Lachrymatory factor

Propanethial 5-oxide (78), the lachrymatory factor of the onion, functions as both a 1,3-dipole and a dipolarophile to give the first isolab le 1,2-dithietane 77, produced by rearrangement of the initially formed unstable... 

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]. 

Shallots Allium ascalonicum) are an allium wherein the bulb laterals separate into individual bulbs. Apparently, shallots do not develop a lachrymatory factor, such as thiopropanal-S-oxide upon maceration [35]. The major aroma constituents in shallots are similar to those found in A.cepa. In raw shallots, the most important aroma compounds appear to be dipropyl disulfide, propyl ( )-prope-nyl disulfide, methyl propyl trisulfide, dimethyl trisulfide and dipropyl trisulfide (Table 7.5, Fig. 7.6) [35, 152, 153]. 

The structure proof of the stereochemistry of the lachrymatory factor of onions was determined by reduction of the 1,2-dithietane dioxide with LAH. The ( )-hexane-3,4-dithiol product establishes the ring stereochemistry as trans (Scheme 43) (80JA2490). 

These are respectively the 5-oxides and 5-dioxides of thiocarbonyl compounds. The lachrymatory factor of onions was shown to be (Z)- ... 

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 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]. 

Tabic I. Sen.sory experience of different onion thio.sulfinales and lachrymatory factor... 

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. 

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

Block, E., Penn, R. E., Revelle, L. K. Structure and origin of the onion lachrymatory factor. 

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). 

When onion is cut or cmshed, ( )-5 -l-propenyl-L-cysteine sulfoxide (isoalliin) is converted into ( )-l-propene-l-sulfenic acid. Different from garlic, (Z)-thiopropanal S-oxide, a lachrymatory factor, is formed from the sulfenic acid by lachrymatory factor synthase (Figure 18.6). The remaining ( )-l-propene-l-sulfenic acid and methyl sulfenic acid produced from 5 -methyl-L-eysteine sulfoxide ean form methyl 1-pro-pene-thiosulfinate and 1-propenyl methane thiosulfinate that are further converted to sulfides such as 1-propenyl methyl disulfide and 1-prope-nyl methyl trisulfide. 

FIGURE 18.6. Formation of a lachrymatory factor and flavor compounds from S-1-prope-nyl cysteine sulfoxide. 

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. 

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