Ethers neighboring group participation

In the reaction of A-Boc derivatives of lactim ethers the desilylation step is aborted by neighboring group participation which leads to 6-trimethylsilylmethyloxazin-2-ones. Allylation is diverted to annulation in cases such as 1-acetylcycloalkenes. ... 

The neighboring group participation was invoked for the preferential capture of a ketonic function of keto aldehyde substrates by enol silyl ethers in TMSOTf-cat-alyzed reactions [42], TMSOTf initially coordinates with the aldehyde, and the ketonic oxygen attacks intramolecularly the activated aldehydic carbonyl, thus resulting in the domino-type activation of ketonic functions (Scheme 2-15). 

The Mitsunobu reaction proved to be useful for the synthesis of aryl alkyl ethers from alcohols and phenols. The method proceeds under mild conditions and tolerates many functional groups with inversion of configuration, as exemplified by the reactions of lactate and ewrfo-5-norbornen-2-ol (equations 4 and Neighboring group participation,... 

TMSOTf, TMSOMe, CICH2CH2CI, 0°C to rt, 98% yield. The MM ether is converted to a simple MOM ether. When the TMSOMe was left out of the reaction, neighboring group participation occurred to give a 1,3-dioxane. ... 

This led us to probe the behavior of the diazonium salt 67, featuring a neighboring silyloxy group (Scheme 11). When subjected to the reaction, this afforded the bicyclic ether product 68, confirming that neighboring group participation had occurred. 

The reported studies of ether oxygen participation in carbohydrate derivatives demonstrate that 0-3,0-4,0-5, and 0-6 participation can occur. In all the cases illustrated, the participation occurs with the oxygen interacting from an antiperiplanar stereochemistry. Since this is the normal stereochemistry required for neighboring group participation, ether oxygen anchimeric assistance would not be expected unless such a configuration is attainable. Participation, of course, may result after ionization has produced a carbocation intermediate. The latter pathway, however, seems to be unimportant for product formation in most cases discussed here. 

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