Synthesis of the furo[2,3-b]chromene ring system of hyperaspindols A and B

• Danielle L. Paterson and
• David Barker

Beilstein J. Org. Chem. 2015, 11, 265–270, doi:10.3762/bjoc.11.29

• it to determine the complete relative stereochemistry of the natural products. Results and Discussion The proposed route to furo[2,3-b]chromene 7 was based around the preparation of benzylic ketone 8 which was hoped to under acidic conditions would undergo cyclisation to give 7 (Figure 2). We have

Electrochemical selenium- and iodonium-initiated cyclisation of hydroxy-functionalised 1,4-dienes

• Philipp Röse,
• Steffen Emge,
• Jun-ichi Yoshida and
• Gerhard Hilt

Beilstein J. Org. Chem. 2015, 11, 174–183, doi:10.3762/bjoc.11.18

• and pyran derivatives under indirect electrochemical conditions generating selenium or iodonium cations. The reactions proceed in good yields and regioselectivities for the formation of single diastereomers. Keywords: cyclic ethers; cyclisation; 1,4-dienes; electrochemistry; iodonium; selenium

• -dienols were transformed into cyclic phenylselenoethers by intramolecular cyclisation using selenium cations generated by indirect electrolysis. The reaction was carried out by electrolysing a mixture of the 1,4-dienol, diphenyl diselenide and tetraethylammonium bromide in CH3CN at room temperature in an

• undivided cell, using platinum foil electrodes (constant current 10 mA). In this investigation only diphenyl diselenide was used as selenium source. These reaction conditions led to the formation of products of type 3 as exclusive diastereomers (Scheme 2). The cyclisation of 2 could lead to a number of

The Shono-type electroorganic oxidation of unfunctionalised amides. Carbon–carbon bond formation via electrogenerated N-acyliminium ions

• Alan M. Jones and
• Craig E. Banks

Beilstein J. Org. Chem. 2014, 10, 3056–3072, doi:10.3762/bjoc.10.323

• are useful reactive synthetic intermediates in a variety of important carbon–carbon bond forming and cyclisation strategies in organic chemistry. The advent of an electrochemical anodic oxidation of unfunctionalised amides, more commonly known as the Shono oxidation, has provided a complementary route

• -acyliminium ion cyclisation strategy in the presence of a disubstituted acetylene nucleophile. Anodic oxidation proceeded in high yield and a smooth cyclisation of the pendant acetylene nucleophile was triggered by treatment with titanium tetrachloride. Ozonolysis of the chloromethyl alkene intermediate

Come-back of phenanthridine and phenanthridinium derivatives in the 21st century

• Lidija-Marija Tumir,
• Marijana Radić Stojković and
• Ivo Piantanida

Beilstein J. Org. Chem. 2014, 10, 2930–2954, doi:10.3762/bjoc.10.312

• from aminobiphenyl and arylaldehyde in dichloromethane in the presence of molecular sieves at room temperature. Radical cyclisation in the presence of (tert-butyl)peroxide in chlorobenzene at 140–150 °C for 48 h, yielded the corresponding phenanthridines in moderate yields. The t-BuO• radical

• second step under the catalysis of Pd(OAc)2 comprised both cyclisation and oxidation in a single step: a dehydrogenative C–H amination with PhI(OAc)2 as oxidant and removal of the picolinamide group followed by oxidation with Cu(OAc)2. This strategy afforded phenanthridines in moderate to good yields (up

• to 65% for the second step). Bowman et al. reported a palladium-mediated route using imidoyl-selenides as precursors besides the radical route. Comparison of the cyclisation yields for the same set of phenanthridine derivatives revealed an overall better efficiency of the t-BuO• radical-assisted

Gold(I)-catalysed synthesis of a furan analogue of thiamine pyrophosphate

• Amjid Iqbal,
• El-Habib Sahraoui and
• Finian J. Leeper

Beilstein J. Org. Chem. 2014, 10, 2580–2585, doi:10.3762/bjoc.10.270

• , involving gold(I)-catalysed cyclisation of an alkynyl alcohol to form the furan ring. The furan analogue of thiamine diphosphate (ThDP) was also made and tested for binding to and inhibition of pyruvate decarboxylase (PDC) from Zymomonas mobilis (overexpressed in E. coli with a N-terminal His-tag). It is a

• very strong inhibitor, with a Ki value of 32.5 pM. It was also shown that the furan analogue of thiamine can be functionalised at the C-2 position, which will allow access to mimics of reaction intermediates of various ThDP-dependent enzymes. Keywords: furan synthesis; gold-catalysed cyclisation

• dehydrative cyclisation reaction of alkynyl alcohols catalysed by simple gold(I) salts [31]. The reaction proceeds rapidly under mild, open flask conditions to provide aromatic heterocycles such as furans, pyrroles and thiophenes in high yield with low catalyst loadings (Scheme 2). Following this synthetic

Palladium-catalysed cyclisation of alkenols: Synthesis of oxaheterocycles as core intermediates of natural compounds

• Miroslav Palík,
• Jozef Kožíšek,
• Peter Koóš and
• Tibor Gracza

Beilstein J. Org. Chem. 2014, 10, 2077–2086, doi:10.3762/bjoc.10.216

• ., Koreničova 1, SK-811 03 Bratislava, Slovakia 10.3762/bjoc.10.216 Abstract The study of Pd-catalysed cyclisation reactions of alkenols using different catalytic systems is reported. These transformations affect the stereoselective construction of mono- and/or bicyclic oxaheterocyclic derivatives depending on

• a starting alkenol. The substrate scope and proposed mechanism of Pd-catalysed cyclisation reactions are also discussed. Moreover, the diastereoselective Pd-catalysed cyclisation of appropriate alkenols to tetrahydrofurans and subsequent cyclisation provided properly substituted 2,5-dioxabicyclo

• process, the terminal carbon–carbon double bond is bis-O-functionalised with two hydroxy groups by sequential intramolecular–intramolecular reaction. Based on our continuous interest in the palladium-catalysed cyclisation reactions and their applications in natural product syntheses [24][25], we have

Comparing kinetic profiles between bifunctional and binary type of Zn(salen)-based catalysts for organic carbonate formation

• Carmen Martín and
• Arjan W. Kleij

Beilstein J. Org. Chem. 2014, 10, 1817–1825, doi:10.3762/bjoc.10.191

• Scheme 2 first the epoxide coordinates to the Zn center allowing Lewis acid activation following the ring opening by nucleophilic attack of X. Then, carbon dioxide insertion into the metal–oxygen bond takes place and a consecutive cyclisation step (ring closure) occurs to give the cyclic carbonate and

Synthesis of trifluoromethyl-substituted pyrazolo[4,3-c]pyridines – sequential versus multicomponent reaction approach

• Barbara Palka,
• Angela Di Capua,
• Maurizio Anzini,
• Gyté Vilkauskaité,
• Algirdas Šačkus and
• Wolfgang Holzer

Beilstein J. Org. Chem. 2014, 10, 1759–1764, doi:10.3762/bjoc.10.183

• afforded the corresponding pyrazolo[4,3-c]pyridine 5-oxides 7a–c by a regioselective 6-endo-dig cyclisation [30] in high yields. Moreover, we tested an alternative approach to access compounds 7 through multicomponent reactions (MCR). Attempts to react chloroaldehyde 2 with hydroxylamine hydrochloride and

Multichromophoric sugar for fluorescence photoswitching

• Stéphane Maisonneuve,
• Rémi Métivier,
• Pei Yu,
• Keitaro Nakatani and
• Juan Xie

Beilstein J. Org. Chem. 2014, 10, 1471–1481, doi:10.3762/bjoc.10.151

• , 9-OF undergoes a cyclisation photoreaction yielding the molecule in its colored and non-fluorescent closed form (9-CF), as revealed by the appearance of a large absorption band peaking in the 500–700 nm range (Figure 3c, full blue line). The photostationary state (PSS) reached under 335 nm

Syntheses of fluorooxindole and 2-fluoro-2-arylacetic acid derivatives from diethyl 2-fluoromalonate ester

• Antal Harsanyi,
• Graham Sandford,
• Dmitri S. Yufit and
• Judith A.K. Howard

Beilstein J. Org. Chem. 2014, 10, 1213–1219, doi:10.3762/bjoc.10.119

• ester is utilised as a building block for the synthesis of 2-fluoro-2-arylacetic acid and fluorooxindole derivatives by a strategy involving nucleophilic aromatic substitution reactions with ortho-fluoronitrobenzene substrates followed by decarboxylation, esterification and reductive cyclisation

• , presumably because of the high solubility of the amino acid intermediate in the aqueous reaction mixture and the well-established difficulty of direct amide bond formation processes. Consequently, before carrying out the nitro group reduction and amide forming cyclisation reactions, the acids 4a–e were

Rapid pseudo five-component synthesis of intensively blue luminescent 2,5-di(hetero)arylfurans via a Sonogashira–Glaser cyclization sequence

• Fabian Klukas,
• Alexander Grunwald,
• Franziska Menschel and
• Thomas J. J. Müller

Beilstein J. Org. Chem. 2014, 10, 672–679, doi:10.3762/bjoc.10.60

• solvent. Pseudo five-component Sonogashira–Glaser cyclisation synthesis of 2,5-di(hetero)arylfurans 2 (aobtained from the THP-protected precursor). Evaluation of different reaction conditions. Selected absorption and emission data (recorded in dichloromethane at T = 293 K). Selected cyclovoltammetrica

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

• combination of deprotection and activation is also possible and is found in the literature as an Ugi Deprotection/Activation–Cyclisation (UDAC). In addition, other MCR-post-condensation reactions, especially for macrocycles, include intramolecular aryl couplings, amidations, SnAr reactions, nucleophilic

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

• , initial deprotection of D (to liberate the Lewis-basic –NHBn moiety) and subsequent reduction passing through a Cram-chelate transition state [54] should deliver the anti-amino alcohol C. After subsequent cyclisation trans-B would result. Noteworthy, this strategy would completely circumvent

• significant amounts in the cyclisation of amino diol 9c. This is explained by steric shielding of the amino function (→ decreased nucleophilicity) through the bulky phenyl group in α-position (compared to the less demanding Me and Bn side chains R of substrates 9a and 9c). As the R-substituent is in the β

• Et3N/CH2Cl2 can be attributed to the lower solubility of iodine in Et3N (which leads to a slower and thus more selective reaction) and general base catalysis: Simultaneous deprotonation (through Et3N) in the cyclisation step strongly favours the desired reaction pathway to piperidines 11. Due to this

Intermediates in monensin biosynthesis: A late step in biosynthesis of the polyether ionophore monensin is crucial for the integrity of cation binding

• Wolfgang Hüttel,
• Jonathan B. Spencer and
• Peter F. Leadlay

Beilstein J. Org. Chem. 2014, 10, 361–368, doi:10.3762/bjoc.10.34

• initially-formed (E,E,E)-triene undergoes stereospecific epoxidation to a tri-epoxide and subsequent ring-opening and cyclisation to generate the polyether rings. This model has been confirmed and extended (Scheme 1) by the results of more recent work in which specific genes have been disrupted or deleted

• cyclisation is not initiated before the full-length chain is produced, and that the initial product of the PKS is a linear enzyme-bound (E,E,E)-triene, “premonensin” (2) [19]. The monensin PKS does not have a conventional C-terminal thioesterase domain that would catalyse polyketide chain release, and instead

• MonACPX, catalysed by the unusual thioesterase MonCII [20]. The evidently tight coupling between PKS-mediated chain assembly and oxidative cyclisation has hampered efforts to unravel the exact sequence and mechanism of events in the late stages of the biosynthesis. Indirect but suggestive evidence for the

Efficient synthesis of dihydropyrimidinones via a three-component Biginelli-type reaction of urea, alkylaldehyde and arylaldehyde

• Haijun Qu,
• Xuejian Li,
• Fan Mo and
• Xufeng Lin

Beilstein J. Org. Chem. 2013, 9, 2846–2851, doi:10.3762/bjoc.9.320

• condensation of imine 5 with substituted acetaldehyde 2. This could then undergo an iodine-catalytic intramolecular cyclisation to afford the final dihydropyrimidinone 4. Based on the observations above, a preliminary investigation on the catalytic asymmetric version was performed. Recently, our group has

[²H₂₆]-1-epi-Cubenol, a completely deuterated natural product from Streptomyces griseus

• Christian A. Citron and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2013, 9, 2841–2845, doi:10.3762/bjoc.9.319

• the homomonoterpene 1 proceeds by S-adenosylmethionine-dependent methylation of GPP followed by a cyclisation reaction [3][4][5]. For biosynthetic studies on secondary metabolites isotopically labelled precursors are frequently used. Historically, the usage of radiolabelled compounds was most

Modulating NHC catalysis with fluorine

• Yannick P. Rey and
• Ryan Gilmour

Beilstein J. Org. Chem. 2013, 9, 2812–2820, doi:10.3762/bjoc.9.316

• the corresponding lactam 13 using a Ru(III)/NaIO4 system proceeded smoothly, followed by TFA-mediated Boc deprotection to yield 14 (75%, 2 steps). Reduction of the methyl ester to the primary alcohol (15, 18%), and subsequent protection as the TBDMS ether delivered the cyclisation substrate 16 in good

• yield (92%). A three step, one pot sequence consisting of methylation, treatment with phenylhydrazine and subsequent cyclisation furnished the triazolium salt 17 in 76% yield (3 steps). Finally, DAST-mediated TBDMS deprotection/deoxyfluorination completed the synthetic sequence to give 7 in 45% yield

• lactam 19 (21% over 2 steps) in preparation for the cyclisation sequence. As previously described, successive treatment with the Meerwein salt, phenylhydrazine and methyl orthoformate yielded the target triazolium salt 8 in 61% over 3 steps. The pre-catalysts 9 and 10 required for control experiments

Stereoselectively fluorinated N-heterocycles: a brief survey

• Xiang-Guo Hu and
• Luke Hunter

Beilstein J. Org. Chem. 2013, 9, 2696–2708, doi:10.3762/bjoc.9.306

• the cyclisation event. 6.4 Enantioselective fluorocyclisation The lack of diastereoselectivity seen in Scheme 7 is attributable to the fluorination event preceding the cyclisation event, and this is a significant issue which inhibits the further development of diastereoselective processes. However

• synthesis this reagent fulfils both functions at different stages: thus, the target 78 is achieved from N-(tert-butylsulfinyl)imine 75 through a nucleophilic addition/radical cyclisation sequence. The selectivity during the radical cyclisation (77→78) can be explained by the Beckwith–Houk transition-state

• illustrates a strategy for achieving stereoselectivity in C–F bond formation. The racemic β-lactam 4b was synthesised as a single diastereoisomer from the Schiff base 79 (Scheme 11), by a Reformatsky addition followed by spontaneous cyclisation; removal of the amine protecting group under oxidative conditions

IBD-mediated oxidative cyclization of pyrimidinylhydrazones and concurrent Dimroth rearrangement: Synthesis of [1,2,4]triazolo[1,5-c]pyrimidine derivatives

• Caifei Tang,
• Zhiming Li and
• Quanrui Wang

Beilstein J. Org. Chem. 2013, 9, 2629–2634, doi:10.3762/bjoc.9.298

• ]pyrimidines by the oxidative cyclisation of benzothieno[2,3-d]pyrimidine hydrazones. The investigation has also suggested the important ability as inhibitors of Shiga toxin trafficking for protecting HeLa cells [9]. The wide range of biological activities shown by various triazolopyrimidines encouraged

• an efficient synthesis of new [1,2,4]triazolo[1,5-c]pyrimidine derivatives from hypervalent iodine (IBD)-mediated oxidative cyclisation of aldehyde pyrimidinylhydrazones and consecutive Dimroth rearrangement in relatively good yields under very mild conditions. Results and Discussion The

• feasible oxidative cyclisation to afford novel triazolopyrimidines. We initiated our investigation by examining the oxidative cyclization of 4a with iodobenzenediacetate (IBD) as an oxidant (Table 2). Thus, 4a was treated with one equivalent of freshly prepared IBD in dichloromethane at room temperature

An overview of the synthetic routes to the best selling drugs containing 6-membered heterocycles

• Marcus Baumann and
• Ian R. Baxendale

Beilstein J. Org. Chem. 2013, 9, 2265–2319, doi:10.3762/bjoc.9.265

• -methylpiperidine (1.21) using zeolites and easily aromatised by catalytic dehydrogenation to 3-picoline in 78% overall yield (Scheme 4) [26]. The overall processing sequence is highly energy-efficient (coupling of the endothermic cyclisation with the exothermic dehydration gives a reasonable energy balance). In

• addition the ammonia liberated during the cyclisation step is later consumed in the ammoxidation of the 3-picoline to the corresponding 3-cyanopyridine (Scheme 3). The value of substituted 3-picoline precursors is illustrated in the synthesis of clarinex (1.22, Desloratadine, Scheme 5), a dual antagonist

• the pyridine ring therefore yielding the corresponding piperidine 2.63. A base-mediated Dieckmann cyclisation and Krapcho decarboxylation [77] then furnishes 2.60. Traditionally, the reduction of 2.60 to prepare 2.59 can be carried out under fairly mild hydrogenation conditions that ultimately produce

Synthesis and characterization of novel bioactive 1,2,4-oxadiazole natural product analogs bearing the N-phenylmaleimide and N-phenylsuccinimide moieties

• Catalin V. Maftei,
• Elena Fodor,
• Peter G. Jones,
• M. Heiko Franz,
• Gerhard Kelter,
• Heiner Fiebig and
• Ion Neda

Beilstein J. Org. Chem. 2013, 9, 2202–2215, doi:10.3762/bjoc.9.259

• ′-carbonyldiimidazole (CDI). In the first step the amidoxime is O-acylated with the activated derivative in a condensation reaction. The O-acylated amidoxime can be isolated or it can immediately undergo the cyclisation to the heterocyclic oxadiazole ring. This cyclodehydration reaction takes place by heating to

Synthesis and antibacterial activity of monocyclic 3-carboxamide tetramic acids

• Yong-Chul Jeong and
• Mark G. Moloney

Beilstein J. Org. Chem. 2013, 9, 1899–1906, doi:10.3762/bjoc.9.224

• ) carboxamides. Results and Discussion Synthesis The synthesis of the required tetramic acid systems 2a–g (Scheme 1), 2h (Scheme 2) and 3a–f (Scheme 3) was achieved by Dieckmann cyclisation of the required N-alkyl-N-malonyl glycine (readily prepared from glycine). A similar strategy for the base-mediated

• cyclisation of N-acetoacetylamino acid esters leading to 3-acetyltetramates has been reported, which give N–H rather than N-alkyl systems [15][16][17]. Tetramate 7 was obtained from amino acid 6, except that the key intermediate 5 was obtained by reductive amination (Scheme 2) of (R)-citronellal (4), which

• generality [20][21]. We found that an approach based upon direct acylation of methyl thioethers 8a,b (these were readily obtained from the required N-acylmethionine by DCC/DMAP coupling with Meldrum’s acid and cyclisation under reflux) was possible, which made use of the high acidity of the tetramate system

Gold(I)-catalysed one-pot synthesis of chromans using allylic alcohols and phenols

• Eloi Coutant,
• Paul C. Young,
• Graeme Barker and
• Ai-Lan Lee

Beilstein J. Org. Chem. 2013, 9, 1797–1806, doi:10.3762/bjoc.9.209

• structural motifs found in a variety of important biologically active natural products such as vitamin E and flavanoids [1][2][3][4][5]. One approach towards chromans [6][7][8][9][10][11][12], which is biosynthetically inspired, is the Friedel–Crafts allylation [13] of phenols followed by cyclisation of the

• necessary to force the in situ cyclisation of 9 to 8. Next, lower equivalents of phenol 5 were investigated. Unfortunately, dropping the equivalents of phenol also appears to be detrimental to chroman formation: only Friedel–Crafts products 9 and 10 are observed with 2 or 1 equivalents of phenol (Table 1

• (Table 3, entries 7 and 8). Substitution at the β-position, however, is not tolerated: the reaction stops at the Friedel–Crafts stage (9t), and is reluctant to undergo further cyclisation to the desired chroman (Table 3, entry 9). Having investigated a series of tertiary allylic alcohols in entries 1–9

Computational study of the rate constants and free energies of intramolecular radical addition to substituted anilines

• Andreas Gansäuer,
• Meriam Seddiqzai,
• Tobias Dahmen,
• Rebecca Sure and
• Stefan Grimme

Beilstein J. Org. Chem. 2013, 9, 1620–1629, doi:10.3762/bjoc.9.185

• −1) and the B3LYP geometries (10.67 kcal mol−1). The total influence of the geometries and vibrations on ΔG‡ is therefore small (0.1–0.2 kcal mol−1 at most) and this technical detail cannot explain the discrepancy of the CCSD(T) barrier and the experimental value. The 5-exo cyclisation of 5-hexenyl

Development of an additive-controlled, SmI₂-mediated stereoselective sequence: Telescoped spirocyclisation, lactone reduction and Peterson elimination

• Brice Sautier,
• Karl D. Collins and
• David J. Procter

Beilstein J. Org. Chem. 2013, 9, 1443–1447, doi:10.3762/bjoc.9.163

• to “switch on” individual steps mediated by the electron transfer reagent. The sequence involves the use of two activated SmI2 reagent systems and a silicon stereocontrol element that exerts complete diastereocontrol over the cyclisation and is removed during the final stage of the sequence by

• Peterson elimination. The approach allows functionalised cyclopentanols containing two vicinal quaternary stereocentres to be conveniently prepared from simple starting materials. Keywords: cyclisation; free radical; Peterson elimination; reduction; samarium; telescoped process; Introduction Samarium

• manuscript, we report studies on SmI2-mediated cyclisation and lactone reduction that culminate in a “telescoped” sequence, i.e., a sequence of steps carried out on a single reaction mixture by the sequential addition of various reagents. In the sequence, additives are used with SmI2 to “switch on