Propanethial 5- oxide, 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... 

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]. 

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). 

The compound which causes tears (the lachrymatory factor) is (Z)-propanethial-S-oxide (II) which, once the onion bulb is sliced, is derived from trans-(-h)-S-(l-propenyl)-L-cysteine sulfoxide (I) by the action of the enzyme al-liinase. Alliinase has pyridoxalphosphate as its coenzyme (cf. reaction sequence 17.11). Chopping of onions releases 3-mercapto-2-methylpentan-l-ol, which, with its very low threshold of 0.0016 pg/1 (water), smells of meat broth and onions. Raw onions contain 8-32pg/kg, and onions which have been cut, stored for 30 minutes and then cooked contain... 

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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