A surprising steric effect on a tandem cycloaddition/ring-opening reaction : rapid syntheses of difluorinated analogues of (hydroxymethyl)conduritols

Difluorinated analogues of (hydroxymethyl)conduritols can be synthesised from selected furans and a difluorinated dienophile in two reaction steps

Highly functionalised difluorinated cyclohexenols

Alkenoate 5 prepared on a multigramme scale from trifluoroethanol underwent cycloaddition with furan in good yield. Reduction of the ester group in the endo-cycloadduct 7a with DIBAl-H also resulted in decarbamoylation to reveal a diol which was protected as an acetonide 10. Various methods for nucleophilic ring opening were explored releasing highly functionalised difluorinated cyclohexenols, and suggesting a novel route to difluorinated analogues of cyclitols and monosaccharides

The interplay of structure and reactivity in a most unusual furan diels-alder reaction

Difluorinated alkenoate ethyl 3,3-difluoro-2-(N,N-diethylcarbamoyloxy)-2-propenoate reacts rapidly and in high yield with furan and a range of substituted furans in the presence of a tin(IV) catalyst. Non-fluorinated congener 2-(N,N-diethylcarbamoyloxy)-2-propenoate fails to react at all under the same conditions. These reactions have been explored using density functional theory (DFT) calculations. They reveal a highly polar transition state, which is stabilized by the Lewis acid catalyst SnCl4 and by polar solvents. In the presence of both catalyst and solvent, a two-step reaction is predicted, corresponding to the stepwise formation of the two new carbon−carbon bonds via transition states which have similar energies in all cases. Our experimental observations of the lack of reaction of the non-fluorinated dienophile, the stereochemical outcomes, and the rate acceleration accompanying furan methylation are all well predicted by our calculations. The calculated free energy barriers generally correlate well with measured reaction rates, supporting a reaction mechanism in which zwitterionic character is developed strongly. An in situ ring opening reaction of exo-cycloadduct ethyl exo-2-(N,N-diethylcarbamoyloxy)-3,3-difluoro-7-oxabicyclo[2.2.1]hept-5-enyl-2-endo-carboxylate, which results in the formation of cyclic carbonate ethyl 4,4-difluoro-5-hydroxy-2-oxo-5,7a-dihydro-4H-benzo[1,3]dioxole-3a-carboxylate by a Curtin−Hammett mechanism, has also been examined. Substantial steric opposition to Lewis acid binding prevents carbonate formation from 2-substituted furans