Ketones pyrolysis

Rate constants and activation energies for liquid- and gas-phase isomerization of a-pinene have been determined.310 The activity of metal sulphate monohydrates in isomerizing a-pinene is correlated with the strength of co-ordination of the water of crystallization to the metal ion.3" Pyrolysis of chrysanthanol acetate (217 R = Ac) gives citronellal and the (E)- and (Z)-3,7-dimethylocta-l, 6-dien-l -ol acetates in 20, 28, and 3% yields respectively formation of the enol acetates is consistent with a biradical or a concerted pathway.312 Further work directed towards C-l—C-7 bond pyrolysis of pinane derivatives shows C-l—C-7 C-l—C-6 bond cleavage ratios of 4 51 for (217 R = Ac), 13 22 for (217 R = H), 6 7 for (218 R = H), and 43 35 for (218 R = Me) the expected acyclic and cyclic alcohol, aldehyde, and ketone pyrolysis products are obtained.313 The ene reaction between /3-pinene and methyl... 

Recently, Vogtle et al. reported the selective ketone pyrolysis as a new synthetic method for mono-and polycyclic hydrocarbons that is, flash vacuum pyrolysis (10"" Torr, 610 to 650°C) of [3.3]CP-2,11-diones 1 afforded [2.2] CPs 2 in moderate yields. This ketone pyrolysis is widely appHcable to the synthesis of tribridged cyclophanes and of cyclophanes with a large cavity starting from [3 ]CP-(one)". "... 

Breitenbach, J., Ott, E, and Vbgtle E, Selective ketone pyrolysis new method for mono- and polycyclic hydrocarbons, Angew, Chem. Int Ed. Engl, 31, 307, 1992. 

By pyrolysis of dibasic acids or their salts to yield cyclic ketones. The slow distillation of adipic acid with about. 5 per cent, of baryta affords cyclo-pentanone in good yield ... 

Electron-deficient alkenes add stereospecifically to 4-hydroxy-THISs with formation of endo-cycloadducts. Only with methylvinyl-ketone considerable amounts of the exo isomer are produced (Scheme 8) (16). The adducts (6) may extrude hydrogen sulfide on heating with methoxide producing 2-pyridones. The base is unnecessary with fumaronitrile adducts. The alternative elimination of isocyanate Or sulfur may be controlled using 7 as the dipolarenOphile. The cycloaddition produces two products, 8a (R = H, R = COOMe) and 8b (R = COOMe, R =H) (Scheme 9) (17). Pyrolysis of 8b leads to extrusion of furan and isocyanate to give a thiophene. The alternative S-elimi-nation can be effected by oxidation of the adduct and subsequent pyrolysis. 

With aldehydes, primary alcohols readily form acetals, RCH(OR )2. Acetone also forms acetals (often called ketals), (CH2)2C(OR)2, in an exothermic reaction, but the equiUbrium concentration is small at ambient temperature. However, the methyl acetal of acetone, 2,2-dimethoxypropane [77-76-9] was once made commercially by reaction with methanol at low temperature for use as a gasoline additive (5). Isopropenyl methyl ether [116-11-OJ, useful as a hydroxyl blocking agent in urethane and epoxy polymer chemistry (6), is obtained in good yield by thermal pyrolysis of 2,2-dimethoxypropane. With other primary, secondary, and tertiary alcohols, the equiUbrium is progressively less favorable to the formation of ketals, in that order. However, acetals of acetone with other primary and secondary alcohols, and of other ketones, can be made from 2,2-dimethoxypropane by transacetalation procedures (7,8). Because they hydroly2e extensively, ketals of primary and especially secondary alcohols are effective water scavengers. 

Thermal Stability. The saturated C —C 2 ketones are thermally stable up to pyrolysis temperatures (500—700°C). At these high temperatures, decomposition can be controlled to produce useful ketene derivatives. Ketene itself is produced commercially by pyrolysis of acetone at temperatures just below 550°C (see Ketenes, ketene dil rs, and related substances). 

Enol ethers of 17-ketones are formed by pyrolysis of the corresponding dialkyl ketals and enol acetates are readily prepared by the exchange procedure.The latter derivatives are widely used as reactive intermediates for the introduction of substituents at C-16. 

Spiro-pyrazoline derivatives (18) are obtained smoothly from 16-methy-lene-17-ketones (17). ° The sole products formed from (18) by pyrolysis or cleavage in the presence of boron trifluoride etherate, are the 16-spiro-cyclopropyl steroids (19). ... 

Oxidation of 17j5-acetoxy-5a-androstan-3-one (1) by chromium trioxide in acetic acid at 55-65° gives the 2,3-seco acid (2). Conversion of the seco acid to its anhydride followed by pyrolysis and distillation gives the A-nor-2-ketone (3) in 45% overall yield. Analogous reactions have been carried... 

AB-Dinorsteroids have been prepared by Dauben et al from B-nor-A" -3-ketones by reactions analogous to the sequence (4) - (7). Pyrolysis of the B-nor-2,3-seco diacid anhydride gives the AB-dinor-A -2-ketone in 5 % yield. However, refluxing the diacid with acetic anhydride containing potassium cyanide for 2 days gives the ketone in 50-60% yield, apparently by base-catalyzed cyclization of an intermediate bis-acylcyanide. 

In a typical Knof procedure, 3jS-hydroxyandrost-5-en-17-one acetate is epoxidized with perbenzoic acid (or m-chloroperbenzoic acid ) to a mixture of 5a,6a- and 5)5,6)5-epoxides (75) in 99 % yield. Subsequent oxidation with aqueous chromium trioxide in methyl ethyl ketone affords the 5a-hydroxy-6-ketone (76) in 89% yield. Baeyer-Villiger oxidation of the hydroxy ketone (76) with perbenzoic acid (or w-chloroperbenzoic acid ) gives keto acid (77) in 96% yield as a complex with benzoic acid. The benzoic acid can be removed by sublimation or, more conveniently, by treating the complex with benzoyl chloride and pyridine to give the easily isolated )5-lactone (70) in 40% yield. As described in section III-A, pyrolysis of j5-lactone (70) affords A -B-norsteroid (71). Knof used this reaction sequence to prepare 3)5-hydroxy-B-norandrost-5-en-17-one acetate, B-noran-... 

Carboxylic acids can be converted to symmetrical ketones by pyrolysis in the presence of thorium oxide. In a mixed reaction, formic acid and another acid heated... 

In a related reaction, pyrolysis of allylic ethers that contain at least one a hydrogen gives alkenes and aldehydes or ketones. The mechanism is also pericyclic"" ... 

Pyrolysis of carboxylic acids Dehydrohalogenation of acyl halides Dehalogenation of a-halo acyl halides Rearrangement of diazo ketones (Wolff)... 

---