One hundred years of benzotropone chemistry

• Arif Dastan,
• Haydar Kilic and
• Nurullah Saracoglu

Beilstein J. Org. Chem. 2018, 14, 1120–1180, doi:10.3762/bjoc.14.98

• ]. Firstly, dichloride 41 was reduced with LiAlH4 in ether to give the monochloride 42. The reaction of 42 with DDQ produced 4,5-benzotropone (11) in 24% yield together with 28% of starting material. The key step for 11 from 42 is the electrocyclic ring expansion of dehydrogenation product 44 to the

• via the oxaphosphetane 92 intermediate by treating 4,5-benzotropone (11) with (diphenylmethylene)(phenyl)phosphine oxide (91) generated thermally or photochemically from 90 [82]. Furthermore, dimeric byproduct 93 is also formed under photochemical conditions. 2.2.2. Ring expansion reactions via a

• benzotropones (Scheme 19) [87]. Firstly, dimethylsulfoxonium methylide addition to 4,5-benzotropone (11) provided the introduction of the cyclopropane ring required for two benzohomoazocines. Beckmann rearrangement of 104 resulted in a mixture of ring expansion products 105 and 106 in nearly equal proportions

Anodic oxidation of bisamides from diaminoalkanes by constant current electrolysis

• Tatiana Golub and
• James Y. Becker

Beilstein J. Org. Chem. 2018, 14, 861–868, doi:10.3762/bjoc.14.72

• ). It has been found that anodic methoxylation of amides (and carbamates) can be utilized to form new carbon–carbon bonds [6][7] (Scheme 2). Furthermore, this anodic route could also be important from a synthetic point of view, affording ring-expansion [8][9][10] and annulation of rings [1][11][12

Continuous multistep synthesis of 2-(azidomethyl)oxazoles

• Thaís A. Rossa,
• Nícolas S. Suveges,
• Marcus M. Sá,
• David Cantillo and
• C. Oliver Kappe

Beilstein J. Org. Chem. 2018, 14, 506–514, doi:10.3762/bjoc.14.36

• ]. An alternative approach consists of the ring expansion of azirines, which can be prepared from vinyl azides 1, by the reaction with carbonyl compounds. Substituted 2-acylazirines rearrange to oxazoles in the presence of bases [24][25][26]. In addition the light-mediated synthesis of oxazoles from

• fully continuous process are described in detail. Results and Discussion Thermolysis of the vinyl azide and oxazole formation. Batch optimization The reaction conditions for the thermolysis of the vinyl azide and the subsequent ring expansion of the intermediate azirine to form the oxazole ring were

• ). HPLC monitoring of the formation of 2-(azidomethyl)oxazole 7a. Continuous sequential thermolysis of vinyl azides 1 and ring expansion of azirines 2 with bromoacetyl bromide to give 2-(bromomethyl)oxazoles 6. Continuous-flow three-step sequential synthesis of 2-(azidomethyl)oxazoles 7a–c from vinyl

Mechanochemical synthesis of thioureas, ureas and guanidines

• Vjekoslav Štrukil

Beilstein J. Org. Chem. 2017, 13, 1828–1849, doi:10.3762/bjoc.13.178

• mechanochemical synthetic strategy and applied it to a previously unknown carbodiimide insertion into sulfonimides, resulting in two-atom ring expansion and chain extension reactions [51]. Saccharin was selected as a model cyclic sulfonimide substrate, while 4-methyl-N-tosylbenzamide was employed as an acyclic

Strategies toward protecting group-free glycosylation through selective activation of the anomeric center

• A. Michael Downey and
• Michal Hocek

Beilstein J. Org. Chem. 2017, 13, 1239–1279, doi:10.3762/bjoc.13.123

• 1-O-phosphofuranosyl hexoses in good to excellent yield (Table 1, entries 1–4) [43]. The conditions provided only a very modest stereoselectivity, however, the α anomer was slightly favored regardless of the stereochemistry of the sugar at C2. Most importantly, very little or no ring expansion to

•  2) allowed for the synthesis of not only simple alcohols (Table 2, entry 1), but also disaccharides, if the accepting hydroxy group is primary (Table 2, entries 2–4). Under the optimized conditions no ring expansion was observed and a very modest preference for the α anomer was detected [49]. In

Synthesis of alkynyl-substituted camphor derivatives and their use in the preparation of paclitaxel-related compounds

• M. Fernanda N. N. Carvalho,
• Rudolf Herrmann and
• Gabriele Wagner

Beilstein J. Org. Chem. 2017, 13, 1230–1238, doi:10.3762/bjoc.13.122

• alkyne cyclisation but the ring expansion has not yet taken place. The sulfonamide reduction and formation of the ketone are just about to occur, as the oxygen atom involved is on its way of being transferred from the sulphur onto the carbon atom. The structure of 23 was established by two-dimensional 1H

An eco-compatible strategy for the diversity-oriented synthesis of macrocycles exploiting carbohydrate-derived building blocks

• Sushil K. Maurya and
• Rohit Rana

Beilstein J. Org. Chem. 2017, 13, 1106–1118, doi:10.3762/bjoc.13.110

• size, shape, heteroatoms, functional groups and stereo chemical complexity for selective binding [4]. The diversity-oriented synthesis (DOS), an algorithm in organic chemistry used to generate diverse molecules that include two-directional coupling, ring expansion methods, multidimensional coupling and

Synthesis of 1-indanones with a broad range of biological activity

• Marika Turek,
• Dorota Szczęsna,
• Marek Koprowski and
• Piotr Bałczewski

Beilstein J. Org. Chem. 2017, 13, 451–494, doi:10.3762/bjoc.13.48

• have decided to replace the double bond at the 6,7 position by the benzene ring in the new abicoviromycin derivative 169 to increase the stability while still retaining the biological activity (Figure 5). Thus, the palladium-catalyzed ring expansion of 2-alkynyl-2-hydroxybenzocyclobutenone 170 allowed

Palladium-catalyzed ring-opening reactions of cyclopropanated 7-oxabenzonorbornadiene with alcohols

• Katrina Tait,
• Oday Alrifai,
• Rebecca Boutin,
• Jamie Haner and
• William Tam

Beilstein J. Org. Chem. 2016, 12, 2189–2196, doi:10.3762/bjoc.12.209

• nucleophile at the internal cyclopropane carbon C, which could induce ring expansion to form seven-membered ring 12 . In this paper, we aim to explore the use of a palladium catalyst with an alcohol nucleophile on the ring opening of cyclopropanated oxabenzonorbornadiene with the goal of determining which

A chiral analog of the bicyclic guanidine TBD: synthesis, structure and Brønsted base catalysis

• Mariano Goldberg,
• Denis Sartakov,
• Jan W. Bats,
• Michael Bolte and
• Michael W. Göbel

Beilstein J. Org. Chem. 2016, 12, 1870–1876, doi:10.3762/bjoc.12.176

• ][18][19] and used for highly enantioselective Strecker [17] and Diels–Alder reactions [19]. Compared to guanidine 7 ring expansion into structure 10 would shift the stereogenic phenyl groups into closer proximity to a hydrogen-bonded guest molecule and thereby might improve the enantioselective

• the chiral TBD analog 10. The cyclization steps in particular, which convert the triamine 19 into the final guanidine 10, give considerably higher yields compared to guanidines with five-membered rings such as compounds 6–9. Apart from the better synthetic accessibility the formal ring expansion to

Rearrangements of organic peroxides and related processes

• Ivan A. Yaremenko,
• Vera A. Vil’,
• Dmitry V. Demchuk and
• Alexander O. Terent’ev

Beilstein J. Org. Chem. 2016, 12, 1647–1748, doi:10.3762/bjoc.12.162

• acetone (171). The Sn(OTf)2 and La(OTf)3-catalyzed reaction afforded neopentyl alcohol (173) in 62 and 70% yield, respectively [320]. The hydrogen peroxide promoted ring expansion for the synthesis of oxabicycles 176a–c was described for the first time in 1985 [321]. The reaction involved the solvolysis

On the cause of low thermal stability of ethyl halodiazoacetates

• Magnus Mortén,
• Martin Hennum and
• Tore Bonge-Hansen

Beilstein J. Org. Chem. 2016, 12, 1590–1597, doi:10.3762/bjoc.12.155

• halodiazoacetates 2a–c (Scheme 1) from 1 and studied their reactivity in Rh(II)-catalyzed reactions [11]. In the presence of Rh(II) catalysts the halodiazoacetates extrude N2 and form the corresponding Rh–carbenes which undergo typical carbenoid reactions such as cyclopropanation [11], cyclopropanation–ring

• expansion [12], and C–H insertion and Si–H insertion reactions [13]. Under Rh(II)-catalysis conditions, 2b extrudes N2 much faster than 1 (15 s vs 8 min to 50% conversion) [14]. The higher reaction rate implies a lower turnover limiting barrier in the catalytic cycle with 2b compared to 1. This example made

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

• mechanism for 4-substituted pyran-2-ones shown in Scheme 2e), Dieckmann condensations as well as oxidative ring expansion of tetramates (212, a–d in Scheme 29). Tetramates 209 are formed by Dieckmann condensation (c in Scheme 29). 2.1 Pyridinones 2.1.1 Condensation between carbonyl groups and nitrogen

The EIMS fragmentation mechanisms of the sesquiterpenes corvol ethers A and B, epi-cubebol and isodauc-8-en-11-ol

• Patrick Rabe and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2016, 12, 1380–1394, doi:10.3762/bjoc.12.132

• opening of A2+ to C2+ followed by a ring expansion to D2+, a proton transfer to E2+ and loss of water to F2+. The fragment ion F2+ is structurally the same as F1+ suggested above for m/z = 161 of 1. This structural identity of F1+ and F2+ is also reflected by the highly similar MS2 spectra of m/z = 161

Synthesis of 2,1-benzisoxazole-3(1H)-ones by base-mediated photochemical N–O bond-forming cyclization of 2-azidobenzoic acids

• Daria Yu. Dzhons and
• Andrei V. Budruev

Beilstein J. Org. Chem. 2016, 12, 874–881, doi:10.3762/bjoc.12.86

• formation of azepines 3. This confirms the suggestions of previous reports [61][62] about the impossibility of ring expansion of such aryl azides. Interestingly, under these conditions (Table 3), the yields for 3H-azepine 3a and 3c were determined as 20 and 18%, respectively. We wondered if the yields of

Opportunities and challenges for direct C–H functionalization of piperazines

• Zhishi Ye,
• Kristen E. Gettys and
• Mingji Dai

Beilstein J. Org. Chem. 2016, 12, 702–715, doi:10.3762/bjoc.12.70

• ]. Carreira et al. have developed a ring expansion of 3-oxetanone to synthesize substituted piperazines [18]. Transition metal (such as Ti, Au, and Pd) catalyzed cyclizations of linear starting materials have been used by several groups including the Schafer, Nelson, Huang, and Wolfe groups to synthesize

The aminoindanol core as a key scaffold in bifunctional organocatalysts

• Isaac G. Sonsona,
• Eugenia Marqués-López and
• Raquel P. Herrera

Beilstein J. Org. Chem. 2016, 12, 505–523, doi:10.3762/bjoc.12.50

• . The final tetracyclic indoles 8 are released from the spirocyclic intermediates 11, following a ring-expansion and rearomatization/final protodeauration cascade process (Scheme 5) [26]. In 2012, the same group reported an additional example of a one-pot multisequence reaction following a similar mode

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

• , several strategies for the preparation of the characteristic nine-membered carbocyclic ring structures have been developed. The synthetic strategies are typically based on ring expansion (Grob-type fragmentation and sigmatropic rearrangements), ring closing (metathesis and transition metal-catalyzed

2-Hetaryl-1,3-tropolones based on five-membered nitrogen heterocycles: synthesis, structure and properties

• Yury A. Sayapin,
• Inna O. Tupaeva,
• Alexandra A. Kolodina,
• Eugeny A. Gusakov,
• Vitaly N. Komissarov,
• Igor V. Dorogan,
• Nadezhda I. Makarova,
• Anatoly V. Metelitsa,
• Valery V. Tkachev,
• Sergey M. Aldoshin and
• Vladimir I. Minkin

Beilstein J. Org. Chem. 2015, 11, 2179–2188, doi:10.3762/bjoc.11.236

• significantly higher than those displayed by the parent 2-(2-hydroxyphenyl)benzoxazole (0.02) and 2-(2-hydroxyphenyl)benzothiazole (0.005) [34]. Conclusion The ring expansion of the 3,4,5,6-tetrachloro-1,2-benzoquinones occuring under coupling with 2-methylbenzoxazoles, 2-methylbenzothiazoles and 2,3,3

Synthesis of quinoline-3-carboxylates by a Rh(II)-catalyzed cyclopropanation-ring expansion reaction of indoles with halodiazoacetates

• Magnus Mortén,
• Martin Hennum and
• Tore Bonge-Hansen

Beilstein J. Org. Chem. 2015, 11, 1944–1949, doi:10.3762/bjoc.11.210

• 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. Keywords: catalysis; cyclopropanation; indole; quinoline; Rh(II); ring expansion; Introduction The

• three reactions and the yields were 90% (X = Cl), 84% (X = Br) and 70% (X = I). The synthetic utility of the heterocyclic cyclopropanation-ring expansion reaction has been limited due to very low yields, strong basic reaction conditions, high temperature and formation of large amounts of polymeric

• ), but the conversion was only ~10%, confirming that indoles with a substituent in the 7-position are poor substrates for this reaction. Methyl 3-indolylacetate (Table 1, entry 8) was a very intereresting substrate as a probe for the scope and limitations of the cyclopropanation-ring expansion since the

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 novel N-cyclopentenyl-lactams using the Aubé reaction

• Madhuri V. Shinde,
• Rohini S. Ople,
• Ekta Sangtani,
• Rajesh Gonnade and
• D. Srinivasa Reddy

Beilstein J. Org. Chem. 2015, 11, 1060–1067, doi:10.3762/bjoc.11.119

• between an azido alcohol and a ketone to provide lactams through an in situ-generated hemiacetal as a temporary tether. This helps the azide addition in an intramolecular fashion, followed by ring expansion (Scheme 1) [1][2][3][4]. Recently, we applied this reaction for the synthesis of sugar–lactam

• intermolecular ring-expansion reaction of hydroxy azides with cyclic ketones. This involves the Lewis acid-promoted formation of an N-diazonium intermediate, which undergoes rearrangement to give an iminium ether intermediate that can be hydrolyzed by a base [8][41]. Along these lines we have proposed the

Enantioselective synthesis of polyhydroxyindolizidinone and quinolizidinone derivatives from a common precursor

• Nemai Saha and
• Shital K. Chattopadhyay

Beilstein J. Org. Chem. 2014, 10, 3104–3110, doi:10.3762/bjoc.10.327

• [29], ring expansion–transannular cyclization [30], Cope–House cyclization [31], etc. as key steps. Although great advances have been made, creation of diverse entities from a single source remains important. Herein, we report a synthetic entry to some polyhydroxylated indolizidine and quinolizidine

Isoxazolium N-ylides and 1-oxa-5-azahexa-1,3,5-trienes on the way from isoxazoles to 2H-1,3-oxazines

• Alexander F. Khlebnikov,
• Mikhail S. Novikov,
• Yelizaveta G. Gorbunova,
• Ekaterina E. Galenko,
• Kirill I. Mikhailov,
• Viktoriia V. Pakalnis and
• Margarita S. Avdontceva

Beilstein J. Org. Chem. 2014, 10, 1896–1905, doi:10.3762/bjoc.10.197

• and kinetic stabilities of isoxazolium N-ylides, probable intermediates in a carbenoid- or carbene-mediated one-atom isoxazole ring expansion. Preliminary calculations at the DFT B3LYP/6-31G(d) level with the PCM solvation model for dichloromethane were performed for the model reaction of isoxazoles A

• “one-step oxazahexatriene mechanism” of carbenoid-mediated isoxazole ring expansion originates from the results of the interaction of diazo compounds 2a–c with the complimentary isoxazole 1a and azirine 5 (Scheme 5). The reaction of isoxazoles with a carbenoid can only give a (3Z)-1-oxa-5-azahexa-1,3,5

Concise, stereodivergent and highly stereoselective synthesis of cis- and trans-2-substituted 3-hydroxypiperidines – development of a phosphite-driven cyclodehydration

• Peter H. Huy,
• Julia C. Westphal and
• Ari M. P. Koskinen

Beilstein J. Org. Chem. 2014, 10, 369–383, doi:10.3762/bjoc.10.35

• ring expansions of 2-(α-hydroxyalkyl)pyrrolidines deduced to proline. 3. Based on the ring expansion of Cossy and Pardo [37], O´Brien [39] realized the synthesis of L-733,060 (80% ee). The 2-(α-hydroxyalkyl)pyrrolidine precursor was prepared from a protected pyrrolidine through sparteine-mediated