Thioalkyl groups

In addition to 1,2-alkyl shifts, sulfonium ylides with P-hydrogen can also undergo fragmentation into an olefin and a thioether [1317,1318]. When allylic thioethers are treated with two equivalents of ethyl diazoacetate in the presence of a catalyst for diazodecomposition, S-alkylation and elimination of the thioalkyl group from the initially formed a-alkylthio-4-alkenoic esters occurs to yield 2,4-dienoic esters [1319]. 

Polarization of double bonds due to the electron releasing effect of thioalkyl groups in thioenol ethers is much weaker than that reported for enol ethers (Table 4.26), as can be verified for the (E) and (Z) isomers of methyl propenyl sulfide in Table 4.40 [326, 327]. 

An electron-withdrawing group, such as nitro, at 4 - position of the benzyl ester determined a significant enhancement of both affinity and selectivity (compound MRS 1334). Any other modifications, including heterocycles at the 4-position, aminoalkyl or thioalkyl groups at the 3- and 5-positions, appeared detrimental in term of both affinity and selectivity. 

Attaching thioalkyl groups on the ethylene-dithiolene unit has been performed by Charlton and co-workers (372, 397). This led to the synthesis of eight neutral compounds with a formula [TTC -Ni(edt)2] (n = 4—11) (15) (Scheme 21). All of them exhibit strong absorption around 1000 nm with an absorbance of 40,000 dm3 mol-1 cm-1. 

Variation of the thioalkyl group, silyloxy group, counterion of the trityl group, or the geometry of the thioketene acetals influences the stereochemistry of the Michael addition. The optimal substituents depend on the nature of the acceptor. In most instances, however, more bulky silyl groups result in higher selectivity. 

The most prominent systems studied thus far are the Fischer carbene complexes of the Group 6 metals, i.e., Cr, Mo and W, e.g., 4. One important process that will be discussed at some length in this chapter is nucleophilic substitution, e.g., the replacement of the MeO group by a group with a different heteroatom such as an amino or thioalkyl group. This reaction proceeds via a tetrahedral intermediate (equation 1) and is similar to nucleophilic substitutions on carboxylic esters. 

Ylide generation from diazo compounds by reaction of carbenoids is a better method than photochemical or thermal dediazoniation in the presence of organic substrates containing heteroatoms, because these dediazoniations without metal catalysis yield, in most cases, not very selective carbenes. Here again, the copper-catalyzed route is in most cases inferior to that with rhodium catalysts. The diazoketo ester with a terminal thioalkyl group (8.145) can be obtained from the... 

Not only compounds with thioalkyl groups, but also those with aliphatic or aromatic sulfine groups react intramolecularly with carbenoid groups, as the examples in Schemes 8-63 and 8-64 demonstrate (Moody et al., 1988). In these reactions, cyclic sulfoxonium ylides are obtained. 

The alkoxy group in 2-alkoxy-5,6 dihydro-2H-pyrans (279) can be exchanged for another OR or NR R group when the substrate is heated at 150-200° with suitable alcohols or amines this opens the way to the nucleoside type of compound (e.g. 323). The alkoxy group in 279 can be also be exchanged for a thioalkyl group in a substitution reaction with mercaptans. The reactions are catalyzed by stannic chloride or diluted hydrochloric acid. The 2-alkylthio derivatives (324) are often accompanied by allylic rearrangement products (325). 

A number of polymer-bound thiols have also been found to be highly effective for ruthenium complexation and removal from polymers formed by ROMP processes. One of the best is the triazine derivative 11 (Figure 13.4), although a simple polymer-bound thioalkyl group can be equally effective [28]. Levels as low as 10-ppm ruthenium can readily be reached, provided that a suitably large excess of the bound ligand is used. 

Thus far we have considered reactions in which there is a net synthesis of a thiol-ester bond. In addition to these, two different types of ester interchange reactions have been demonstrated by which one thiol ester can be used to synthesize another. One of these methods involves a transfer of the acyl group to other mercaptans, whereas the other involves a transfer of the thioalkyl group to various acids. 

As mentioned above, ester interchange can also be accomplished through the enzymatic transfer of the thioalkyl group. Thus it has been demonstrated that extracts of C. kluyveri contain a coenzyme A-transphorase (CAT) which catalyzes a reaction between acetyl-SCoA and butyrate to form butyryl-SCoA [reaction (12)]. 

Through variation of the conditions employed for the hydrolysis, in the presence of mercuric compounds, of the thioalkyl groups from aldose or ketose dithioacetals (mercaptals), pyranosides, furanosides, 1-thioglyco-sides, or acetals may be obtained 6, 38-40). Several natural 1-thioglyco-... 

As shown earlier in this chapter, the thioalkyl groups may be removed from the sugar dithioacetals, and, according to the conditions employed, furanosides, pyranosides, thioglycosides, or acetals may be obtained. 

This finding was of unique biochemical interest because it represented a new type of energy transfer process (i.e. a thioalkyl group transfer) by means of which one activated carboxylic acid could be converted to another. Moreover, the coupling of reactions (25) and (27) offered an attractive explanation for the acetyl-P requirement in the oxidation of butyrate by dialyzed extracts of C. A / yoeri,< > and focused attention also on the possible role of CoA derivatives as the activated intermediates in fatty acid metabolism. 

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