Alkynones were treated with boron trifluoride diethyl etherate to generate β-iodoallenolates, which underwent intramolecular aldol reactions to produce cycloalkenyl alcohols. Diastereoselective oxa-Michael ring closure could then be induced by treatment with a catalytic amount of gold(III) chloride, affording highly functionalized tetrahydropyran-containing ring systems.
Mukaiyama aldol reactions in aqueous media have been surveyed. While the original Mukaiyama aldol reactions entailed stoichiometric use of Lewis acids in organic solvents under strictly anhydrous conditions, Mukaiyama aldol reactions in aqueous media are not only suitable for green sustainable chemistry but are found to produce singular phenomena. These findings led to the discovery of a series of water-compatible Lewis acids such as lanthanide triflates in 1991. Our understanding on these beneficial effects in the presence of water will be deepened through the brilliant examples collected in this review.
A Ritter-like coupling reaction of cyclic alcohols and both aryl and alkyl nitriles to form amides catalyzed by copper (II) triflate is described. These reactions proceed in good yields under mild and often solvent-free conditions. With 2- and 3-substituted cycloalkanols, amide products are formed with near complete retention of configuration. This is likely due to fast nucleophilic capture of a non-planar carbocations (hyperconjomers) stabilized by ring hyperconjugation. A critical aspect of this novel catalytic cycle is thein situactivation of the alcohol substrates by thionyl chloride to form chlorosulfites.
Herein, an efficient method for the Ullmann C?O coupling reaction between various kinds of phenols and aryl halides, including amino, ketone, cyano, methyl, methoxy, fluoro, chloro and bromo derivatives, is described. The catalyst used, copper ferrite (CuFe2O4) nanoparticles, are easily made, air-stable, and of low cost. The catalyst can be recycled easily just by using an external magnet. Even in the presence of sensitive substituents, the reaction proceeds successfully to provide the desired products in high yields without protection of other functional groups.
A one-pot cascade reaction for the synthesis of polysubstituted benzofurans and naphthopyrans from simple phenols and propargylic alcohols catalyzed by iron(III) is presented. The results demonstrate that the structural specificity for the formation of furan and pyran products is controlled by the structural nature of the propargylic alcohols. Namely, benzofurans could be synthesized efficiently from phenols and secondary propargylic alcohols in the presence of 5mol% of iron(III) chloride hexahydrate (FeCl36H2O) catalyst. On the other hand, pyran derivatives were obtained exclusively when tertiary propargylic alcohols were employed. Mechanistic studies revealed that presumably due to the discriminated steric effect of secondary and tertiary propargylic alcohols, the Fe-catalyzed FriedelCrafts (FC) reaction of phenols with the two types of alcohols proceeds via different models. Most importantly, we have demonstrated for the first time that fully 2,3,4-substituted naphthopyrans could be synthesized efficiently via the iron-catalyzed one-pot cascade reaction. Consequently, the results presented herein provide straightforward pathways for versatile syntheses of valuable benzofuran and pyran derivatives from simple phenolic compounds and propargylic alcohols.