RESUMEN
A new protocol for the deoxygenation of alcohols and the hydrogenation of alkenes under Brønsted acid catalysis has been developed. The method is based on the use of either a benzyl or isopropyl ether as a traceless hydrogen-atom donor, and involves an intramolecular hydride transfer as a key step, which is achieved in a regio- and stereoselective manner.
RESUMEN
Most asymmetric phase transfer reactions are Brønsted base reactions, and the inorganic bases used greatly influenced the profile of the reaction. Alkoxide salts are able to activate substrates with high pKa values, but background reactions are often unavoidable. On the other hand, carbonate and phosphate salts are milder, but their low basicity limits the scope of their reactions. This presents a difficult situation whereby fragile substrates such as lactone will be hydrolyzed by a stronger base but will not be activated with a weaker one. Thus, a Brønsted probase strategy is devised, in which a strong base can be generated in situ from silylamide (probase) through the use of fluoride. In this approach, the strong base produced will be transient and not be in excess, thus reducing background and side reactions. We demonstrate this strategy using pentanidinium and bisguanidinium as catalysts; highly enantioselective phase transfer alkylation of several types of substrates including dihydrocoumarin (lactone) can be achieved. We found that the probase also acts as a silylation reagent, generating silyl enol ether or silyl ketene acetal, which are key intermediates in the reaction. We further propose that hypervalent silicates form ion-pairs with pentanidinium and bisguanidinium as intermediates in the reaction, and it is through these ion-pairs that the selective enantiofacial approach of the electrophile is determined.
RESUMEN
A diastereoselective hydride transfer process has been developed under Brønsted acid-catalyzed reaction conditions using methyl ethers or acetals as hydride donors and tertiary alcohols or alkenes as precursors of carbocation. The method enables construction of complex molecules having multiple stereogenic centers from rather simple and readily available starting materials with predictable diastereoselective control.
RESUMEN
Diastereoselective hydride transfer could be triggered by electrophilic halogenation (bromination or fluorination) of homoallylic alcohol O-Bn ethers. The resulting diastereomerically enriched haloalkyl alcohols underwent subsequent intramolecular nucleophilic substitution to afford the corresponding tetrahydrofurans.