Carbonyl sulfide pathway

The fate of thiocyanate in soil is largely uncharacterized. Early studies have shown that thiocyanate can undergo both aerobic (Betts et al. 1979) and anaerobic microbial degradation (Betts et al. 1979 Stafford and Callely 1969 Youatt 1954) however, the degradation pathway has not been defined (Brown and Morra 1993). Saturated soils treated with thiocyanate were found to emit carbonyl sulfide (COS) (Minami 1982 Minami and Fukushi 1981). Katayama et al. (1992, 1993) have reported the formation of carbonyl sulfide from the biodegradation of thiocyanate by pure and mixed cultures of Thiobacillus thioparus. 

Special attempts were made to identify carbonyl sulfide as a metabolite of dichlofluanld because it would help us understand the metabolic pathway. In a previous investigation into the metabolism of captan, CS was found as a metabolite (4), while in a latter study, COS was reported ( 17). Viles reagent (18) often used by many investigators for this purpose, proved to be unsuitable because of the possible presence of carbon disulfide. Both COS and CS2 give colored copper chelates that can not be quantitatively separated by tic. Furthermore, analysis of the mixtu-... 

For each of the three precursors of hydrogen sulfide, i.e. sulfur dioxide/sulfite, sulfate, and cysteine, a different biosynthetic pathway has been established. Figure 1 gives an overall view of these three pathways a suggestion for a path of synthesis of hydrogen sulfide from carbonyl sulfide is included. 

In a separate study of the decomposition of sulfinic acids in the absence of solvent at 200 °C the major products were sulfur dioxide and alkenes . Minor products were water, carbon dioxide, carbon monoxide, carbonyl sulfide and sulfur. The reaction is considered to proceed by a unimolecular free radical mechanism, although kinetic evidence is lacking. Olefin formation results from transfer reactions followed by elimination and one plausible pathway is... 

Limited information is available on the biotransformation of carbon disulfide in humans, and the metabolic products of carbon disulfide are not completely known. In animals and humans the proposed metabolic pathways involved in the metabolism of carbon disulfide (Beauchamp et al. 1983) are depicted in Figure 2-3, reactions i-x. Reaction i has been demonstrated in in vivo animal studies and in in vitro assays. Reactions ii-v are proven by in vitro studies, while products of reactions vi-ix are the results of proposed metabolic pathways of carbon disulfide in animals and humans. Carbon disulfide is metabolized by cytochrome P-450 to an unstable oxygen intermediate (reaction i). The intermediate may either spontaneously degrade to atomic sulfur and carbonyl sulfide (reaction ii) or hydrolyze to form atomic sulfur and monothiocarbonate (reaction iii). The atomic sulfur generated in these reactions may either covalently bind to macromolecules (reaction iv) or be oxidized to products such as sulfate (reaction v). 

Recent work on the role of solvated electrons in intra-DOM reduction processes has demonstrated the importance of trapped e in reactions with species adsorbed on the DOM matrix [98-100]. Modeling of DOM mediated photoreactions indicated the importance of sorption of molecules to DOM for reaction to occur [98, 99]. This is consistent with the lifetime of e" precluding escape from the aqueous DOM matrix into bulk solution. Since many important reactions with environmental implications involve binding or adsorption to DOM - see, for example, [3,101,102] - the role of matrix effects and the caged electron could be very significant. Some workers have suggested that since e remains primarily trapped within the DOM matrix, Oj must be formed by direct electron transfer from the excited triplet state of DOM to O2 [14]. However, it is equally if not more plausible that Oj may be produced by the reduction of Oj by radicals or radical ions produced by intramolecular electron transfer reactions from irradiated DOM [25]. The participation of radicals in the production of carbonyl sulfide and carbon monoxide from irradiated DOM in South Florida coastal waters was recently demonstrated by Zika and co-workers [81-83] and potential pathways for the formation of free radicals from irradiated DOM were discussed. Clearly, the relative contribution of e q and associated transients to the photochemistry of DOM has not been unequivocally resolved in the literature. 

Commercial hexythiazox is a racemic mixture of the two trans enantiomers Scheme 26.2.2 shows the main synthetic pathways [11, 17, 19]. Starting from 4-chloro propiophenone the key intermediate erythro amino alcohol may be obtained by stereoselective catalytic reduction of the corresponding hydroxy imi-noketone or by sodium borohydride reduction of the aminoketones obtained via Gabriel synthesis. Different routes lead from this aminoalcohol to the trans-thiazolidinone system the basis of all routes is activation of the hydroxy group, e.g., in form of the sulfonate and a ring forming reaction with carbon disulfide or carbonyl sulfide. The final acylation of the NH group with cyclohexyl isocyanate leads to hexythiazox. 

Another product resulting from DMS oxidation is methane sulfonic acid, CH3SO3H. Laboratory studies under near-atmospheric conditions have shown that the yield of SO2 is 90% and that of CH3SO3H about 10%. The pathway leading to CH3 SO3H is not yet established, however. The CH3SO radical appears to be an intermediate. Methane sulfonic acid is a component of the marine aerosol, and it also is present in marine cloud and rain waters. Carbonyl sulfide is another product resulting from... 

However, TMEDA unlike HMPA, does not cause flow over from a carbonyl to conjugate addition manifold for many lithiated systems. For example, lithiated allylic sulfides undergo conjugate (or 1,4 ) addition to cyclopent-2-enone in the presence of HMPA (see Allyl Phenyl Sulfide), but in the presence of TMEDA, carbonyl addition only is observed. The perception that TMEDA is unable to form solvent-separated ion pairs required for conjugate addition in this case now requires reevaluation. j In the reaction of lithio a-trimethylsilylmethyl phenyl sulfide with cy-clohexenone, HMPA promotes predominant conjugate addition, whereas TMEDA has little effect on the normal carbonyl addition pathway taken in THF alone (eq 8). ... 

Probably the most important reactant in the formation of volatile meat flavor compounds is hydrogen sulfide. It can be formed by several pathways during meat cookery, but one mechanism is Strecker degradation of cysteine in the presence of a diketone as established by Kobayashi and Fujimaki (29). The cysteine condenses with the diketone and the product in turn decarboxylates to amino carbonyl compounds that can be degraded to hydrogen sulfide, ammonia and acetaldehyde. These become very reactive volatiles for the formation of many flavor compounds in meat and other foods. 

Thermal decomposition of azolines of type 13 follows an analogous pathway to that already discussed for azolones (see Section 6.04.5.2). As depicted in Scheme 11, those azolines having X = S follow a typical retro-l,3-dipolar cycloaddition process (path a) affording carbonyl compounds and nitrile sulfide intermediates, which in the absence of a trapping... 

Molecular orbital theory also predicts that a nucleophile of the sulfide type will bond at the carbon terminus of a conjugated ene carbonyl system that is, the nucleophile will bond with the electrophile in the Michael addition mode of reaction (20). Thus, the reaction of polysulfide dianion with an enone represented by a chalcone may proceed initially in such a manner as shown in Scheme 2, which reproduces one of the several pathways... 

In addition to enol silyl ethers, an optically active boryl enolate underwent the highly anri-stereoselective aldol reaction with a wide variety of aldehydes in the presence of TiCU (Eq. 34) [120]. The vinyl sulfides shown in Eq. (35) reacted with a,fi-unsaturated ketones via the 1,4-addition pathway in the presence of a titanium salt, but the reaction was followed by the cleavage of a carbon-carbon bond in the cycloalkane to give open chain products in a stereoselective manner [121]. The 1,2-type addition was observed, if the olefinie acetal was used instead of the corresponding carbonyl compound, as shown in Eq. (36) [121], The successive scission of the carbon-carbon bond took place analogously to give the same type of products as shown in Eq. (35). 

Barton has devised such a pathway for converting carboxylic acids to hydroperoxides via A -hydroxypyridine-2-thione esters [16a,b] (Scheme 5). The yields of hydroperoxides are 45-89% [16]. Reduction of the crude products with trimethyl phosphite (rt) or dimethyl sulfide (80 °C) readily provides the corresponding alcohols. On the other hand, tosylation and ensuing Kornblum-De La Mare type fragmentation [17] leads to carbonyl compounds [16b] (Scheme 5). 

Cysteine may react with carbonyls to yield flavor compounds (e.g., trithiolanes) or be decarboxylated to give cysteamine, deaminated to provide a-keto-3-thiopro-pionic acid or degraded to free HjS. While each of these pathways may yield significant aroma compounds, the formation of free H2S is particularly important. H2S is a flavor compound in its own right and also is very reactive with carbonyls and free radicals to form very potent aroma compounds (e.g., ethyl sulfide, diethyl disulfide, amyl mercaptan, and 3-methyl-2-butenethiol). 

There have been several papers dealing with the oxidation reactions of nitrogen and sulfur-based compounds. Hindered amines, such as substituted 2,2,6,6-tetramethylpiperidines, are easily oxidized by electron-transfer reactions to the corresponding cation, by the sulfate radical anion, and by sensitized electron transfer to carbonyl triplets. Radicals derived from tertiary piperidines were observed directly by optical spectroscopy and deprotonated to a-alkylamine radicals. The amine radical cation derived from secondary piperidines deprotonated to give aminyl radicals. In the presence of oxygen, both classes were oxidized to give nitroxyl radicals, but by different proposed mechanisms. Both oxidation and fragmentation pathways have been observed in the photochemical reaction of alkyl phenyl sulfides with tetranitromethane. The oxidation of various A-(arylthio)-4-substituted-2,6-diarylanilines (18) with PbOa yielded, in most cases, persistent radicals that could... 

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