Conformational furfural

Early work on the experimentally established conformational preferences in solution for a variety of other 2-substituted heterocycles is summarized in Table 30. Most of these conclusions have been deduced either from dipole moment measurements in benzene or by the use of lanthanide induced shifts for chloroform solutions. The aforementioned MO studies correctly predict the preferred conformations, (63, R = H) or (64, R = H), of pyrrole-2-carbaldehyde, thiophene-2-carbaldehyde and furfural in the gas phase. 

The equilibria between the 00-cis- and 00-/ra t-conformers of 3-, 4-, and 5-substituted furfurals have been examined by MP2/6-31G(d,p) calculations. The geometry of the furan nucleus appears little disturbed by the substitution pattern <2003JCC429>. 

More recently, 13C-NMR studies have been used to investigate rotational isomerism. At lower temperatures line broadening is observable and below — 60 C furfural in dichloromethane shows two C-3 and two C=0 resonances the relative intensities and chemical shifts of which suggest a ratio of oxygen-cis to oxygen-trans isomers of 5 1, in conformity with the earlier work.362... 

After protons, C is the most widely detected nucleus in NMR. Proton cross-polarization and decoupling are usually applied to increase the S/N, and these types of experiment can result in substantial sample heating. Many forms of C-based NMR thermometers have been proposed. The first such system was based on the cis-trans conformational equilibrium of furfural, with the linewidths of carbon-3 and the aldehyde carbon being temperature-dependent. There are many disadvantages of linewidth-based measurements, and subsequent developments concentrated almost wholely on temperature-dependent C chemical shifts. The first such system utilized a temperature-dependent lanthanide-induced pseudocontact shift in a complex of acetone-de and ytterbium(III)1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octadionate (Yb (fod)3). The 6co of the acetone-dg, measured with respect to a CS2 standard, was almost linearly dependent on 1 / T with a small quadratic term over a range 200-315 K. If a small amount of protonated acetone was added, then the proton resonance, measured with respect to the protons of TMS, was also found to be temperature dependent ... 

The fT-alkylidene derived from furfural undergoes cycloaddition to form a major diastcrcomer with the opposite absolute configuration at the (3-carbon. However, it is found that the extent of diastereofacial selectivity in addition to -acceptor is around 90%, so a more efficient method to reverse the sense of induction is to employ <7-ephedrine as the source of chirality. In each example, the stereochemistry of addition to the /i-carbon of the acceptor is the result of attack syn to the phenyl and methyl groups and explained by a conformational effect69. 

Complementary to the previous work, the effect of substituents located in a five-membered ring on the conformational equilibrium between planar forms of furfural has also been studied. The results obtained show that not only properties of a substituent but also its position have an important effect on the equilibrium between OO-cis and OO-trans isomers (2004JCC(25) 429). The electron-donating substituents at position 4 stabilize the OO-trans conformer, whereas the electron-withdrawing substituents show the opposite effect (Scheme 33). In turn, the effect of substituents at position 5 is contrary to that typical for the position 4. 

Glucose exists in chair and bed conformations and Byrne et al [72] believe that the conformation of the elementary cellulose unit controls the route in which the reaction will proceed, where if an initial chair conformation exists, then the breakdown proceeds (Scheme Via) via furfural derivatives, but the bed conformation will form levoglucosan. Unfortunately, the original cellulose conformation cannot be determined. 

Ground state gas and solution phase conformational d3mamics of polar processes Furfural systems ... 

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