TMSOTf-Induced Reactions

Acid-induced reactions of silicon-containing unsaturated compounds with A -acyliminium intermediates have proven particularly useful. Allylsilanes lead to the corresponding allyl-substituted products. This reaction proceeds well even for -lactams (equation 49). 5.86 j, g penicillin-derived 4-acetoxyazet-idinone (91) reacts with excess allylsilane in refluxing dichloroethane, catalyzed by TMSOTf, to produce as the sole product trans compound (92). A -Acyliminium ions generated from cyclic ureas and carbamates (93) likewise yield allylated products with virtually complete trans stereoselectivity (equation SO). Allylstannanes react under somewhat milder conditions than allylsilanes, requiring only a catalytic amount of BFsZ EtjO at room temperature for a 3-lactam system (equation 51). This allylstannane procedure was applied to cyclic carbamate systems for the stereoselective synthesis of several isomers of statine. Thus, ethoxycarbamate (94) reacts with methallyltributylstannane in excellent yield and dia-stereoselectivity (equation 52). ... 

The role of the ligand has been found to be crucial in the silyl Lewis acid Mukaiyama aldol reaction, which opens interesting applications for synthetic organic chemistry. When TMSOTf induces the reaction, the silyl group of TMSOTf remains in the product and that of the silyl enol ether becomes the catalyst for the next catalytic cycle however, if the reaction is promoted by TMSNTf2, the silyl group of the catalyst is not released from... 

The TMSOTf-induced intramolecular reaction of allylstannanes with epoxides causes a regio- and stereoselective cyclization to five- or six-membered carbocycles (eq 94), depending on the substituents of the starting material. TiCU also gives high yields and slightly higher selectivities catalytic amounts of Lewis acids are insufficient for completion of the reactions. 

The critical comparisons between TMSOTf-induced acetal cyclization and TfOH-induced enol ether cyclization are combined in Table V. For clarity only the Z-isomers of 14 are included. Since we assume that reactions of 14 must proceed through the oxocarbenium ion i we conclude that, based on the disparity in stereochemical results, reactions of methyl (5a), ethyl (5b) and most likely also isobutyl (5c) acetals do not occur via an S l mechanism. However, the similarity in stereochemical outcome in the isopropyl series (d) strongly suggests that 5d does, to a large extent, react via an S l mechanism. 

SnCU-induced cyclizations between alkenes and enol acetates result in cycloalkanes or bicycloalkanes in high yield (eq 23). It is interesting to note that the TMSOTf-catalyzed reaction can yield fused products rather than bicyclo products. Alkenic carboxylic esters, allylic alcohols, sulfones, and sulfonate esters are also cyclized in the presence of SnC however, alkenic oxiranes often cyclize in poor yield. ... 

Intramolecular nucleophilic substitution reactions of phenol ether derivatives induced by activated iodine(III) reagents such as PIFA-TMSOTf and PIFA-BF3 Et20 [Eq. (7) - (11)] have been applied to total or formal synthesis of various types of bioactive natural products. 

As another example of novel catalysis employing montmorillonite, the clay was found to show excellent catalytic activity for the addition reaction of trimethylsilyl ketene acetal to a, -acetylenic esters (ynoates), which contrasted strikingly with the reactions induced by a homogeneous acid catalyst, trimethylsilyl triflate (TMSOTf), as well as the addition reactions of lithium enolates with ynoates [Eq. (17)] (89). Table XXIII summarizes the results of the reactions of the silicon and lithium enolates of methyl propionate (21) with ynoates (22a-c). Except for the reaction of 22c, ferric ion-exchanged montmorillonite (Fe-Mont), which is more acidic than Al-Mont, catalyzed exclusive 1,2-additions of trimethylsilyl ketene acetal to 22a and 22b to give 23 in... 

TMSOTf promotes the Diels-Alder cycloaddition of /7-tolyl vinyl sulfoxide with furan or cyclopentadiene the reaction is slower and less stereoselective than when induced by ethyl Meerwein s reagent (EtsOBFa). It also promotes the [4 + 2] cycloaddition between 2,2-dimethoxyethyl acrylate and several... 

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