• Eelco Ruijter and
• Romano V.A. Orru

Beilstein J. Org. Chem. 2014, 10, 544–598, doi:10.3762/bjoc.10.50

• β-amino acids, in which the ring was varied, were combined with a variety of aldehydes and isocyanides in methanol to obtain the desired β-lactams. In Scheme 6, a plausible mechanism of this reaction is shown. The β-lactam ring herein is formed via a ring contraction of the seven-membered

Advancements in the mechanistic understanding of the copper-catalyzed azide–alkyne cycloaddition

• Regina Berg and
• Bernd F. Straub

Beilstein J. Org. Chem. 2013, 9, 2715–2750, doi:10.3762/bjoc.9.308

• as well. The C–N bond is formed concomitantly to the formation of a double bond between the copper ion and the C1 atom of the acetylide. This unusual six-membered copper(III) metallacycle then undergoes a transannular ring contraction to give the copper triazolide. The latter can be protonated to

Gold(I)-catalyzed enantioselective cycloaddition reactions

• Fernando López and
• José L. Mascareñas

Beilstein J. Org. Chem. 2013, 9, 2250–2264, doi:10.3762/bjoc.9.264

• experimental data as well as theoretical calculations suggested that both cycloadducts, 14 and 15, arise from the same intermediate, the cycloheptanyl metal–carbene species VIII, which might evolve through a 1,2-hydrogen shift to give the seven-membered carbocycles 14 (Scheme 8, a), or by a ring-contraction

• π-acceptor ligands, such as phosphites or phosphoramidites favor the formation of the [4 + 2] adduct of type 15, DFT calculations indicated that the activation barriers for the ring-contraction process leading to these six-membered systems, and the 1,2-H shifts that retain that seven-membered ring

• , are quite similar [60]. Therefore, it seemed plausible that this type of chiral catalysts could also provide an enantioselective entry to the seven-membered adducts of type 14, provided that the ring-contraction route could be slightly deactivated. The viability of this hypothesis was validated by

The chemistry of isoindole natural products

• Klaus Speck and
• Thomas Magauer

Beilstein J. Org. Chem. 2013, 9, 2048–2078, doi:10.3762/bjoc.9.243

• methylate 48, an unprecedented benzilic acid-type rearrangement (ring contraction) was discovered. Exposure of 48 to copper(I) chloride in methanol furnished (+)-46 in 95% yield, which after removal of the protecting group gave (+)-K252a (27). Staurosporine (26) could be synthesized in five steps from 48

The preparation of several 1,2,3,4,5-functionalized cyclopentane derivatives

• André S. Kelch,
• Peter G. Jones,
• Ina Dix and
• Henning Hopf

Beilstein J. Org. Chem. 2013, 9, 1705–1712, doi:10.3762/bjoc.9.195

• chiral (C2-symmetrical) configuration (trans-orientation of hydrogen atoms) of 24. Fortunately, recrystallization of a 24/25-mixture from diethyl ether provided single crystals of 24 suitable for an X-ray structure investigation which proved the chiral configuration (Figure 2). The ring contraction of a

• Tolbert [7]. The preparation of derivatives of 16 better suited for nucleophilic substitution and elimination. Preparation of the pentaacetate 21 from 16. Preparation of the cycloheptadiene octaesters 24/25 according to Diels [11] and Le Goff [13], respectively, followed by ring contraction to the

4-Pyridylnitrene and 2-pyrazinylcarbene

• Curt Wentrup,
• Ales Reisinger and
• David Kvaskoff

Beilstein J. Org. Chem. 2013, 9, 754–760, doi:10.3762/bjoc.9.85

• . 1 mbar due to a shock wave induced by rapid distillation of the azide into the furnace under high vacuum, results in ring contraction to cyanocyclopentadiene 6 [12]. Similar pyrolysis of 4-azidopyridine results in ring contraction to a mixture of 2- and 3-cyanopyrroles 16 and 17. Because the ring

• pyrolysis tube. The exact mechanisms of ring contraction in arylnitrenes are under investigation, but in the case of 2-pyridylnitrene (Scheme 2) three routes to the cyanopyrroles have been described [10], as has their thermal interconversion [9][10]. The analogous, concerted ring contraction of

• cyanopyrroles 16 and 17 and azo compound 25, but the overall yield is again poor, ca. 30%, due to heavy charring in the pyrolysis tube. It is worth noting that ethynylimidazoles, which could be thought of as ring-contraction products of carbene 21, were not detectable. Photolysis of either 4-azidopyridine (18

3-Pyridylnitrene, 2- and 4-pyrimidinylcarbenes, 3-quinolylnitrenes, and 4-quinazolinylcarbenes. Interconversion, ring expansion to diazacycloheptatetraenes, ring opening to nitrile ylides, and ring contraction to cyanopyrroles and cyanoindoles

• Curt Wentrup,
• Nguyen Mong Lan,
• Adelheid Lukosch,
• Pawel Bednarek and
• David Kvaskoff

Beilstein J. Org. Chem. 2013, 9, 743–753, doi:10.3762/bjoc.9.84

• similarly afford 3-cyanoindoles. 2-Pyrimidinylcarbenes rearrange to 3-pyridylnitrenes, but 4-pyrimidinylcarbenes and 4-quinazolinylcarbenes do not necessarily rearrange to the corresponding 3-pyridylnitrenes or 3-quinolylnitrenes. The ring contraction reactions are interpreted in terms of ring opening of

• to the reaction of 2-pyridylnitrene, where 2-cyanopyrrole is formed preferentially [11]. In the analogous case of 3-quinolylnitrene, 3-cyanoindole is formed exclusively under mild FVT conditions [21]. A direct, concerted ring contraction in 3-pyridylnitrene would be possible (via 17 and 18, Scheme 5

• ), but such reactions have considerably higher activation barriers, ca. 30 kcal/mol in the case of phenylnitrene [22]. A transition state for the concerted ring contraction of 3-pyridylnitrene (10) to 3H-3-cyanopyrrole (18) was not found, but we calculate a barrier of 23 kcal/mol for the concerted ring

A computational study of base-catalyzed reactions of cyclic 1,2-diones: cyclobutane-1,2-dione

• Nargis Sultana and
• Walter M. F. Fabian

Beilstein J. Org. Chem. 2013, 9, 594–601, doi:10.3762/bjoc.9.64

• ] leads to ring contraction, e.g., the rearrangement of cyclobutane-1,2-dione (1) to 1-hydroxycyclopropanecarboxylate (2) [5][6][7]. In contrast, cyclobut-3-ene-1,2-diones 3 react to 2-oxobut-3-enoates 4 (at least formally according to path B) [8], whereas benzocyclobutene-1,2-diones 5 lead to 2

Synthesis of 2,3-dihydronaphtho[2,3-d][1,3]thiazole-4,9-diones and 2,3-dihydroanthra[2,3-d][1,3]thiazole-4,11-diones and novel ring contraction and fusion reaction of 3H-spiro[1,3-thiazole-2,1'-cyclohexanes] into 2,3,4,5-tetrahydro-1H-carbazole-6,11-diones

• Lidia S. Konstantinova,
• Kirill A. Lysov,
• Ljudmila I. Souvorova and
• Oleg A. Rakitin

Beilstein J. Org. Chem. 2013, 9, 577–584, doi:10.3762/bjoc.9.62

• ., ring contraction and fusion products. A plausible mechanism was proposed for the formation of the products. Keywords: anthracene-1,4-diones; 1H-carbazole-6,11-diones; fused thiazoles; fusion reaction; heterocycles; naphthoquinones; ring contraction; sulfur–nitrogen; Introduction The 1,4

• -ethyldiisopropylamine in the reaction of 2-(methylamino)anthracene-1,4-diones with sulfur monochloride was discovered and explained. 3H-Spiro(thiazol-2,1'-cyclohexanes) underwent a new ring contraction and fusion reaction resulting in the formation of tetrahydroindoles. Experimental Melting points were determined on a

Valence isomerization of cyclohepta-1,3,5-triene and its heteroelement analogues

• Helen Jansen,
• J. Chris Slootweg and
• Koop Lammertsma

Beilstein J. Org. Chem. 2011, 7, 1713–1721, doi:10.3762/bjoc.7.201

• of aromatic substrates [61][62][63][64]. In addition, also photo-oxidation of benzene creates this isomeric pair [65][66]. Depicted in Scheme 4 are the most important reactions that the parent oxepine (3) and benzene oxide (4) can undergo. Irradiation of oxepine results in ring contraction yielding 2

Use of mixed Li/K metal TMP amide (LiNK chemistry) for the synthesis of [2.2]metacyclophanes

• Marco Blangetti,
• Patricia Fleming and
• Donal F. O'Shea

Beilstein J. Org. Chem. 2011, 7, 1249–1254, doi:10.3762/bjoc.7.145

• possible explanation for this is the cumbersome methods required for their synthesis. Typical approaches have utilised Wurtz coupling, the oxidation and thermal (500–600 °C) extrusion of SO2 from dithia[3.3]cyclophanes or the photochemical ring contraction of diselena[3.3]cyclophanes [17][18][19][20][21

Recent developments in gold-catalyzed cycloaddition reactions

• Fernando López and
• José L. Mascareñas

Beilstein J. Org. Chem. 2011, 7, 1075–1094, doi:10.3762/bjoc.7.124

• with a gold(I) catalyst bearing a π-acceptor ligand, such as a triarylphosphite (Au8/AgSbF6, Scheme 28) [98][99][102][103]. Several experimental results as well as theoretical calculations suggest that the observed (4 + 2) cycloadducts 53 are indeed the result of a ring contraction process (1,2-alkyl

• is formed. Nonetheless, the formation of adducts 46 by a ring contraction process in intermediate XXXI cannot be fully discarded. More recently, Fürstner and coworkers have further demonstrated this type of dichotomy depending on the electronic characteristics of the ligands at gold (Figure 2) [106

Gold-catalyzed regioselective oxidation of terminal allenes: formation of α-methanesulfonyloxy methyl ketones

• Yingdong Luo,
• Guozhu Zhang,
• Erik S. Hwang,
• Thomas A. Wilcoxon and
• Liming Zhang

Beilstein J. Org. Chem. 2011, 7, 596–600, doi:10.3762/bjoc.7.69

• aminoimidazoles [34], the generation of cyclopropanone–oxyallyl intermediates [35], and ring contraction [36]. Their direct synthesis from corresponding ketones can be realized via oxidation by using either CuO/MsOH [37][38] or PhI(OH)OMs [39]. However, the former method uses stoichiometric amounts of copper

Molecular rearrangements of superelectrophiles

• Douglas A. Klumpp

Beilstein J. Org. Chem. 2011, 7, 346–363, doi:10.3762/bjoc.7.45

• formation of a tertiary-carbenium ion 45c which ultimately leads to the stable enone structure 47. When 2-cyclohexen-1-one (47) is reacted with HF-SbF5, a ring contraction occurs to give 3-methyl-2-cyclopenten-1-one (50, Scheme 11) [23]. This conversion involves diprotonation of 48 to give a