Cyclopropenes acidity

Perkin1 attempted to prepare a cyclopropene acid by eliminating two molecules of hydrobromic acid froiL,y,di. ethyldibroiuglutar c ester. All the reagents which he tried failed to give h-u. the desired product, an ethoxy derivative (I) being forced, instead. 

This type of fatty acid is usually found at low levels (around 1%) in plant oils that contain cyclopropene acids. Cyclopropene acids are characteristic for oils from the Malvalaceae, Stercu-liaceae, Bombaceae, Tiliaceae, and Sapicidaceae families with sterculic acid (9,10-methyleneoctadec-9-enoic acid) and mal-valic acid (8,9-methyleneheptadec-8-enoic acid) as two major compounds. Sterculic acid is more abundant (about 50% of the total fatty acids in Sterculiafoetida oil). 2-Hydroxysterculic acid may also be present in this oil as a possible intermediate in the biosynthesis of malvalic acid (through a-oxidation of sterculic acid). 

Decarbonylation of cyclopropene acids. In a study of the synthesis of methyl sterculate (6) from methyl stearolate (1), Gensler et al.1 were unable to repeat the apparently straightforward synthesis based on addition of the Simmons-Smith reagent described in 1, 1021-1022. They were also unable to eifect addition of methylene generated by cuprous bromide decomposition of diazomethane. However, the reaction of (1) with diazoacetic ester in the presence of copper bronze, followed by hydrolysis, gives the cyclopropene diacid (2) in 70-90% yield. 

Cottonseed oil contains up to 0.5% of a pair of unique fatty acids malvalic (18 1) and sterculic (19 1). These acids are characterized by the presence of a cyclopropene group at or near the center of the fatty acid chain. Under appropriate conditions these give colored compounds and the development of a red color in the Halphen test (reaction with sulfur in carbon disulfide in the presence of amyl alcohol) is due to the cyclopropene acids and therefore characteristic of cottonseed oil (and other minor oils containing cyclopropene acids). This test for cottonseed oil was developed over a century ago in 1897. Inclusion of cyclopropenoid acids in animal diets causes undesirable physiological effects such as reduced egg production, poor hatching, and pink egg whites in chickens, and in rats, decreased growth and sexual development and... 

Jojoba oil consists primarily of wax esters of the acids listed here and long-chain alcohols. Kapok oil also contains up to 15% cyclopropene acids. 

Cyclopropane fatty acids occur frequently in bacterial membrane phospholipids. Also, they generally accompany the cyclopropene acids in seed oils (see following paragraph). Though other chain lengths have been reported, the most common cyclopropane acids are Cu and C19 (lactobacillic acid) compounds. They are probably formed from appropriate olefinic acids (16 1 9c and 18 111c) which are widely distributed in bacterial lipids. The cyclopropane acids have cis configuration but it is not clear whether they are individual enantiomers or racemic mixtures. 

Methods of determining cyclopropene acids continue to be developed (Fisher and Schuller, 1981 Vickery, 1980). Methyl malvalate and methyl sterculate have been separated by spinning band distillation (Pawlowski a/., 1981). 

The presence of cyclopropene acids in 40 seed oils including a number of the Malvales is reported (Vickery, 1980). 

Cyclopropene acids, 53,165,475,520 Cyclothone acclinideney 148 Cylindrocarbon radicicohy 151 Cytidylyltransferase, 504,505,550... 

The analysis of cyclopropane acids was reviewed by Minnikin (1978) and by Christie (1970), who also covered cyclopropene acids. Subsequently, Sebedio and Grandgirard (1989) discussed naturally occuring cyclic acids and cyclic acids formed in heated vegetable oils. Cyclic acids were the subject of a recent review (Le Quere and Sebedio, 1996). Useful information on gas chromatography (GC), mass spectrometry (MS) and high-pressure liquid chromatography (HPLC) of cyclic fatty acids can be found in books by Christie (1987, 1989). In the present chapter the aim is to concentrate on the more recent literature and those publications considered to be particularly... 

Figure 5.1 Structures of naturally occurring cyclic fatty acids. I, Cyclopropane acids a, 9,10-methylenehexadecanoic acid b, 11,12-methyleneoctadecanoic (lactobacillic) acid c, 8,9-methy-leneheptadecanoic (dihydromalvalic) acid d, 9,10-methyleneoctadecanoic (dihydrosterculic) acid. II, Mycolic (2-alkyl-3-hydroxy) acid. Ill, Cyclopropene acids a, 9,10-methyleneoctadec-9-enoic (sterculic) acid b, 8,9-methyleneheptadec-8-enoic (malvalic) acid. IV, Cyclopentenyl acids a, ll-cyclopent-2-enyl-undecanoic (hydnocarpic) acid b, 13-cyclopent-2-enyl-tridecanoic (chaulmoogric) acid c, 13-cyclopent-2-enyl-tridec-6-enoic (gorlic) acid. V a, 11-cyclohexyl-undecanoic acid b, 11-cycloheptylundecanoic acid.

Traditional procedures for quantification of total cyclopropene acid content have been reviewed by Christie (1970) and include titration with hydro-bromic acid and GC of the methyl mercaptan derivatives, products from reaction with silver nitrate/methanol and from methanethiol addition. However, a colorimetric test involving reaction with sulphur/carbon disulphide (the Halphen test) was found to be one of the most reliable methods for determining the small levels of cyclopropene acids present in oils containing cottonseed oil (Coleman, 1970). Another approach is GC of the relatively stable cyclopropane acids after hydrogenation with hydrazine (Conway, Ratnayake and Ackman, 1985)... 

A popular method is to analyse the total fatty acid methyl esters by CjC after reaction with silver nitrate/anhydrous methanol for 2 h at 30°C (Bianchini, Ralaimanarivo and Gaydou, 1981 Eisele et aL, 1974 Gaydou, Bianchini and Ralaimanarivo, 1983 Ralaimanarivo, Gaydou and Bianchini, 1982). Most fatty esters remain unchanged, but cyclopropene esters are converted to later-eluting methoxy ether derivatives and small amounts of ketone derivatives. Two partially resolved peaks, those from sterculic acid eluting later, are observed for each type of derivative from each cyclopropene acid and can be quantified and used to determine the proportion of sterculic and malvalic acids in the untreated oil. Verification of the identities of the acids can be determined from GC-MS of the methyl esters of the products (Ahmad et al., 1979 Eisele et ai, 1974) but prominent allylic ions in the mass spectra of the DMOX derivatives of the methoxy ethers are more readily interpretable to reveal the positions of the cyclopropene rings (Spitzer, 1995). 

In the NMR spectrum of the total fatty acid methyl esters from seed oils, a specific signal at 0.76 ppm for the cyclopropene methylene protons has been used, relative to the protons of either the terminal methyl or methyl ester, to give quantification of total cyclopropene acids down to about the 1% level (Boudreaux, Bailey and Tripp, 1972 Pawlowski, Nixon and Sinn-huber, 1972). However, because of incomplete resolution of signals, quantitation becomes increasingly problematic as the proportion of cyclopropene acids decreases. 

In the infra-red (IR) spectrum a band at 1010 cm characteristic of the cyclopropene system has been used to determine high levels of cyclopropene acids (Bailey, Boudreaux and Skau, 1965). There is another specific band at 1870 cm" which is weak in the IR spectrum but intense in the Raman spectrum (Kint et aL, 1981). Using peak height ratios of this band relative to that at 1745 cm (for triacylglycerol carbonyls), quantification of cyclopropene acids down to 0.03% was possible. The method may have potential for detecting vegetable oil contamination with, for example, cottonseed oil, but information is needed on its accuracy and precision. 

HPLC methods for quantification do not suffer from the same problems as GC methods, and cyclopropene acids can be analysed directly as methyl... 

Analysis of cyclopropene acid-containing triacylglycerols is not possible by GC because they are polymerized at high temperatures. One approach, applied to the seed oils of munguba (Bombax munguba Mart Schuch, Ahmad and Mukherjee, 1986) and Sterculia foetida (Pasher and Ahmad,... 

Ralaimanarivo, A., Gaydou, E. M. and Bianchini, J.-P. (1982) Fatty acid composition of seed oils from six Adansonia species with particular reference to cyclopropane and cyclopropene acids. Lipids, 17 (1), 1-10. 

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