Production of isothiocyanates

The brassicas of importance as foods include turnips, rutabagas, mustards, and the cole crops — cabbage, broccoli, cauliflower, and brussels sprouts. The production of isothiocyanates in brassicas is via an enzymatic reaction on specific glycosides. Some of the isothiocyanates, especially allylthiocyanate, are highly pungent and are mainly responsible for the odors of brown mustard, horseradish, cabbage, and other crucifers. Any process that destroys or inactivates enzymes in these plants will cause decreases in aroma production, resulting in a less distinctive flavor. This is usually the case when brassica foodstuffs are commercially preserved. 

On the other hand, isothiocyanates are known to be reactive with a variety of food constituents, such as amines, amino acids, proteins, thiols, and alcohols, due to their electrophilic properties to give a variety of compounds, including thioureas and dithiocarbamates [32]. Despite their chemical nature, only limited information has been made available regarding the stability of isothiocyanates and the biological activity of reaction products of isothiocyanates with food constituents [33-35]. 

ThiO 4-oxoselenazolidines substituted in the 3-position, have been described in a number of patents, (79, 80). They are used as intermediates in the manufacture of pharmaceuttcal products and color sensitizers in photography. They are obtained by action of isothiocyanates on a-hydroselenoacetic acid in the presence of strong base and in the absence of air (Scheme 77). 

The reaction of enamino ketones with isocyanates and isothiocyanates has not been studied extensively. The enamino ketone (162) has been shown to react with phenyl isothiocyanate to give 163, the product of C acylation 114). Enamino ester derivatives of acetoacetic ester react similarly with isothiocyanates, also giving the C-acylated products 115). 

Schiff s bases also underwent C or N acylation with isocyanates (698) and isothiocyanates (698,701). Further studies provided 2 1 and 2 2 reaction products of arylisothiocyanates and enamines (702) and polymers derived from enamines and bisisocyanates (703). 

The thiono derivatives of tetrahydro-1,3-oxazine became a subject matter of some interest since Kjaer and Jensen discovered that products of enzymatic hydrolysis of Malcolma maritima contain 6-methyl- and 6,6-dimethyl-2-thionotetrahydro-l,3-oxazine (26). The authors proved the identity of these compounds with the products of cyclization of 3-hydroxypropyl-isothiocyanate in an alkaline medium. 

Reaction of dimsyl anion with isothiocyanates gives a-thioamidosulphoxides 478 in 12-59% yield, whereas with isocyanates it affords a mixture of a-amidosulphoxides 479 and methylsulphinylmalonoamides 480, the products of a double addition549 (equation 289). 

As in the case of silanone 9, the reaction of the silylene bis[2-(dimethylamino-methyl) phenyl]silanediyl (8) with phenyl isothiocyanate was examined.29 In this reaction the expected silanethione 36 was obtained as a single product even in the presence of (Me2SiO)3, no insertion product of 36 into a Si-O bond of (Me2SiO)3 was observed (Scheme 10). 

In analogy to the thiohydantoin synthesis, the PEG-bound diamines have also been treated with various alkyl and aryl isothiocyanates and, after applying traceless cyclative cleavage, the desired thioxotetrahydropyrimidinones were obtained in excellent yields. In contrast to the thiohydantoin synthesis, purification of the products was more complicated if the excess of isothiocyanate amounted to more than 2.2 equivalents [85],... 

The conversion of isothiocyanates to isonitriles under microwave conditions has been studied by Ley and Taylor using a polymer-supported [l,3,2]oxaphospholidine [119]. The use of 3-methyl-2-phenyl-[l,3,2]oxaphospholidine in solution is less favored [120] due to the associated toxicity and instability of the phosphorus-derived reagent, as well as the need to isolate the products from a complex reaction mixture by vacuum distillation. This drawback has been resolved by attaching the active [l,3,2]oxaphospholidine to a polymer matrix. 

Thus, (dimorpholinophosphoryl)formonitrile oxide undergoes 1,3-addition reactions with HC1, HI, primary and secondary amines, acylhydrazines, and even with thiourea or thiosemicarbazide (Scheme 1.13) (98). The former gives (dimor-pholinophosphoryl)isothiocyanate and urea. Those products might arise from a retro destruction of the unstable 1,3,5-oxathiazoline. The latter transforms to the isothiocyanate, the product of addition of a second molecule of thiosemicarbazide. (98). 

The interaction of biguanide (CL) and isothiocyanate esters in dimethyl-formamide under mild conditions (376) gives excellent 3delds of 1-sub-stituted hexahydro-4,6-di-imino-s-triazine-2-thiones (CLII), together with small quantities of thioammeline (CLIII) and monosubstituted melamines (CLIV). Under more severe conditions, the latter (CLIV) become the main products. The production of these triazines is explained by a mechanism involving the primary formation of addition products (CLI), followed by their cydisation with loss of either ammonia, hydrogen sulphide, or primary amine (376). 

Because of the fraction in which the inhibitor(s) were present, the allelochemicals in campestris are most likely isothiocyanate derivatives such as allyl isothiocyanate, a breakdown product of the thioglucoside, sinigrin (21). 

It is very likely that the inhibitors are breakdown products of glucosinolates, which are considered nontoxic in themselves (23) but can yield physiologically active products upon hydrolysis by the enzyme myrosinase. Through hydrolysis glucosinolates can yield isothiocyanates (24-28). Ju et al. (29) demonstrated the capacity of thiocyanates as allelopathic agents, and isothiocyanates also have allelopathic activity (30). 

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