Triflates thioesters

When treated with dialkylboron triflates, thioesters preferentially form (E)-(O) boron enolates, which react with aldehydes to afford good yields of the 2,3-anti aldol products. 

When a mixture of aldehydes and (Z)-l-ethylthio-l-trimethylsilyloxy-l-propene is added slowly to a solution of tin(Il) triflate and 10-20 mol% of the chiral diamine 4 in acetonitrile, /1-silyloxy thioesters 5 are obtained in high simple diastereoselection and induced stereoselectivity. Thus, the chiral auxiliary reagent can be used in substoichiometric amount. A rationale is given by the catalytic cycle shown below, whereby the chiral tin(II) catalyst 6 is liberated once the complex 7 has formed33. 

Dialkylboron trifluoromethanesulfonates (triflates) are particularly useful reagents for the preparation of boron enolates from carbonyl compounds, including ketones, thioesters and acyloxazolidinones.4 Recently, the combination of dicylohexylboron trifluoromethanesulfonate and triethylamine was found to effect the enolization of carboxylic esters.5 The boron-mediated asymmetric aldol reaction of carboxylic esters is particularly useful for the construction of anti (3-hydroxy-a-methyl carbonyl units.6 The present procedure is a slight modification of that reported by Brown, et al.2... 

It was first observed that reactions of imines with ketene silyl acetals proceeded smoothly in the presence of 5 mol % Yb(OTf)3 (a representative lanthanide triflate) to afford the corresponding /3-amino ester derivative in a moderate yield. The yield was improved when Sc(OTf)3, rather than Yb(OTf)3, was used as catalyst (Eq. 3) [10]. Not only silyl enolates derived from esters, but also one derived from a thioester worked well to give the desired /3-amino esters and thioester in high yield. In the reactions of the silyl enolate derived from benzyl propionate, anti adducts were obtained with good selectivity. In addition, the catalyst could be recovered when the reaction was complete, and could be re-used. 

P, P] A single example of a stereoselective, Lewis-acid-assisted addition of silyl enol ether to an a,/J-unsaturated thioester has been reported by Mukaiyama and co-workers (76). In this report, silyl enol ether 36.1 was added to thioester 36.2 under the influence of an antimony(V) chloride-tin(II) triflate mixture to give a 94 6 (anti/syn) mixture of diastereomers in 70% yield (Scheme 36). 

P, P] By using antimony(V) chloride and tin(II) triflate, monothioketene acetals can be induced to add to a,/J-unsaturated thioesters (76). Interestingly, neither antimony(V) chloride nor tin(II) chloride by themselves effectively promote the reaction, implying that the combination of Lewis acids produces a new species. The stereochemistry of these additions is summarized in Scheme 44 and Table 16. For all cases studied, the reaction uniformly provides the anti diastereomers with good selectivity. The selectivity observed is somewhat lower than the optimized results observed with thioketene acetals and enones (vide supra). 

In contrast to all known chiral boron enolates, the external reagent (95) derived from ethanethioate (96) and 2,5-dimethylborolanyl triflate (2S,5S)-(90) effects asymmetric induction in the aldol reaction to provide 3-hydroxy thioesters of approximately 90% ee for most aldehydes (Scheme 41 Table 8).48... 

Another catalytic system has been developed by Kobayashi and Mukaiyama. Specifically, tin triflates ligated by chiral diamines (Figure 5.6e,f) activate aldehydes toward addition by silyl enol ethers of acetate and (0)-propionate thioesters (Table... 

High selectivity for either the anti or the syn aldol product can be obtained by using an appropriate thioester and a boron-mediated aldol reaction. The trans-enolate and hence the anti aldol product can be obtained from the 5-tert-butyl ester and dibutylboron triflate (1.68). The trans-enolute (note the convention that the O-metal substituent takes a higher priority than the SR substituent) is favomed owing to the steric interaction of the bulky tert-butyl group with the ligands on the boron... 

Aliphatic nitriles react slowly with phenols and phenyl ethers in the presence of trifluoromethanesulphonic acid to give ketones after hydrolysis, in a variation of the Houben-Hoesch reaction. The crystalline complex of copper(i) triflate and benzene induces the acylation of aromatic substrates with selenol esters, affording a transition-metal mediated version of the Friedel-Crafts reaction. Aromatic carboxylic acids can be converted into symmetrical diaryl ketones in good yield by treatment of their 5-(2-pyridyl)thioesters with bis-(l,5-cyclo-octadiene)nickel [equation (15)]. In contrast to other methods for preparing symmetrical aromatic ketones, this method allows their preparation from a single starting material. 

Thioesters give trans-hoion enolates 96 when treated with dicyclopentylboron triflate in the presence of Hiinig s base, as shown in Scheme 2.27 for t-butyl propanethioate [109] — a protocol that has been applied for the generation of the chiral boron enolate 97 featuring a C2-symmetric borolane moiety [110]. However, the stereoselectivity in enolate formation can be reversed by starting from phenyl (rather than from alkyl) propanethioate so that cis-enolate 98 forms exclusively [111] - a result that might originate from thermodynamic control [100]. 

Tm(II) enolates are accessible according to Mukaiyama s method [114] by deprotonation of ketones by means of tin(II) triflate in the presence of a tertiary amine. The latter seems to play a crucial role for successful deprotonation. It turned out that Af-ethylpiperidine proved itself to be the most appropriate reagent for this purpose. According to this procedure, which seems to be not applicable to carboxylic esters and thioesters, tin enolates became accessible from ketones and imide-type substrates the latter mainly derived of Evans auxiliaries. If a-substituted tin enolates were generated, their configuration was hardly determined spectroscopically but deduced from the stereochemical outcome of... 

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