Acid derivatives, pyrolysis

The best yields are obtained when the ketene has an electronegative substituent, such as halogen. Simple ketenes are not very stable and must usually be generated in situ. The most common method for generating ketenes for synthesis is by dehydrohalo-genation of acyl chlorides. This is usually done with an amine such as triethylamine.167 Other activated carboxylic acid derivatives, such as acyloxypyridinium ions, have also been used as ketene precursors.168 Ketene itself and certain alkyl derivatives can be generated by pyrolysis of carboxylic anhydrides.169... 

A third category of syn eliminations involves pyrolytic decomposition of esters with elimination of a carboxylic acid. The pyrolysis of acetate esters normally requires temperatures above 400° C and is usually a vapor phase reaction. In the laboratory this is done by using a glass tube in the heating zone of a small furnace. The vapors of the reactant are swept through the hot chamber by an inert gas and into a cold trap. Similar reactions occur with esters derived from long-chain acids. If the boiling point of the ester is above the decomposition temperature, the reaction can be carried out in the liquid phase, with distillation of the pyrolysis product. 

The main lines of this approach were later embodied in an enantioselective synthesis of (—)-a-allokainic acid (Scheme 34) (179). The sole stereo center of die ene reaction starting material was derived from a glutamic acid derivative (132) to avoid loss of optical activity via double bond migration (see Scheme 33), the a acid function of kainic acid had to be reduced before the pyrolysis step... 

Reaction of l,3-bis(methylthio)-2-methoxypropane with 2 moles of lithium diisopropylamide5 (or w-butyllithium) effects (a) the elimination of methanol to form l,3-bis(methylthio)propene and (b) the lithiation of this propene to generate l,3-bis(methylthio)allyllithium in solution. Its conjugate acid, l,3-bis(methylthio)propene, can be regenerated by protonation with methanol, and has also been prepared (a) in 31% yield by reaction of methylthioacetaldehyde with the lithio derivative of diethyl methylthiomethylphosphonate,5 (b) in low yield by acid-catalyzed pyrolysis of l,l-bis(methylthio)-3-methoxypropane,6 and (c) in low yield by acid-catalyzed coupling of vinyl chloride with chloromethyl methyl sulfide.7... 

One synthetic route to the damascones starts with an appropriate cyclogeranic acid derivative (halide, ester, etc.). This is reacted with an allyl magnesium halide to give 2,6,6-trimethylcyclohexenyl diallyl carbinol, which on pyrolysis yields the desired l-(2,6,6-trimethylcyclohexenyl)-3-buten-l-one. Damascone is obtained by rearrangement of the double bond in the side-chain [98]. 

A convenient method for generating and trapping reactive ketenes is the flash-vacuum pyrolysis (FVP) of substituted Meldrum s acid derivatives. The dispirocyclobutane-l,3-dione 9 is obtained from the transient ketene 8 generated by FVP of the diester precursor 7.56... 

There is less evidence for the participation of azomethine ylides 88 in the early examples of the thermal cyclization reactions of Meldrum s acid derivatives 86. This reaction, conducted under flash vacuum pyrolysis conditions, may proceed via the methyleneketene 87. Hydrogen transfer from this highly unstable species may lead to the dipolar intermediate 88, which could cyclize either in a 6zr, to give 89, or 8tt, to give 90, manner (Scheme 27) [83JCS(CC)988 85TL833 87JCS(CC)140]. 

In 1776, Scheele first prepared cyanuric acid by pyrolysis of uric acid (7) (B-1793MI22000). In 1830, the first correct analysis was obtained by Liebig and Wohler (1835MI22000), but the molecular formula was incorrectly assigned as C6H6N606. Dreschel deduced the correct molecular formula, C3H3N3O3, in 1875 (1875JPR(11)289). Cyanuric acid does not have many industrial applications itself, but many of its derivatives are valuable monomers (see Section 2.20.5). 

Pyrolysis of Meldrum s acid (2,2-dimethyl-l,3-dioxane-4,6-dione) 362 proceeds by loss of acetone and CO2 to give ketene. Because of the ready availability of the starting material and the ease with which it can be functionalized at the acidic 5-position, pyrolysis of Meldrum s acid derivatives has been widely studied. Pyrolysis of the 5-formyl and 5-acyl derivatives 363 gives formyl or acylketenes 364, which can be trapped in a number of ways170-171. in many cases, loss of acetone and CO2 is accompanied by loss of CO to give a carbene, and this is illustrated by FVP of 365 at 560 °C which affords the Q -diketone 368 by way of ketene 366 and carbene 367172. 

A convenient conversion of homopiperazine into the 1-methyl derivative involves reductive methylation with formaldehyde and a Raney nickel catalyst.199-199a A small amount of the 1,4-dimethyl derivative is also obtained in this reaction. Use of benzaldehyde with homopiperazine gave 177 which was hydrolyzed to starting materials with hydrochloric acid.199 Pyrolysis of the dihydrobromide of 148 (R = H, R = CH2CH2Br) gave 178, 179, homopiperazine, and piperazine.140... 

Many rDA reactions are carried out at temperatures of 150 C or more in solution phase and often at temperatures of 400-600 C using the flash vapor pyrolysis (FVP) method individual conditions are referenced throughout the text. However, an accelerating effect by anionic, cationic and radical substimtion on either the dienophile or at the termini of the diene fragments has been predicted by Carpenter.Experimentally, this prediction has been substantiated only for anionic substitution. In 1967, Hart reported what is likely the first example of an oxyanion-accelerated rDA reaction. Both oxyanionic " and car-banionic substituents accelerate the cycloreversion reaction such that they proceed rapidly at room temperature (for example, equation 3). In addition, acid-catalyzed rDA reactions have been reported in which protonation effectively makes the dienophile fragment of the adduct more electron deficient. Grieco has utilized a room temperature retro aza DA reactitm useful for the N-methylation of dipeptides and amino acid derivatives (equatitm 4). ... 

It has already been noted that the esters of arsinous and arsonous acids disproportionate of heating. The formation of As(OR)3 in the pyrolysis of arsonic acid derivatives is thought to be the product of the secondary disproportionation reaction of the primary product RAs(OR)2-... 

Itaconic acid was isolated in 1836 from the pyrolysis products of citric acid (7) and the pol3mierization of the ethyl ester was observed by SwAETS in 1873 (2). While many patents relating to the acid and its esters as monomers have issued since that time, only recently have reports begun to appear in the scientific journals. The voluminous patent literature describes the use of polymeric itaconic acid derivatives in such applications as protective and decorative coatings, synthetic fibers, oil additives and rigid plastics as well as many others. Several summaries of the patent art and present commercial applications are available (3). Such information has been omitted from this review, which is directed primarily toward chemical behavior of the itaconic monomers and polymers. 

The pyrolysis oil collected between 200 to 275 C in the second step was 18.14 % by wt. on an anhydrous wood basis and corresponded to the hemicelluloses degradation . In general, hemicelluloses decompose to yield furans and its derivatives as well as a series of aliphatic carboxylic acids. Hardwoods are composed of pentosan-based hemicelluloses, while hexosan-based hemicelluloses are found in softwoods. Hardwood produces more low molecular weight carboxylic acids and furans than softwood upon decomposition . In contrast, hardwood contains a lower quantity of fatty acids than softwood. Cl - C6 carboxylic acids, fatty acids (e.g. Cia), fiiran and derivatives were identified in this fraction. In the second pyrolysis step, a little rearrangement of the lignin polymer occurred which was based on the formation of small amount of phenolic compounds 0,395 % by wt., anhydrous wood basis). The main components of this fraction are listed in Table 2. Betulin and lupeol were abundant in this fraction. Furthermore, levoglucosan which has been reported earlier as a major compound in the cellulose-derived pyrolysis oils, was identified in the second... 

In general, pyrolysis of poly(methacrylic acid derivatives) have a thermal behavior similar to those of the corresponding poly(acrylic acid derivatives). However, the amount of monomer is typically higher for the methacrylic series. 

Derivation Pyrolysis of acetone or acetic acid by passing its vapor through a tube at 500-600C. 

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