Cyclotetrasilazane als precursor für cyclotrisilazane und bis(cyclodisilazan-1-yl)silane

Di-lithiated octamethylcyclotetrasilazane (OMCTS, 1) reacts with halosilanes in different ways. Ring contraction with formation of the isomeric cyclodisilazanes 2, 3 occurs in the reaction with chloro- and fluorotrimethylsilanes. Substitution (6) and ring contraction with formation of the isomeric six-membered ring 7 occurs with chlorodimethylsilane. 2, 3, 6 and 7 are excellent precursors of silyl-bridged, SiH-functional, four-membered ring systems (4, 5, 9–11). The mechanism of the isomerization reactions are discussed.     

Synthesis of new 14-membered macrolide antibiotics via a novel ring contraction metathesis

[reaction: see text] A novel ring opening ring closing metathesis (ROM-RCM) was demonstrated for cyclic conjugated dienes, effecting the excision of a C(2)H(2) unit and a net ring contraction. Applying the ring contraction metathesis, new 14-membered ring macrolide antibiotics were synthesized in a single step from existing 16-membered ring macrolides. This new class of macrolide antibiotics will provide access to new therapeutics for the treatment of macrolide-resistant bacterial infections.     

Flash vacuum thermolysis of acenaphtho[1,2-a]acenaphthylene. Thermal behaviour of a polycyclic aromatic hydrocarbon containing two abutting pentagons

FVT of acenaphtho[1,2-a]acenaphthylene (1) gave acenaphtho[1,2-e]acenaphthylene (2), cyclopenta[cd]perylene (3) and cyclopenta[rst]benzo[hi]chrysene (4). The formation of 3 and 4 indicates that, besides ring contraction/ring expansion of 1 giving 2, homolytic scission of a five-membered ring carbon---carbon single bond of 1 is an important competitive process.     

Synthesis of (+/-)-3H-epivincamine via a Rh(II)-triggered cyclization/cycloaddition cascade

A synthesis of (+/-)-3H-epivincamine is reported. Important steps include (1) a Rh(II)-catalyzed intramolecular [3+2]-cycloaddition of an alpha-diazo indolo amide, (2) a reductive ring opening of the cycloadduct, (3) a decarboethoxylation reaction, and (4) a base-induced keto-amide ring contraction.     

Hetarylnitrenes V. Reactions of Tetrazolopyrazine Ring Contraction of Nitrenodiazines in Solution. Preliminary communication

Thermolysis of tetrazolopyrazine ( 1 ) in organic solvents gives pyrazinylnitrene ( 2 ) which undergoes ring contraction to 1-cyanoimidazole ( 3 ). 7-Methyl-5-methylthio-tetrazolo[1,5-c]pyrimidine ( 4 ) likewise gives 1-cyano-2-methylthio-4-methyl-imidazole ( 6 ). The two tetrazoles also undergo ring contraction to 1-cyanoimidazoles by gas chromatography, and 1 gives a low yield of 3 by photolysis. Thermolysis of 1 and 4 in cyclohexane gives aminopyrazine ( 7 ) and 6-amino-4-methyl-2-methylthio-pyrimidine ( 8 ), respectively. Tetrazolo[1,5-a]pyrimidines ( 9 ) give only 2-aminopyrimidines ( 10 ). 1-Cyanoimidazole, formed by thermolysis of 1 in acetic acid, reacts further to give 1-acetylimidazole, which with more acetic acid gives imidazole and acetic anhydride. An earlier report [2] of ring expansion of pyrazinylnitrene in acetic acid is discredited. In protic deuteriated solvents (D₃O, CH₃OD), tetrazolopyrazine reacts as an enamine, specifically exchanging H? C(6) for deuterium.     

Thallium trinitrate mediated oxidation of 3-alkenols: ring contraction vs cyclization

The reaction of a series of six-membered ring 3-alkenols with thallium trinitrate (TTN) in three different experimental conditions was studied. Either cyclization products or ring contraction products were obtained, depending on the structure of the substrate as well as the nature of the solvent. The reaction of a seven-membered ring 3-alkenol with TTN led to the ring contraction product exclusively.     

Stereodefined ring contraction-rearrangement of thiocoumarins to new fused benzo[b]thiophene derivatives

Mesoionic 1,3-oxazolium-5-olates (münchnones) react with thiocoumarins having an electron-withdrawing group at the 3-position to afford stereodefined fused polycyclic thienopyrroles. The reaction sequence seems to be triggered by a regiospecific dipolar cycloaddition followed by ring opening of the initial 1:1 cycloadduct and intramolecular rearrangement with an unusual ring contraction.     

Novel ring chemistry of vitamin B(6) with singlet oxygen and an activated ene: isolated products and identified intermediates suggesting an operable [3 + 2] cycloaddition

Pyridoxine reaction with (1)O(2) in aqueous solution at neutral pH resulted in oxidation at the 2- and 6-positions of the pyridine ring and unprecedented ring contraction. Kinetic and low temperature studies provided observable intermediates by NMR spectroscopy. In addition, novel cycloaddition between pyridoxine and N-methylmaleimide, without N-alkylation and in water, suggest a common [3 + 2] cycloaddition with the 3-hydroxypyridine ring.     

Thermal Interconversion of Phenylcarbene and Tropylidene. Preliminary communication

Phenylcarbene and tropylidene interconvert in the gas-phase, as evidenced by the formation of stilbene and heptafulvalene ( 5 ) from both at 300°. Heptafulvalene itself rearranges to stilbene and anthracene above 500°. Phenylcarbene, but not tropylidene, undergoes ring contraction to fulvenallene ( 9 ) at 900°. Formation of fluorene from diphenylcarbene involves a complete ring expansion to 2-phenyltropylidene ( 14 ), and subsequent ring contraction to 2-biphenylylcarbene ( 15 ).     

Synthesis of functionalized tetrahydro-1,3-diazepin-2-ones and 1-carbamoyl-1H-pyrroles via ring expansion and ring expansion/ring contraction of tetrahydropyrimidines

A general approach to 6-phenylthio-substituted 2,3,4,5-tetrahydro-1H-1,3-diazepin-2-ones based on the ring expansion reaction of 1,2,3,4-tetrahydropyrimidin-2-ones under the action of nucleophiles has been developed. The first step of the synthesis was preparation of N-[(2-benzoyloxy-1-tosyl)ethyl]urea by three-component condensation of 2-benzoyloxyethanal, urea and p-toluenesulfinic acid. Nucleophilic substitution of the tosyl group in the obtained sulfone with sodium enolates of α-phenylthioketones followed by cyclization-dehydration, and debenzoylation gave 4-hydroxymethyl-5-phenylthio-1,2,3,4-tetrahydropyrimidin-2-ones which were transformed into the 4-mesyloxymethyl-derivatives. Treatment of the latter with nucleophilic reagents, such as NaCN, sodium diethyl malonate, PhSNa, MeONa, NaBH(4), sodium succinimide, or potassium phthalimide, afforded the target multi-functionalized diazepinones. The obtained 6-phenylthio-diazepinones and their 6-tosyl-substituted analogues were converted into 3-substituted 1-carbamoyl-1H-pyrroles under acidic conditions as a result of ring contraction. Effective one-pot synthesis of the latter from 4-mesyloxymethyl-pyrimidines was realized using a ring expansion/ring contraction sequence.     

The synthesis of phthalazin-1(2H)-ones and 3-hydroxyisobenzofuran-1(3H)-ones via the ring contraction of tropones

Phthalazin-1(2H)-ones and 3-hydroxyisobenzofuran-1(3H)-ones were synthesized by the reactions of tropones with hydrazines and alcohols, respectively, via the ring contraction.     

A novel intramolecular reversible reaction between the hydroxyl group and isobutenylene chain in a cyclophane-type macrocycle

A novel Hg²⁺ ion induced reversible ring contraction was achieved employing the intramolecular reaction of isobutylene with an aromatic hydroxyl group of cyclophane; reversibility of the reaction was facilitated by excess addition of NaBH₄ which also resulted in complexation. The ring contraction and expansion was monitored by UV-VIS absorption, and by fluorescence and ¹H NMR spectra. Switchable fluorescence behavior (on—off—on) was observed when the ring-size was tuned from a 19-membered ring to an 18-membered and vice versa. This fine tuning has the potential to be applied in the construction of new supramolecular devices.     

Understanding the fate of the oxyallyl cation following Nazarov electrocyclization: sequential Wagner-Meerwein migrations and the synthesis of spirocyclic cyclopentenones

A general reaction sequence is described that involves Nazarov cyclization followed by two sequential Wagner-Meerwein migrations, to afford spirocyclic compounds from divinyl ketones in the presence of 1 equiv of copper(II) complexes. A detailed investigation of this sequence is described including a study of substrate scope and limitations. It was found that after 4π electrocyclization, two different pathways are available to the oxyallyl cation intermediate: elimination of a proton can give the usual Nazarov cycloadduct, or ring contraction can give an alternative tertiary carbocation. After ring contraction, either [1,2]-hydride or carbon migration can occur, depending upon the substitution pattern of the substrate, to furnish spirocyclic products. The rearrangement pathway is favored over the elimination pathway when catalyst loading is high and the copper(II) counterion is noncoordinating. Several ligands were found to be effective for the reaction. Thus, the reaction sequence can be controlled by judicious choice of reaction conditions to allow selective generation of richly functionalized spirocycles. The three steps of the sequence are stereospecific: electrocyclization followed by two [1,2]-suprafacial Wagner-Meerwein shifts, the ring contraction and then a hydride, alkenyl, or aryl shift. The method allows stereospecific installation of adjacent stereocenters or adjacent quaternary centers arrayed around a cyclopentenone ring.     

Etude des petits cycles—XXXI : Regression de la cyclobutanone et extension de cycle du methylenecyclopropane par oxydation

On treatment with SeO₂ + H₂O₂, cyclobutanone undergoes either oxidation to γ-butyrolactone or ring contraction to cyclopropanecarboxylic acid, depending on the experimental conditions. Oxidation with thallium (III) salts leads to ring contraction. Reaction of methylenecyclopropane with thallium (III) or mercury (II) salts leads to ring fission giving methylethylketone or derivatives thereof.     

Quinazolines and 1,4-benzodiazepines. XLVI. Photochemistry of nitrones and oxaziridines

The cyclic nitrones 7-chloro-1,3-dihydro-5-phenyl-2H-1,4-benzodiazepin-2-one 4-oxide ( 5a ) and 1,3-dihydro-7-methylthio-5-phenyl-2H-1,4-benzodiazepin-2-one 4-oxide ( 5b ) are photoisomerized to readily isolable oxaziridines, 7-chloro-4,5-epoxy-5-phenyl-1,3,4–5-tetrahydro-2H-1,4-benzodiazepin-2-one ( 6a ) and 4,5-epoxy-5-phenyl-1,3,4,5-tetrahydro-7-methylthio-2H-1,4-benzo-diazepin-2-one ( 6b ). Oxaziridine 6b upon further irradiation gave ring expansion and ring contraction products, 4,6-dihydro-2-phenyl-9-methylthio-5H-1,3,6-benzoxadiazocin-5-one ( 7b ) and 4-benzoyl-3,4-dihydro-6-methylthioquinoxalin-2(1H)-one ( 8b ) respectively. The ring contraction product, 4-benzoyl-6-chloro-3,4-dihydroquinoxalin-2(1H)-one ( 8a ), was obtained from irradiation of oxaziridine 6a .     

Photoisomerization of 2H,6H-thiin-3-one 1-oxides to 3H,7H-[1,2]oxathiepin-4-ones

Oxidation of 2H, 6H-thiin-3-ones 1a – c with 3-chloroperbenzoic acid affords the corresponding 1-oxides 2a – c . On irradiation (350 nm) in either benzene or MeCN, these cyclic sulfoxides 2 isomerize to 3H, 7H-1,2-oxathiepin-4-ones 3 . The tetramethyl derivative 3a is isolated by flash chromatography at ?10°, but, at higher temperatures, it undergoes ring contraction and H₂O elimination to give 4,4-dimethyl-2(2-methylprop-2enylidene)thietan-3-one ( 4 ). Diemthyloxathiepinones 3b and 3c undergo ring contraction in MeOH to afford 1-(4-methylthiophen-2-yl)ethanone ( 5 ) and two diastereoisomeric 4,4-dimethyl-2-methoxy-2-(1-methoxyethyl)thietan-3-ones ( 6 and 7 , respectively).     

Recyclization of 2-(3,6-diaryl-2,5-dihydropyridazin-4-yl)-1H-benzimidazoles to 2-[(3,5-diarylpyrazol-4-yl)methyl]-1H-benzimidazoles

2-(3,6-Diaryl-2,5-dihydropyridazin-4-yl)-1H-benzimidazoles undergo a previously unknown type of recyclization of the pyridazine ring to a pyrazole to give 2-[(3,5-diarylpyrazol-4-yl)methyl]-1H-benzimidazoles. It is suggested that the mechanism of this conversion, which includes formation of a secondary enhydrazine group in the diazine ring and its subsequent contraction, occurs after the intramolecular formation and opening of a cyclopropane ring. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 853–861, June, 2008.     

Iodine(III)-promoted ring contraction of 1,2-dihydronaphthalenes: a diastereoselective total synthesis of (+/-)-indatraline

[reaction: see text] A new approach for the synthesis of (+/-)-indatraline, which is a 3-phenyl-1-indanamine that displays several biological activities, is described. The strategy features as the key step a diastereoselective ring contraction of a 1,2-dihydronaphthalene promoted by PhI(OTs)OH, to construct the indan ring system. The oxidative rearrangement of other 1,2-dihydronaphthalenes was also investigated, generalizing this method to obtain indans.     

Efficient synthesis of 2-(pyrazol-3-yl)benzimidazoles from 3-arylacylidene-3,4-dihydroquinoxalin-2(1H)-ones and hydrazine hydrate via a novel rearrangement

A highly efficient method for the synthesis of 2-(pyrazol-3-yl)benzimidazoles has been developed on the basis of the novel ring contraction of 3-arylacylidene-3,4-dihydroquinoxalin-2(1H)-ones with hydrazine hydrate.     

Isomerization of pyridazino [4,5-b] indoles to pyrrolo [3,4-b] indoles

The dihydropyridazine ring undergoes contraction to a dihydropyrrole ring to give 1-phenyl-2-benzylideneaminodihydropyrrolo[3,4-b]indol-3-ones when 1-phenyldihydropyridazino[4,5-b]indol-4-ones are treated with aromatic aldehydes under acid catalysis conditions. The reaction mechanism consists in the formation of a (3-indolyl)phenylmethyl cation, which leads to opening of the pyridazine ring and subsequent development of a bond between the carbonium center and the amide nitrogen atom.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1406–1408, October, 1976.