Flash vacuum thermolysis derivatives

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. 

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. 

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]. 

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). 

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). 

Preparative flash vacuum thermolysis (FVT) of organic ester derivatives of 6-chloro-l-hexanol yielded as final product 1,5-hexadiene55. Thus, when R = H the process proceeds as described in equation 11. 

The low-temperature, Mn(III)-oxidation of unsaturated derivatives of Meldrum s acid provides a route to cyclohexenes <02T25> and the readily available 5-substituted Meldrum s acids are a convenient source of a-substituted acrylate esters <02JOC7365>. Flash vacuum thermolysis of 5-aminomethylene derivatives of Meldrum s acid 26 generates imino-propadienones, RN=C=C=C=0 <02JOC8558>. 

Ring expansion of the keto ester 148 by flash vacuum thermolysis at 520 °C at 0.01 mmHg afforded the l-benzoxocin-6-one derivative 149 in an excellent yield (95%) (Scheme 38) <2004TL9653>, which was used as intermediate in the synthesis of helianane, a novel heterocyclic sesquiterpene isolated from the marine sponge Haliclona fascigera <1997JOC2646>. 

Under flash vacuum thermolysis conditions (SSO-600 C/0.01 mmHg), even an acetoxy group can be induced to rotate inward, as shown by the conversion of (30) to the ketoaldehyde (31). The presence of a suitably placed dienoj ile, even the weak terminal vinyl group as in (30a), results only in the intramolecular Diels-Alderjnoduct (32), thereby demonstrating the large preference for outward motion by an acyloxy derivative." Similar thermolysis of 1-acetoxy-l-methylbenzocyclobutene (30 R = = Me)... 

Flash-vacuum thermolysis of thietane 1-oxide affords the reactive intermediate, sulfine, CH2=S=0. Field-ionization mass spectrometry of the thermolysis products also indicates the formation of thietane, propenal, ethylene, CaHsO, CaHg, and hydrogen sulfide. A 1,2-oxathiolane intermediate was suggested. The exo sulfoxide 118 is thermally stable, but the endo derivative 115 decomposes around 200°C, probably because of the ease of -elimination ... 

The 4-methylene-l,3-dithietane-2-ones lose carbon oxysulfide to give thio-ketenes that dimerize to 2,4-dimethylene-l,3-dithietanes, as exemplified by the flash-vacuum thermolysis of 551. Reaction of several methylene 1,3-dithietane-2-ones with secondary amines also appears to proceed via loss of carbon oxysulfide followed by addition of the amine to the thioketene. An exception to the loss of carbon oxysulfide is the reaction of a guanidine derivative with 551 to give the ring-opened product 552. ... 

Sulfines derived from simple aldehydes and ketones are rather unstable and generally appear as transient species. Sulfine (36) has been generated by flash vacuum thermolysis of, for instance, 1,3-dithietan-l-oxide (37) or methanesulfinyl chloride (38) (Scheme 20). However, the dehydrochlorination of a sulfinyl chloride, like (39), by a tertiary amine is a common route to obtain a stable sulfine, like (40) (Scheme 21). 

The route developed by Fowler and his associates (287) involved an ingenious application of the aza-Cope rearrangement, in which the bridged hydroxamic acid derivative 468, prepared as shown in Scheme 48, was subjected to flash vacuum thermolysis. The product, the enol ether 469, was not isolated but immediately hydrolyzed to the ketone 470, which was then hydrogenated and cyclized to the racemic ketone 466. This appears to be the first application of the aza-Cope rearrangement in synthetic chemistry, since the reaction is normally not thermodynamically favored when C-1 is replaced by nitrogen. However, it is clearly successful when the nitrogen is acylated, as in the present example. 

Perhaps flash vacuum thermolysis of 342 should be tried, for the 2-furyl-carbenes derived from 343 do give ring-opened acetylenes (344-345) in 20-60% yields under such conditions (Eq. 98).381... 

Jullien, J. Pechine, J.M. Perez, F Piade, J.J. Flash vacuum thermolysis of 3-keto-trimethylsilyl-enol-ethers. Synthesis of allenic and furanic derivatives. Tertrahedron 1982, 38, 1413-1416. 

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