Carbonyls trimethylsilyl trifluoromethanesulfonate

High enantioselectivities may be reached using the kinetic controlled Michael addition of achiral tin enolates, prepared in situ, to a,/i-unsaturated carbonyl compounds catalyzed by a chiral amine. The presence of trimethylsilyl trifluoromethanesulfonate as an activator is required in these reactions236. Some typical results, using stoichiometric amounts of chiral amine and various enolates are given below. In the case of the l-(melhylthio)-l-[(trimethylsilyl)thio]ethene it is proposed that metal exchange between the tin(II) trifluoromethanesulfonate and the ketene acetal occurs prior to the 1,4-addition237,395. 

Treatment of the enantiopure substrate 4-341 containing a sila-ene moiety with trimethylsilyl trifluoromethanesulfonate (TMS OTf) gave 4-342 in a highly stereoselective fashion and 52% yield. It can be assumed that the reaction passes through the two chairlike transition structures 4-345 and 4-346 (Scheme 4.76). It is of interest that the stereochemistry of the two C-C-double bonds in 4-341 did not influence the configuration of the product. Moreover, when using EtAlCl2 as mediator, a 3 l-mixture of 4-343 and 4-344 is obtained in 40% yield. This can be explained by an axial orientation of the carbonyl moiety in the transition state. 

Reaction of benzaldehyde and its 4-nitro derivative with the enantiopure diene 796 in DCM and in presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf) afforded the condensed enantiopure oxathiocanes 777 and 778 (50% yield). The reaction path began with the ionic addition of the diene 796 onto the carbon atom of the aldehyde function in 797a,b activated by trimethylsilyl cation attack to the carbonyl oxygen. 

An example that illustrates the advantages that an enol ether can offer vis-d-vis the parent carbonyl compound in iminium ion cyclization is found in Overman and Goldstein s construction of the allopumi-liotoxin A alkaloid intermediate (32). A variety of Mannich conditions failed to produce (32) from amino ketone (28), but yielded only cyclopentaoxazolidine (29). However, intramolecular Mannich cyclization was accomplished in 52% yield at low temperature by treatment of the bicyclic trimethylsilyl enol ether (30) with 1.1 equiv. of trimethylsilyl trifluoromethanesulfonate (Scheme 15). Remarkably, (32) produced in this manner was racemic, a result ascribed to facile [3,3] sigmatropic rearrangement of intermediate (31). 

Cyclizations can be initiated by an oxonium ion generated from a carbonyl, acetal or ketal precursor. When dimethylacetal is treated with trimethylsilyl trifluoromethanesulfonate in dichloromethane. 6-exo cyclization takes place forming piperidine 11 with the methyl and vinyl group stereoselectively synls. The remaining methoxy substituent gives a mixture of diastereomers. 

A-Methyl Nitrone Formation. Aldehydes or ketones react with a stoichiometric amount of Me3SiN(Me)OSiMe3 (1) in benzene at 50 °C for 24 h to give A-methyl nitrones in good to excellent yields (eq 1). For deactivated carbonyl corrpounds, such as p-nitrobenzaldehyde, p-(dimethylamino)benzaldehyde, and 2-furaldehyde, addition of trimethylsilyl trifluoromethanesulfonate (0.03-0.04 equiv) as a catalyst is necessary for the formation of A-methyl nitrones otherwise accumulation of hemiaminal intermediates (2) occur. Sequential intra- and intermolecular [3 + 2]... 

Aceto(carbonyl)cyclopentadienyl(tri-phenylphosphine)iron. 3-Acylthiazolidine-2-thiones. Bis(cyclopentadienyl)titana-cyclobutanes. Bromomagnesium diisopro-pylamide. Cerium(III) chloride. Dichloro-phenylborane. Dimethylphenylsilyllithium. Ethylene chloroboronate. Ketene bis(trimethylsilyl)ketals. Mandelic acid. Norephedrine. Potassium fluoride-Alumina. (S)-(—)-Proline. Tetra-n-butylam-monium fluoride. Tin(IV) chloride. Tin(II) trifluoromethanesulfonate. Titanium(IV) chloride. Tri-n-butyltin fluoride. Trityl perchlorate. 

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