Methyl ketene, pyrolysis

Flash vacuum pyrolysis of methyl imidazol-2-yl carboxylate 290 at 750°C gave 20% yield of 268 via the corresponding ketene 291. Similar pyrolysis of methyl imidazol-l-yl carboxylate 292 gave 20% of a 1 1 2 mixture of compounds 267,268, and 296. This fact can be rationalized by the pathway depicted in Scheme 71. Ketenes 291 and 295 may be intermediates formed from 293 and 294, respectively. They are products of rearrangement of 292. Similar pyrolysis of 4-imidazole carboxylic acid anilide performed at 800°C gave 267 in 20% yield (86JOC306). 

Methylbenzofuran-3-carbaldehydes undergo ready condensation with Meldrum s acid (isopropylidene malonate) to afford arylmethylene derivatives 83. These on flash vacuum pyrolysis at 500-600 C give 3-dibenzofuranols 84 (Scheme 21). The arylmethylene derivative, e.g., 85, presumably undergoes conversion to a methylene ketene (86, Scheme 22) on pyrolysis, which undergoes a [1,5-H] shift and subsequent cyclization and tautomerization, yielding the dibenzofuranol 87. The derived methyl ether 88 has been converted by mild acetylation with acetyl chloride and aluminum chloride and subsequent boron trichloride-induced demethylation to the natural product ruscodibenzofuran (8). A limitation is imposed on this method because 3-acetyl-2-methyldibenzofurans fail to condense with Meldrum s acid so that l-methyl-3-dibenzofuranols are not available by this method. ... 

Pyrolysis of the ethylene acetal of bicyclo[4.2.0]octa-4,7-diene-2,3-dione yields a-(2-hydroxyphenyl)-y-butyrolactonc 11 a mechanism involving a phenyl ketene acetal is proposed. Tartrate reacts with methanediol (formaldehyde hydrate) in alkaline solution to give an acetal-type species (9) 12 the formation constant was measured as ca 0.15 by H-NMR. Hydroxyacetal (10a) exists mainly in a boat-chair conformation (boat cycloheptanol ling), whereas the methyl derivative (10b) is chair-boat,13 as shown by 1 H-NMR, supported by molecular mechanics calculations. 

Flow pyrolysis of enaminones 312 at 400°C gives the pyrazolinones 315 in moderate yields, Scheme 88. The pyrazolinium ylides 314 can be isolated if the pyrolysis is conducted at a lower temperature (380 C). This suggested a concerted [1,4] elimination of ethanol to give the intermediate ketene 313, which undergoes electrocyclization to the ylide 314. A [1,3] methyl shift from nitrogen to carbon finally gives 315 [83JCS(CC)1144]. 

Interestingly, preliminary reports indicate that bis(trifluoromethyl)keten, which is known to react with DMF giving a butenamine, can itself be prepared by treating tetrakis(trifluoromethyl)allene with DMF. Another new synthesis of this keten involves the pyrolysis of bis(tri luoromethyl)diazine [a source of bis(trifluoro-methyl)carbene] in carbon monoxide the yield was quantitative. ... 

The synthesis of racemic olivomycose derivative 11 has been achieved from dihydropyrone 12 by treatment with methylmagnesium bromide then borane-methyl sulfide complex followed by sodium perborate. The dihydropyrone. in turn, was prepared by tr ping die acyl ketene produced on pyrolysis of dioxinone 13 with butyl vinyl ether. 

Thermal extrusion of CO from the furandione 409 gives methoxycarbo-nyl(pivaloyl) ketene 410 which dimerizes to the dioxinone 411 in a high yield (Scheme 83 2001T6757). Co-pyrolysis of the methyl ester 409 with the ieri-butyl ester 412 gives the hetero-dimer 413 with a different dioxinone structure (2002JCS(P1)599). 

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