Lachrymatory factor in onions

SpAre, G., and A. I. Virtanen On the lachrymatory factor in onion (Allium cepa) vapours and its precursor. Acta Chem. Scand. 17, 641 (1963). 

The lachrymatory factor in onion (thiopropanal-S-oxide) is considered to come from the S-l-propenyl-cysteine sulfoxide precursor [40]. Thiopropanal-S-oxide (CH3-CH2-CH=S=0) is unstable reacting with pyruvate to form propanol, 2-methyl pentanal and 2-methyl pent-2-enal [41]. It is of interest that no lachrymatory properties were found in 25 species of wild onions [42]. This would suggest that the 1-propenyl precursor may be absent from the wild onion. 

Brodnitz, M.H., J.H.R. Pascale, Thiopropanol-S-oxide-a lachrymatory factor in onion, J. Agric. Food Chem., 19, p. 269, 1971. 

The lachrymatory factor of onions, propanethial 5-oxide, dimerizes slowly in a benzene solution in the dark to form /ra .y-3,4-diethyl-l,2-dithietane 1,1-dioxide (12)122. 

These are respectively the 5-oxides and 5-dioxides of thiocarbonyl compounds. The lachrymatory factor of onions was shown to be (Z)-propanethial 5-oxide (R1 = Et, R2 = H), and a good deal of the chemistry of sulfines was carried out in connection with the study of the organosulfur chemistry of alliaceous plants [186]. 

Block et al. (1979) have recently shown that the lachrymatory factor of the onion is a sulfine, namely, CH3CH2CH=S=0. This sulfine is also thought to be formed from a sulfenic acid precursor, in this case by the intramolecular shift (14) of the sulfenic acid proton of trans- 1-propene-l -sulfenic acid. 

The most important flavour compound in raw onions is thiopropanal-S-ox-ide, the lachrymatory factor [145,146]. Other important flavour compounds are 3,4-dimethyl-2,5-dioxo-2,5-dihydrothiophene and alkyl alkane thiosulfonates such as propyl methanethiosulfonate and propyl propanethiosulfonate with a distinct odour of freshly cut onions [35, 36, 147]. Various thiosulfinates that have a sharp and pungent odour may also contribute to the flavour of onions. These compounds, however, are rapidly decomposed to a mixture of alkyl and alkenyl monosulfides, disulfides and trisulfides (Scheme 7.3) of which dipropyl disulfide, methyl ( )-propenyl disulfide, propyl ( )-propenyl disulfide, dipropyl trisulfide and methyl propyl trisulfide are the most important contributors to the aroma of raw and cooked onions (Table 7.5, Fig. 7.6) [148-150]. Recently, 3-mercapto-2-methylpentan-l-ol was identified in raw and cooked onions eliciting intense meat broth, sweaty, onion and leek-like odours [142, 151]. 

Thiocarbonyl oxides are a subject of active investigation. The natural occurrence of sulfines and related compounds in plants of the genus Allium (onion, garlic, etc.) is included in a superb and extensive review by Block [91]. Two detailed papers [92, 93] report the isolation of zwiebelanes from onions and their chemical synthesis involving intermediate sulfines produced by oxidation of di-l-propenyl disulfide, subsequent sulfoxide accelerated [3.3] sigmatropic shift and the [2+2] cycloaddition of the C=S and C=S=0 moieties. A further article [94] provides a great deal of information on the mechanism of formation of (Z)-propanethial S-oxide, the lachrymatory factor of the onion, as well as its chemical synthesis and reactions. Techniques of analysis of the volatiles of onions have been further improved [95]. 

The alkene-l,l-dithiolates (111), on phosgenation, give labile dithietanones (112), which give reactions typical of thioketens. The structure of the dimer of the lachrymatory factor (113) of onion has been re-assigned as (114), i.e. the first stable 1,2-dithietan derivative. Procedures for the oxidation of tetrafluoro- and tetrachloro-l,3-dithietans to the corresponding disulphones have been developed.Infrared and Raman spectral studies indicate that 1,3-dithietan is puckered (C21, symmetry) when free but planar (D2,-,) in the solid state. 

The principal component of the lachrymatory factor of the onion (Allium cepa) has been identified by flash vacuum pyrolysis (FVP)/microwave techniques as (Z)-propanethial 5-oxide, CH3CH2CH=S=0. It functions as a 1,3-dipole and as a dipolarophile, and it dimerizes in cold benzene to give principally trans-3,4-diethyl-1,2-dithietan 1,1-dioxide rather than 2,4-diethyl-1,3-dithietan 1,3-dioxide, as previously proposed. Formation of this sulphine from its cellular precursor, rran5-(+)-S-(prop-l-enyi)-L-cysteine sulphoxide, is suggested to involve the formation of a Schiff base and the elimination of ( )-prop-l-enesulphenic acid, which then rearranges to the sulphine (Scheme 2). 

Even considering the difference in the amount of 5 -alk(en)yl-L-cysteine sulfoxide, the total amount of thiosulfinates produced from onion is significantly lower than that from garlic being approximately 0.01-0.035 mg/g-fresh weight (Table 18.2). The reason for this is because most of the ( )-l-propene-l-sulfenic acid produced from E)-S-1-propenyl-L-cysteine sulfoxide is converted into a lachrymatory factor, and not so much can be used for the production of thiosulfinates. 

The first stable 1,2-dithietan has also been isolated. The sulphoxide (200), which is the onion lachrymatory factor, has been shown to dimerize when kept in benzene for 7 days in the absence of light, to the rra/w-dithietan (201). This has been rationalized as a [4 -f 2]cycloaddition in which the sulphine functions as a dipole and as a dipolarophile. 

Ethylsulfine (propanethial 5-oxide) was spectroscopically identical to the natural onion lachrymatory factor . The configuration of the natural ethylsulfine was established to be Z by the anisotropic deshielding effect of the CSO group on the C(l)-H in combination with benzene induced shifts . The lachrymatory effect of sulfines diminishes when the substituent is more bulky t-butylsulfine is devoid of lachrymatory activity. Also, 2,3-dimethylbutanedithial 5 ,5 -dioxide, a disulfine, was isolated from an onion extract . 

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