Flash vacuum thermolysis

Thiophene-2,3-dicarboxylic anhydrides flash vacuum thermolysis, 4, 750 reactions, 4, 72... 

Compound 85 was also prepared by flash-vacuum thermolysis of 79a (250°C, 10 mm Hg) (97TL2501). At room temperature the telluroketone 85, isolated as... 

Flash vacuum thermolysis (FVT) of 2-substituted 4//-pyrido[l,2-n]pyrimidin-4-ones 126 above 800 °C afforded (2-pyridyl)iminopropadie-none (130) (99JCS(P2)1087). These reactions were interpreted in terms of reversible ring opening of 4//-pyrido[l,2-n]pyrimidin-4-ones to imidoyl-ketenes 127. A 1,5-H shift in 127 generated the N(l)H-tautomeric methylene ketene 128, in which facile elimination of HX took place via a six-membered cyclic transition state 129 to yield 130. In the case of 2-methoxy derivative 126 (X = OMe) another competing pathway was also identified at lower temperature, which resulted in the formation C3O2 and 2-methylaminopyr-idine via mesoionic isomer 131 (Scheme 9). The products were identified by IR spectroscopy. 

Flash vacuum thermolysis of 2-dialkylamino-4//-pyrido[l, 2-u]pyrimidin-4-ones 126 (X = NMe2, NEt2) at 850 °C led to the formation of 4H-pyrido[],2-u]pyrimidin-4-one (Scheme 13). The product was identified by NMR and GC-MS (99JCS(P2)1087). 

Heating diethyl (2-pyridylamino)methylenemalonates 304 (R = COOEt, = Me, OH) in AcOH afforded 4-oxo-4//-pyrido[l, 2-n]pyrimidine-3-carboxylates 305 (R = Me, OH) (96JHC1041). Flash vacuum thermolysis of 2-substituted 3-(2-pyridylamino)acrylates 304 (R = CN, COOEt, R = H) through a packed silica tube (530 °C, 0.01 mmHg) gave 3-substituted 4//-pyrido[l,2-n]pyrimidm-4-ones 306 (R = CN, COOEt) (94AJC1263). Ethyl 7-methyl-4-oxo-l, 4-dihydro-1,8-naphthyridine-3-carboxylate (79%) was... 

No 3,4-dihydroquinolmes of type 29 have ever been isolated, or even detected by any spectroscopic method however, their intermediacy was postulated for the disproportionation reaction of 1,2-dihydroquinolines (85CJC412) and for flash-vacuum thermolysis of triazoles (98JOC5779). 

The formation of 2-(t-butyl)-2-trimethylsilyl-2//-thiopyran has recently been reported from 1-substituted 1,3-butadienes with /-BuCSSiMe3 (92MI1) (see Scheme 2). An unusual product with proposed structure 47b has been reported (84JOC5143) from benzothiopyrone 46 on reaction with sec-butyl lithium (46 - 47a — 47b, Scheme 3) on the basis of spectroscopic data. 6-Methylthio-2//-thiopyran was isolated after the flash vacuum thermolysis of more complex starting precursors (93TL2605). 

Keywords retro-Diels-Alder reaction under flash vacuum thermolysis conditions... 

Retro-ene reaction of an N-allylsilanamine under flash vacuum thermolysis (FVT) conditions. The iminosilane has been identified not only by reaction with the trapping agent /-butyl alcohol, but also directly by coupling the oven with a high-resolution mass spectrometer.7... 

Flash vacuum thermolysis (FVP) at 600°C or microwave excitation of 1-substituted perhydropyrido[l,2-f][l,3]oxa-zines afforded 1-substituted 1,4,5,6-tetrahydropyridines <2005TL5451>. Perhydropyrido[l,2-f][l,3]oxazin-l-ones were hydrolyzed with 2M ethanolic KOH to 2-(2-hydroxyalkyl)piperidines <1996CJC2434, 2005EJ01378>. (+)-9- />z -6-Epipinidinol 102 was similarly obtained from 3,8-dimethylperhydropyrido[l,2-f][l,3]oxazin-l-one 101 (Equation 16) <1998T13505>. 

Cycloreversion of the 277-pyrido[l,2- ][l,3,5]triazine-2,4(3//)-dione derivatives could be achieved by flash vacuum thermolysis (FVT). FVT of 89 at 530 °C gave two isocyanates 90 and 91, which were not isolated but detected by IR spectroscopy <2002ARK438>, whereas FVT of 3-(2-pyridyl) analog of 89 afforded 91 as the only product (Equation 5) <2000J(P2) 1841 >. 

Retro Diels-Alder reactions often require severe conditions, high temperatures, and, sometimes, flash vacuum thermolysis. Microwave irradiation has been used as an alternative to these harsh conditions [12], even using graphite as a support [45]. Similarly, the use of higher input power has enabled retro-Diels-Alder reactions of anthracene derivatives to occur in 3-5 min [41]. This method is an alternative to the use of flash thermolysis. The use of graphite is a prerequisite for obtaining high temperatures in a short time. 

The major fragmentation in mass spectra of 1,2,5-oxadiazoles is attributed to the loss of nitrile and nitrile oxide or expulsion of NO. The conversion of 3,4-dicyano-l,2,5-oxadiazole-2-oxide (3,4-dicyanofuroxan) 10 to cyanogen iV-oxide 11 (Equation 5) was investigated under the conditions of collisional activation (CA) and neutralization-reionization (NR) mass spectrometry. Flash vacuum thermolysis mass-spectrometry (FVT-MS) and flash vacuum thermolysis infra-red (FVT-IR) investigations of furoxans 10, 12, and 13 reveal that small amounts of cyano isocyanate accompany the formation of the main thermolysis product 11 <2000J(P2)473>. 

Upon short contact time flash vacuum thermolysis (FVT), compound 10 is cleaved almost quantitatively into the metastable cyanogen /V-oxidc (NC-CNO 11 Equation (5) <2000J(P2)473>. The isolation and characterization of the trimers 12 and 13 of 11 was reported. 

By using flash vacuum thermolysis, 12 was obtained from 1,3,4-telluradiazo-line as green crystals.25,26 The visible spectrum (CHC13 at 80°C) showed kmax 825 nm assigned to the transition of the C=Te bond. In the 13C NMR spectrum, tellurocarbonyl carbon resonates at 301 ppm, which is the most deshielded carbon observed in a neutral molecule. Similarly, the 125Te-NMR shift at 2858 ppm is in the extreme downfield shift range.245... 

Wentrup and co-workers have carried out systematic flash vacuum thermolysis studies with a series of fused tetrazoles. Investigations of the isomeric tetrazolo[l,5- ]pyrazine 17 and tetrazolo[l,5-f]pyrimidine 20 showed that, in both cases, ring contraction takes place to afford imidazoles in high yields, but isotope labeling experiments revealed that the mechanisms of the openings of the two ring systems are different <2002JOC8538>. 

If both ethylene subunits of a l,3-hexadien-5-yne derivative are members of benzene entities, the cycloaromatization, caused by flash vacuum thermolysis, may give rise to bowl-shaped molecules such as corannulene or semibuckminsterfullerene. However, in those cases, the initial step is not a Hopf cydization but an isomerization of the ethynyl to a vinylidenecarbene group [125]. 

The flash vacuum thermolysis of l-(isopropylamino)methylenemalonate (1713) at 600°C yielded iminoacrolein (1715) via imidoylketene (1714). 1,3-Methyl migration, similar to 1,3-hydrogen shift between 1711 and 1712, could not be detected giving cyanoacetone by elimination of propene (see Scheme 60). 

Direct evidence has been reported for the formation of methoxyvinyl- and methylthiovinyl-(carboxy)ketenes (55c and 55d) upon flash vacuum thermolysis of Meldrum s acid derivatives (54c) and (54d), respectively " the intermediates decarboxylate readily to give (56c) and (56d), respectively, and are more transient than those obtained previously from (54a,b). 

Allenic esters (165) have been found to undergo a retro-ene reaction on flash vacuum thermolysis above 800 °C to give unsubstituted vinylketene together with formaldehyde or acetaldehyde (see Scheme 59). 

Flash vacuum thermolysis of 6-chloro-3-diazoindazole (2b) resulted, upon loss of nitrogen, in the formation of carbene 102, which could intra-molecularly rearrange to the nitrene 103, or to the azabenzocyclopropene 104, or to azacycloheptatrienylidene 105 (Scheme 29). The only isolable product was 106, formed by dimerization of the nitrene 103 (78CB2258). 

Enamine 106 (derived from Meldrum s acid), in a process of mono-decarboxylating transesterification and subsequent intramolecular alkylation, is cyclized to form enamino ester 107 (90H(31)1251). The direct route by flash vacuum thermolysis does not work in the case of 6/7 bicyclic 107. Methylene compound 109 originates (analogously to bicyclic 40b) from... 

Enaminone 128 (Scheme 33) is obtained, together with an isomeric indo-lizine derivative, by flash vacuum thermolysis of aminomethylene Meldrum s acid derivative through intermediate ketene and delocalized azomethine ylide (85TL833). The thermally induced cyclization of semi-cyclic dienamines to afford, for instance, tricyclic 129 is also believed to start with an azomethine ylide (97JOC7744) the p-chlorophenyl substituent is essential for the reaction. Unstabilized ylide 130, on the other hand, is generated from pipecolinic acid and /1-phenylcinnamaldehyde by the decarboxylation method target base 131 is formed by 1,7-electrocycliza-tion and [l,5]-hydrogen shift (99J(P1)2605). 

Thermal rearrangement of 0-acyl Af-hydroxycarbamates carrying a cyclopropane substituent was reported (equation 255). When subject to flash vacuum thermolysis at 500 °C the carbamate 573 generates the Af-acyl imine 574 that rearranges to pyrroline 577 in 21-37% yield. The formation of a biradical intermediate 575 or a polar zwitterionic structure 576 was proposed. 

Flash vacuum thermolysis of 1,2,3-oxadithiolane 2-oxide has been reported to proceed via the intermediacy of 1,2-oxathietan <82JCS(P2)279>. The gas-phase thermolysis of 1,2,3-benzoxadithiole 2-oxide (37) proceeds in an analogous manner to the thermolysis of 1,3,2-benzodioxathiolane 2-oxide (49), affording cyclopentadienethione <81AG(E)570>. 

Silylated 1,2,3-trisilacyclopentanes (73) under photolysis split off their central Si atom as a silylene (trapped by a scavenger) <85JOM(292)l67>. l,2,3-Trisilacyclopent-4-enes (74) split off only their central silicon atom as silylene by flash vacuum thermolysis (650-720 °C), whereas by photolysis both central and terminal Si atoms may be extruded depending on the nature of the substituents (Ar better than Alk-stabilized silylene) <84CL393>. Pyrolysis of hexamethyl-1,2,4-trisilacyclopentane (75) is unique for Si-compounds since different products arise at normal <76ZAAC(419)249> and low <820M1453> pressure (Scheme 2). 

The flash vacuum thermolysis of 4-hydroxy-2,5-diphenyl-l,3-oxazin-6-one resulted in carboxy(phenyl)ketene and benzonitrile as the major and benzoyl isocyanate as the minor product. This was interpreted in terms of the influence of the tautomeric forms on the thermal fragmentation pathways <2007JOC1399>. 

Azomethine ylide generation from oxazolidines has also been achieved by flash vacuum thermolysis (20,21). During synthetic efforts toward alkaloid central skeletal cores, Joucla and co-workers (22) revealed that flash vacuum thermolysis of oxazolidine (84) led to an intramolecular [3 + 2] cycloaddition furnishing pyrrolidine 85 in 82% as a single regio- and stereoisomer. Subsequent Dieckmann... 

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