Silyl triflic imide

Although [2+2] cycloaddition of alkyl enol ether with acrylate was promoted in the presence of TMSNTf2, it is hard to handle and isolate TMSNTf2 owing to its moisture sensitivity. As an alternative and concise method, in situ generated silyl triflic imides were available in the [2+2] cycloaddition of alkyl enol ethers. Thus, after mixing EtsSiH and Tf2NH to... 

A possible mechanism for the catalytic [2+2] cycloaddition reaction catalyzed by Tf2NH is depicted in Scheme 4.9. The Mukaiyama-type Michael addition of silyl enol ether to enoate catalyzed by silyl triflic imide aHbrds the corresponding silyl ketene acetal, and then it proceeds successively to the intramolecular silyl oxonium carbon to... 

SCHEME 4.8 [2+2] Cycloaddition of alkyl enol ether using in situ generated silyl triflic imide from triethylsUane and Tf2NH. 

The problem of the nucleophilicity of amides in glycosylation reactions is not limited to the sulfoxide method and has been shown to result in the formation of glycosyl imidates from intermolecular reaction with activated donors. It appears that this problem may be suppressed by the prior silylation of the amide [348,349]. Accordingly, it may be sufficient to operate the sulfoxide method with an excess of triflic anhydride when amides are present so as to convert all amides into O-triflyl imidates, which are then hydrolyzed on work-up. Despite these problems, several examples have been published of successful sulfoxide glycosylation reactions with acceptors carrying remote peptide bonds [344,345] and with donors coupled to resins via amide-based linkages [346,347], with no apparent problems reported. Sulfonamides and tertiary amides appear to be well tolerated by the sulfoxide method [340,350],... 

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