High temperature bromination. Part 18: Bromination of benzonorbornadiene derivatives: Polybrominated benzonorbornenes and benzonorbornadienes

The low and high temperature bromination reactions of bromobenzonorbornadiene derivatives were studied and the possible role of a neighboring group in rearrangements was investigated. New polybrominated benzonorbornadiene and benzonorbornene derivatives were synthesized. All compounds were characterized properly using NMR spectroscopy.     

Bromination of 3-substituted benzo[b]thiophenes: access to Raloxifen intermediate

A practical route to prepare halogeno-derivatives is described, starting from easily available 3-cyano-benzo[b]thiophene. Main efforts have been devoted to the optimization of the experimental procedures (solvent, bromination) to promote selectivity. Synthetic studies investigate the potential access to an advanced intermediate of Raloxifen.     

Functionalization of saturated hydrocarbons. High temperature bromination of octahydropentalene. Part 19

The synthesis and thermal bromination of octahydropentalene was studied. The reaction afforded 1a,3a,4b,6b-tetrabromo-1,2,3,4,5,6-hexahydropentalene (14) with remarkable regio- and stereospecificity. The structure of the product was determined by ¹H and ¹³C NMR data and single X-ray structural analysis. The treatment of octahydropentalene with tenfold bromine gave the octabromopentalene derivative. The formation mechanism of the products is discussed.     

Multibromo-N-alkylcarbazoles: synthesis, characterization, and their benzo[b]thiophene derivatives

We present an efficient method for the selective bromination of N-alkylcarbazoles using N-bromosuccinimide (NBS) in THF. Four brominated N-alkylcarbazoles including 3-bromo-, 3,6-dibromo-, 1,3,6-tribromo- and 1,3,6,8-tetrabromo-carbazoles were prepared and identified by ¹H NMR spectroscopy. Subsequent cross-coupling of these carbazoles with benzo[b]thien-2-ylboronic acid gave the corresponding benzo[b]thien-2-yl functionalized carbazoles in good yields.     

The formation and decomposition of 1,1-dichloro-1H-cyclopropa[l]phenanthrene

Selenoxide elimination from (8) delivers 1,1-dichloro-1H-Cyclopropa[l]-phenanthrene (9) which is converted into the ester (11) upon reaction with methanol.The intervention of (9) is supported by labelling studies.     

An efficient synthesis of spiro[dibenzo[b,i]xanthene-13,3′-indoline]-pentaones and 5H-dibenzo[b,i]xanthene-tetraones

Spiro[dibenzo[b,i]xanthene-13,3′-indoline]-pentaones and 5H-dibenzo[b,i]xanthene-tetraones are synthesized from the condensation of 2-hydroxynaphthalene-1,4-dione with isatins or aldehydes.     

High-Temperature Bromination XIII [1]: Bromination of Dimethyl 7-Oxabicyclo[2.2.1]hepta-2,5-diene-2,3dicarboxylate

The electrophilic addition of bromine to dimethyl 7-oxabicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxylate 10 at 0 and –60°C led in high yield to the formation of dibromide 11. However, high-temperature bromination of 10 in carbon tetrachloride at 77°C gave four products 12–15, three of them with nonrearranged skeleton. Similarly, bromination of the dibromides 14 and 15 at 77°C yielded the nonrearranged tetrabromides 13 and 16, respectively. The structure of the tetrabromide 13 was solved by single-crystal X-ray analysis. The formation mechanism is discussed and supported by calculations.     

Reductive electrochemical formation of 6H-dibenzo[b,d]pyran-6-one and 2-benzopyran-1(1H)-one

In the present Letter several carbolactones (oxidative products) are obtained under aprotic cathodic conditions in the preparative scaled electrolysis of 1,2-quinones in a divided electrochemical cell and in the presence of oxygen. When 9,10-phenanthrenequinone is reduced 6H-dibenzo[b,d]pyran-6-one and [1,1′-biphenyl]-2,2′-dicarboxylic acid are obtained as major products. In the reduction of 1,2-naphthoquinone, 2-benzopyran-1(1H)-one, and 2-(2-carboxyethenyl)-benzoic acid were formed as main products. The proposed mechanism to explain the formation of these and other products, that involves an electron-transfer reaction to the oxygen in air, is now discussed.     

A general approach toward Janus diones: synthesis of dicyclopenta[b,g]naphthalene-1,3,6,8(2H,7H)-tetraone

A general approach toward Janus diones involving the reaction of pyromellitic or naphthalenetetracarboxylic dianhydrides with ethyl(triphenylphosphoranylidene)acetate and subsequent rearrangement of the products is described. This approach allows avoiding the formation of poorly soluble intermediates, in which purity cannot be effectively controlled. Dicyclopenta[b,g]naphthalene-1,3,6,8(2H,7H)-tetraone 3, a promising precursor of molecular and polymeric advanced materials is described.     

Regioselective synthesis of 1H,3H,6H[2]benzopyrano[4,3-d]pyrimidine-2,4-diones and 12H-benzopyrano[3,2-c][1]benzopyran-5-ones by radical cyclization

5-Hydroxy uracils or 4-hydroxy[1]benzopyran-2-ones were refluxed with 2-bromobenzyl bromides in acetone in the presence of anhydrous potassium carbonate to afford a number of 5-(2′-bromobenzyloxy) pyrimidine-2,4-dione (80-92%) or 4-(2′-bromobenzyloxy) benzopyran-7-ones (70-82%) respectively. These were then refluxed with tri-n-butyltin chloride and sodium cyanoborohydride in the presence of a catalytic amount of azobisisobutyronitrile (AIBN) for 3-4 h to give 1H,3H,6H [2]benzopyrano[4,3-d]pyrimidine-2,4-diones (75-85%) or 12H-benzopyrano[3,2-c][1]benzopyran-5-ones (70-85%) respectively.     

Synthesis and characterization of benzo[c]thiophene analogs incorporating benzo[b]thiophene/1-hexylindole/benzo[b]furan

Synthesis of 1,3-disubstituted benzo[c]thiophene analogs incorporating heterocycles such as benzo[b]thiophene/1-hexylindole/benzo[b]furan and thiophene units is described. Optical and electrochemical studies of the benzo[c]thiophene analogs are also reported.     

Use of cyclobutyl derivatives as intermediates in the synthesis of 1,2a,3,8b-tetrahydro-2H-cyclobuta[c]chromenes

Starting from cyclobutanones and tertiary cyclobutanols several new chromenes containing a cyclobutane ring have been prepared by acid catalyzed intramolecular alkylation of an oxygen-carrying aromatic ring. Interestingly one of the cyclobutanols leads to 3-[(4-methoxyphenoxy)methyl]-2,2,3-trimethylcyclopentanone that is a precursor in the synthesis of the carotenoid pigment capsorubin.     

Synthesis of 2,5-dihydrobenzo[b]oxepines and 5,6-dihydro-2H-benzo[b]oxocines based on a ‘[3+3] cyclization-olefin-metathesis’ strategy

Functionalized 2,5-dihydrobenzo[b]oxepines and 5,6-dihydro-2H-benzo[b]oxocines were prepared based on a ‘[3+3] cyclization-olefin-metathesis’ strategy.     

An unexpected reaction of phosphorous tribromide on chromanone, thiochromanone, 3,4-dihydro-2H-benzo[b]thiepin-5-one, 3,4-dihydro-2H-benzo[b]oxepin-5-one and tetralone derived allylic alcohols: a case study

Treatment of chromanone, thiochromanone, tetralone and benzo[b]thiepinone derived allylic alcohols with phosphorous tribromide furnished unexpected products instead of desired bromo derivatives. Reaction of 7-methoxy-2,2-dimethylchromenyl-aryl-methanols and 7-methoxy-2,2-dimethylthiochromenyl-aryl-methanols with PBr₃ gave saturated carbonyls and exocyclic olefins depending on the substituents at R⁴, whereas allylic alcohols without 7-methoxy substituent furnished rearranged products. Several related analogs varying in ring size, heteroatom in the ring and substituents at various positions were synthesized and studied. Reactions of other phosphorous-based reagents like PPh₃, PPh₃/I₂ and PPh₃Br₂ on allylic alcohols were also attempted. A case study is presented here.     

Synthesis, properties, and oxidizing function of 6-substituted 7,9-dimethylcyclohepta[b]pyrimido[5,4-d]pyrrole-8(7H),10(9H)-dionylium tetrafluoroborates

Uracil-annulated heteroazulenes, 6-substituted 7,9-dimethylcyclohepta[b]pyrimido[5,4-d]pyrrole-8(7H),10(9H)-dionylium tetrafluoroborates 7a,b·BF₄⁻, which are the isoelectronic compounds of 5-dezazaflavin, were synthesized. X-Ray crystal analysis and MO calculations were carried out to clarify the structural characteristics of 7a,b·BF₄⁻. The stability of cations 7a,b is expressed by the pK_R+ values which were determined spectrophotometrically to be 10.9 and 11.2, respectively. The electrochemical reduction of 7a,b exhibited high reduction potentials at −0.84 and −0.87 (V vs Ag/AgNO₃) upon cyclic voltammetry (CV). A good linear correlation between the pK_R+ values and reduction potentials (E1_red) of 7a,b·BF₄⁻ and reference compounds 4·BF₄⁻ and 5·BF₄⁻ was obtained. In a search of the reactivity, reactions of 7a,b·BF₄⁻ with some nucleophiles, hydride and diethylamine, were carried out to clarify that the introduction of nucleophiles to give regio-isomers is dependent on the nucleophile. The photo-induced oxidation reactions of 7a,b·BF₄⁻ toward some alcohols under aerobic conditions were carried out to give the corresponding carbonyl compounds in more than 100% yield [based on compounds 7a,b·BF₄⁻], suggesting the oxidizing function of 7a,b·BF₄⁻ toward alcohols in the autorecycling process.     

A novel transformation of 1-exo-substituted 2a-aroyl-1,2,2a,8b-tetrahydro-3H-benzo[b]cyclobuta[d]pyran-3-ones with sulfoxonium ylide to highly strained 2a-(1-arylethenyl)-1,2,2a,7b-tetrahydrocyclobuta[b]benzofurans

When 1-substituted 2a-aroyl-1,2,2a,8b-tetrahydro-3H-benzo[b]cyclobuta[d]pyran-3-ones (1) were treated with dimethylsulfoxonium methylide, 1-endo isomers (endo-1) gave 1-substituted 3-aroyl-1,2,4a,9b-tetrahydrodibenzofuran-4-ols (2) exclusively as expected. On the other hand, 1-exo isomers (exo-1) underwent a novel transformation to 1-substituted 2a-(1-arylethenyl)-1,2,2a,7b-tetrahydrocyclobuta[b]benzofurans (3), together with 2.     

Synthesis of 1,3-disubstituted benzo[c]thiophene analogs containing benzo[b]thiophene/benzo[b]pyrrole

The synthesis of 1,3-disubstituted benzo[c]thiophene analogs incorporating benzo[b]thiophene/benzo[b]pyrrole units is described.     

The cyclopropa[b]naphthalene electron donor: nonplanar 8π 7C cycloheptatrienylidene derivatives

Dipole moment measurements and calculations show the cyclopropa[b]naphthalene moiety to be an electron donor that is stronger than cycloheptatriene. Crystallographic analyses of cycloheptatrienylidenecyclopropa[b]naphthalenes, with and without electron donating substituents in the cycloproparene, have the seven-membered ring resisting imposition of 8π 7C antiaromaticity by bending significantly out of the plane that carries the cycloproparene. The extent of this distortion depends upon the level of electron donation from the cycloproparene.     

Galanthamine analogs: 6H-benzofuro[3a,3,2,-e,f][1]benzazepine and 6H-benzofuro[3a,3,2-e,f][3]benzazepine

The known cholinesterase inhibitory capability of the Amarylidaceae alkaloid galanthamine prompted preparation of analogs in which the position of the nitrogen within the azepine ring is altered. The analogs 6H-benzofuro[3a,3,2-e,f][1]benzazepine and 6H-benzofuro[3a,3,2-e,f][3]benzazepine were prepared in 19 and 2.5%, respectively, following Kametani and Shimizu approaches, respectively. The aniline derivative 6H-benzofuro[3a,3,2-e,f][1]benzazepine failed to undergo most of the reactions typical for galanthamine. Thus, it neither oxidized to the analogous narwedine, nor epimerized to the analogous epigalanthamine, nor reduced to the lycoramine analog, under the conditions used for galanthamine.     

One-pot synthesis of 12H-benzo[b]xanthen-12-ones and naphtha[2,3-b]furans from non-naphthalene substrates

An efficient one-pot synthesis of 12H-benzo[b]xanthen-12-ones via an oxidant initiated and base promoted tandem reaction of 2-(4-(2-haloaryl)-4-hydroxybut-1-ynyl)benzaldehydes has been developed. By using similar strategy, a one-pot synthesis of naphtha[2,3-b]furans from the cascade reaction of 2-(4-hydroxy-but-1-ynyl)benzaldehydes with 2-bromoacetonitrile or α-bromocarbonyl compounds has also been achieved.