NMR spectroscopy

1H and 13C NMR spectroscopy

About 60 mono- and 30 di-substituted furans were prepared by known methods and their NMR spectra recorded. Increments were calculated from the monosubstituted compounds and furan itself and combined with the shifts of disubstituted derivatives. Thus it was possible to calculate the shift values (especially carbon shifts) of higher substituted furans very accurately.
This method was used to prove the structures of target compounds by comparing the shifts of reaction products with those calculated for all possible isomers. This method allowed unambiguous identification of the target structures in all cases.

17O NMR spectroscopy

The influence of single substituents on the furan oxygen atom was calculated as the difference of 17O-shift of the monosubstituted compound versus furan 17O-shift. The following table shows some examples for these increments for both possible substituent positions:

Substituent Position 2 Position 3
Methyl 7.8 -6.2
Trimethylsilyl 15.6 10.4
Methylthio 14.8 -0.6
Bromo 12.7 2.4
-2.8 -7.6
-1.2 -6.2
Aldehyde -0.1 6.7

It is well known that 17O NMR is very sensitive to steric interactions [10]. This effect caused some deviations when using the above increments to calculate 17O-shifts of furans with ortho substitution patterns. As these deviations are nearly constant for all compounds with identical neighbouring groups, a steric correction can be introduced for each pair of substituents. Some examples are given below:

Subst. in posn. 2 Subst. in posn. 3 Steric
Bromo Bromo -3.0
Methyl Bromo -1.5
Bromo + 4.0
Bromo Trimethylsilyl + 2.8
Bromo + 6.8

By this method the furan oxygen shift can be calculated with a maximum deviation of ca. 2 ppm, provided the steric correction - if necessary - is known. This can be illustrated by the following example:

Measured shift 271.3 ppm
Furan parent 237.5
2-Bromo + 12.7
2-Methyl + 7.8
3-Trimethylsilyl + 10.4
Steric correction +2.5
Calculated value 270.9 ppm