Water, reaction with glycals

This approach was adopted by Daves for the coupling of iodo derivatives of nitrogen heterocycles with cyclic enol ethers and furanoid glycals in a water/ethanol mixture, using tetrabutylammonium chloride as a promoter (Eq. 5) [10]. Surprisingly, the use of absolute ethanol as reaction solvent was ineffective. 

The glycals react with water, alcohols, phenols, carboxylic acids, and certain bases, in the presence of an acidic catalyst, in the same way as does 2,3-dihydro-4/f-pyran (which gives, for example, 2-hydroxy-, 2-alkoxy-, and 2-aryloxy-tetrahydropyrans in high yield ). (Tetrahydro-pyranyl ethers are useful for protecting alcoholic groupings during reactions in basic media, and are readily hydrolyzed with acid,) The first step of these additions would be expected to be protonation at C-2, followed by attack of the nucleophilic reagent on the resonance-stabilized, C-1 carbonium ion. 

A positive effect of water was observed in Heck reactions of 2,3-dihydrofuranes (e.g., glycals) with iodo- and bromoderivatives of pyrimidines and other nitrogen-containing heterocycles (Scheme 31). In several cases the reactions that could not be achieved using a standard method (in anhydrous DMF with EtsN and NaOAc as base) took place in aqueous ethanol (1 1, v/v) with NaHCOj-EtsN mixture in the presence of BU4NCI with suppression of double bond migration. ... 

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