Desilylation iminium ions

This TiCU-promoted allene cycloaddition has also been extended to the synthesis of five-membered ring heterocycles. In this case, increasing the steric shielding about the silicon atom seems to improve the cyclization process by suppressing the unproductive desilylative alkylation. For example, 1,3-di-methyl(r-butyldimethylsilyl)allene (39) reacts smoothly with cyclohexanecarbaldehyde to give in high yield the dihydtofuran predominantly as one stereoisomer (equation 31). Similar reaction with an N-acyl-iminium ion precursor (40) produces the pyrrolizidine system as a mixture of bicyclic isomers (equation... 

Use of trimethylsilylmethyl triflate enables the effective formation of intermediate iminium salts in the reaction mixture because the counteranion, triflate ion, is nonnucleophilic both to carbon and silicon atoms. N-Silyl-methylation can also be performed with other alkylating agents, such as silymethyl chloride, bromide, and iodide. However, the resulting iminium salts desilylate immediately after they are formed by the attack of the halide counteranions, leading to a serious decomposition of the requisite iminium intermediates. The final step of desilylation generating azomethine ylides is effected by a fluoride anion which is selectively nucleophilic to a silicon atom. 

As a variation of the process according to Eq. (2), the process of N-protonation of N-silylmethylimines and subsequent desilylation also works well. N-Protonated iminium intermediates are first formed and their desilylation by the counter ion (X ) leads to N-unsubstituted azomethine ylides of nonstabilized types 32. Although the N-unsubstituted azomethine ylides 32 would be able to isomerize irreversibly into the corresponding N-methylimine tautomers, these unusual azomethine ylides show a remark-... 

N-Silylation with trimethylsilyl triflate forms N-silylated iminium triflates, which are subsequently desilylated in situ with fluoride ion to generate N-silylated azomethine ylides (82TL2589 84CL2041 85CPB1975). 

Achiwa reported a short synthesis of pyrrolizidine derivatives by the cycloadditions using a nonstabilized azomethine ylide 23 (m = 1) (82CPB3167). When the trimer of 1-pyrroline is treated with a silylmethyl triflate, N-alkylation of the 1-pyrroline takes place. Then the resulting iminium salt is desilylated with fluoride ion in the presence of ethyl acrylate to give ethyl pyrrolizidine-l-carboxylate 295 as a mixture of stereoisomers (28%). After the epimerization of 295 with LDA, the ester moiety is reduced with lithium aluminum hydride in ether to provide (+ )-trachelanthamidine (296). A double bond can be introduced into 295 by a sequence of phenyl-selenylation at the 1-position, oxidation with hydrogen peroxide, and elimination of the selenyl moiety. The 1,2-dehydropyrrolizidine-l-carboxylate 297 is an excellent precursor of (+ )-supinidine (298) and (+)-isoretronecanol (299). Though in poor yield, 297 is directly available by the reaction of 23 with ethyl 3-chloropropenoate. 

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