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Relevant academic research and scientific papers

Difluorocyclopentenone synthesis

A cyclization for the rapid assembly of gem-difluorinated cyclopentenones is described. 2,2,2-Trifluoroethanol is the cheap and convenient source of both fluorine atoms.

Cyclopropenium Enhanced Thiourea Catalysis

An integral part of modern organocatalysis is the development and application of thiourea catalysts. Here, as part of our program aimed at developing cyclopropenium catalysts, the synthesis of a thiourea-cyclopropenium organocatalyst with both cationic hydrogen-bond donor and electrostatic character is reported. The utility of the this thiourea organocatalyst is showcased in pyranylation reactions employing phenols, primary, secondary, and tertiary alcohols under operationally simple and mild reaction conditions for a broad substrate scope. The addition of benzoic acid as a co-catalyst facilitating cooperative Br?nsted acid catalysis was found to be valuable for reactions involving phenols and higher substituted alcohols. Mechanistic investigations, including kinetic and 1H NMR binding studies in conjunction with density function theory calculations, are described that collectively support a Br?nsted acid mode of catalysis.

Palladium(0)-catalysed hydrostannylation of 1-alkoxy-1-alkynes: A synthesis of α-alkoxyalkenylstannanes and their transmetallation to α-alkoxyalkenyllithiums

1-Alkoxy-1-alkynes undergo rapid hydrostannylation with trialkylstannanes in the presence of catalytic amounts of palladium(0). The reaction gives clean cis-addition with the tin being placed preferentially in the α-position. The resultant α-alkoxyalkenylstannanes transmetallate efficiently with BuLi to α-alkoxyalkenyllithiums which were previously accessible only with difficulty.

EXPERIMENTAL STUDIES OF THE ANOMERIC EFFECT. PART IV. CONFORMATIONAL EQUILIBRIA DUE TO RING INVERSION IN TETRAHYDROPYRANS SUBSTITUTED AT POSITION 2 BY THE GROUPS ETHOXY, 2'-FLUOROETHOXY, 2',2'-DIFLUOROETHOXY, AND 2',2',2'-TRIFLUOROETHOXY

Equilibrium constants (K) for ring inversion equilibria in 2-(RO)-tetrahydropyrans (R = Et, CH2CH2F, CH2CHF2, CH2CF3) have been determined from 13C nmr spectra recorded at 145-160 K in CD2Cl2 and CFCl3/CDCl3 (85/15 by volume).Additional values of K were obtained at 250-270 K from the acid-catalysed equilibration of cis- and trans-2-(RO)-4-methyl-tetrahydropyran (R as above).Plots of lnK against T-1 gave values for ΔH0a->e of -0.26, -0.12, -0.05 and 0.13 kcal mol-1 for R = Et, CH2CH2F, CH2CHF2 and CH2CF3, respectively, in CD2Cl2.The corresponding values of ΔH0a->e for CFCl3/CDCl3 as solvent were -0.58, -0.15, -0.07 and 0.21 kcal mol-1, respectively.The derived ΔS0a->e values were -2.33, -2.22, -2.25 and -2.24 cal K-1mol-1, respectively, in CD2Cl2 and -4.65, -3.37, -3.30 and -3.03 cal K-1mol-1, respectively, in CFCl3/CDCl3. The trends in ΔH0 values are attributed to modifications of anti-periplanar n->o* stabilisation (itself partly responsible for endo- and exo-anomeric effects) which occur as the number of electronegative fluorine substituents increases.For all substituents, axial conformations are the most abundant in the temperature range 120 K to infinite.However, this finding is a consequence of the higher entropy of axial conformations; in at least two cases (R = Et and CH2CH2F) it is the equatorial conformation which has the lower enthalpy.The results confirm the importance of studying conformational equilibria over a wide range of temperature.

Influence of solvent on the magnitude of the anomeric effect

A novel application of 13C nuclear magnetic resonance provided the effects of solvent polarity and hydrogen-bond formation on the conformational equilibria for a range of 2-substituted tetrahydropyrans and the results are interpreted in terms of how solvent affects the competition between the endo- and exo-anomeric effects in determining the magnitude of the anomeric effect.In accord with the generally accepted origin of the endo- and exo-anomeric effects (anti-periplanar n-?* interaction of the oxygen lone-pair orbital with the antibonding orbital of the adjacent C-O bond), the exo-anomeric effect for the α anomer is expected to be weaker because charge delocalization from the glycosidic oxygen to anomeric center is in competition with delocalization from the ring-oxygen atom.The effect of solvent on the relative magnitudes of the endo- and exo-anomeric effects are then considered to arise from the formation of specific complexes with the solvent, and the exo-anomeric effect of a β-glycoside is more strongly influenced.It is contended that hydrogen bonding of solvent to the ring oxygen increases the exo-anomeric effects.For this reason water is particularly effective for the strengthening of the exo-anomeric effect and, thereby, the conformational rigidity of glycosides.Experimental evidence is presented that indicates that the anomeric hydroxyl groups of free sugars dissolved in water tend to prefer the equatorial orientation because these provide stronger hydrogen bonds as proton donors to water.