The assignment of ¹H and ¹³C chemical shifts for dihydro-3(2H)-furanone derivatives by means of specific ²H labelling experiments and ¹³C{¹H} decoupling experiments

The 1H nmr spectrum of dihydro-2,2,5,5-tetramethyl-3(2H)-furanone (1) in 80% sulfuric acid shows a time-dependent decrease in the intensity of the lower-field gem-dimethyl signal relative to the upper-field gem-dimethyl signal. This is interpreted as involving reversible opening of 1 to 2-hydroxy-2,5-dimethyl-4-hexen-3-one, resulting in deuterium exchange at the C-5 methyl groups of 1. The lower-field gem-dimethyl signal of 1 in this medium is therefore assigned to these methyl groups. A similar relationship between the methyl proton signals of 1 in organic solvents has been demonstrated by (i) synthesis of 1-d8 (8), (ii) comparison of the 1H nmr spectra of 1 and its 2,2- and 5,5-dimethyl analogs, and (iii) observation of long range coupling in the spectrum of 1. Assignment of 13C chemical shifts for 1 has been made based on protoncoupled spectra, showing that the signal of the C-5 methyl carbons appears at ca. 4 ppm towards lower field than that of the C-2 methyl carbons. Both the 1H and 13C chemical shift assignments for 1 were confirmed from 13C{1H} spectra using low power single frequency 1H decoupling, demonstrating the utility of this technique for the assignment of chemical shifts.