Pyrolytic elimination from esters

Carboxylic esters are usually decomposed by heating the fused steroid at 300 to 500°, often under vacuum. Benzoates decompose at lower temperatures than acetates, and some selectivity of elimination can be achieved from steroids carrying both functionalities. The high temperatures necessary for these eliminations preclude their use with sensitive compounds, and successful applications have usually been with naked steroids, in which case yields may be quite good. °°  

Xanthates decompose at much lower temperatures than do carboxylic esters (100 to 250°), but their preparation is lengthy.  

Carbonates and carbamates are reported to be intermediate in stability and easier to prepare than xanthates. They can conveniently be prepared directly from the alcohol in high yield and give clean conversion to olefins. Cholesta-3,5-diene, for example, can be readily obtained via the phenylcar-bamate (114) or ethylcarbonate (115) of cholesterol. Such esters appear to have been somewhat neglected as synthetic intermediates. 

A rule for the correlation of ease of pyrolytic elimination with structure has been suggested by DePuy. The order of ease of decomposition among the commonly used esters is  

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Pyrolysis of N-oxides, 338 Pyrolytic elimination from esters, 334 3-(N-pyrrolidinyl) -androsta-3,5-diene-lla,17/3-diol, 410... 

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. 

Evidently, on changing the ethyl ester to methyl ester in bromoacids, the pyrolytic eliminations will be limited only to a debromination process. In this respect, Table 21 describes the results on the pyrolysis of methyl bromoesters under maximum inhibition170 and the data provide further evidence for COOCH3 participation in the elimination from methyl co-bromoesters. By analogy with previous papers16 168,169 the mechanisms of these reactions were also explained in terms of an intimate ion-pair intermediate as for equation... 

An important group of alkene-forming reactions, some of which are useful in synthesis, are pyrolytic eliminations. Included in this group are the pyrolyses of carboxylic esters and xanthates, of amine oxides, sulfoxides and selenoxides. These reactions take place in a concerted manner, by way of a cyclic transition state and therefore proceed with syn stereochemistry, such that the hydrogen atom and the leaving group depart from the same side of the incipient double bond (in contrast to the eliminations discussed in Section 2.1) (2.14). 

While there is a paucity of information on neighboring sulfur involvement in elimination reactions, the results of McCabe and Livingston suggest participation in the solvolytic elimination of hydrogen chloride from the chloroalkene (91) and in the pyrolytic elimination of adipic acid from the adipate ester (92). 

Reactions.—A variety of interesting and useful syntheses have been published involving the reaction of dimsyl anion [MeS(0)CHa ] with esters and lactones,with disulphides, with chlorosilanes, with sulphinate esters, with organoboranes, and with stilbenes. Simple and functionalized a-sulphinyl carbanions can be condensed with carbonyl compounds or alkylated, often in a stereocontrolled manner, as in a nicely conceived synthesis of biotin. Considerable attention has been given to methods for the removal of the sulphoxide function following carbon-carbon bond formation. Among the methods used are reduction by aluminium amalgam (with j3-keto-sulphoxides), reduction with Raney nickel, pyrolytic elimination of sulphenic acid, elimination of sulphur dioxide from sultines, e.g. (64), and sulphoxide-... 

FIGURE 10.11 Pyrolytic elimination of a carboxylic acid from an ester. 

Cellulose xanthate is another modified cellulose obtained from alkali cellulose RceiiOH NaOH + CSj RceiiO-C(S)SNa + HjO Xanthates undergo a pyrolytic reaction with elimination, similar to other esters ... 

A useful synthesis of functionalized dienes from alkenes used the initial formation of a stable rr-allyl palladium complex, followed by reaction with a-sulfinyl ester enolate and pyrolytic sulfoxide elimination (Scheme 55). ... 

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