Monocrotic acid

Monocrotaline on alkaline hydrolysis yields retronecine and monocrotic acid, CjHijOg, b.p. 145-6°/18 mm., [a]p 0°, which forms a p-bromo-phenacylester, m.p. 78°,and a methyl ester, b.p. 94-6°/18 mm., characterised by a 2 4-dinitrophenylhydrazone, m.p. 95-6° see below). The acid gives the iodoform reaction and is oxidised by sodium hypobromite to a mixture of dl- and mcso-aa -dimethylsuccinic acids (I). These and other reactions show that monocrotic acid is a -dimethyllaevulic acid (II) and this has been eonfirmed by comparison with a synthetic specimen of the acid. The methyl ester of the synthetie acid forms a mixture of 2 4-dinitrophenylhydrazones, m.p. 108-9° and 121-2°, into which the analogous produet, m.p. 95-6°, first made from methyl monocrotate see above), has also been separated. 

When monocrotaline is hydrogenolysed the acid scission product is monocrotalic acid, CgHigOj, m.p. 181-2°, [a]p ° — 5-33° (HgO), which provides a methyl ester, m.p. 79-80°, [ ]d°° — 16-2° (EtOH), containing one active H atom and a p-bromophenacyl ester, m.p. 162-3°. It is a lactonic acid, which on boiling with sodium hydroxide solution loses carbon dioxide and produces a/3-dimethyllaevulic acid (monocrotic acid, II). 

Monocrotic acid, CiHnOt. A mixture of 20 g. of monocrotaline and 40 g. of barium hydroxide octahydrate in 250 cc. of water was refluxed for one hour. After cooling, the solution was saturated with carbon dioxide and the barium carbonate was filtered. The filtrate was made just acid to congo red with hydrochloric acid and was then submitted to continuous extraction with ether for twelve hours. The ether extract was dried, the ether was removed, and the residue was distilled in vacuo at 145-146° (18 mm.) yield, 4.2 g. (48%). 

Retronecine hydrochloride, CiHuNOi-HCl. The aqueous solution obtained after ether extraction of monocrotic acid was evaporated to dryness in vacuo. The sirup thus obtained was dissolved in 100 cc. of absolute ethanol and the solvent was removed by evaporation. A crystalline mass resulted and this was extracted with three 30-cc. portions of boiling absolute ethanol to separate the alkaloid salt from barium salts. The combined ethanolic extracts were concentrated to 20 cc. and allow ed to cool. The salt, retronecine hydrochloride, separated and the filtrate yielded an additional amount by the addition of ether. Recrystallization from absolute ethanol gave white prisms, m.p. 161-162° yield, 10.3 g. (87%). 

The establishment of the structure of monocrotic acid as a, 3-dimethyl-levulinic acid (LXIV) and its formation from monocrotalic acid by loss of carbon dioxide limits very dehnitely the possible structures of the latter. Monocrotalic acid was known to contain one carboxyl group and the presence of a lactone group was indicated on back titration of the acid... 

After Adams and Long (71) had established the structure of monocrotic acid unequivocally by the synthesis of a,/3-dimethyllevulinic acid (LXIV), these workers established some grounds for the elimination of the unsatisfactory structures among those three postulated for monocrotalic acid. One of the intermediates prepared in the synthesis of a,/3-dimethyl-levulinic acid was the substituted malonic ester LXXV. By acid hydrolysis, the malonic acid derivative (LXXVI), which would be formed first, might then isomerize spontaneously to LXIX, one of the possibilities for mono-... 

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