Furans, acyl, conformation

Measurements of dipole moments, Kerr constants, and dielectric absorption have been employed (81RCR336) widely to obtain information on the conformational equilibrium in acyl heterocycles. Details on conformer structures and populations depend on the choice of additive scheme, group moments, or polarizability tensor in the case of Kerr constants. Several early conclusions, especially for furan- and thiophene-2-carboxaldehyde, appeared contradictory, owing to the choice of these quantities. A more precise definition of polarizability tensors for several heterocycles and a choice of group moments and additive schemes tested on a large amount of available experimental results and supported by accurate theoretical calculations have led to more confidence in the use of experimental dipole moments and Kerr constants in conformational analysis. A limitation of the method is that the... 

Substituents at C(2) and C(4) may affect the conformational equilibrium of 3-acyl derivatives of five-membered heterocycles and examples have been reported for furan, pyrrole, and thiophene compounds (81RCR336). Stabilizing electrostatic interactions may be the origin of the conformational behavior (80JST(67)251) of the thiophene derivatives 21 and 22, which adopt the conformation depicted. 

The situation in 2-acetylfuran (63, R = Me) resembles qualitatively that of the 2-formyl (63, R = H). Accurate NMR measurements (85JCS(P2)1839) at low temperature and in dimethyl ether performed on 2-acyl derivatives of furan have enabled direct detection of the two conformers. At 173K the (E)-conformer population for 2-acetylfuran amounts to 53%. 

Methylsulfinyl derivatives of furan show a rotational isomerism similar to that observed in acyl derivatives. For 2-methylsulfinyl furan (74) and 3-methylsulfinyl furan (75), theoretical calculations at ab initio level <9iJST(228)7l) indicate two conformational minima corresponding to the two orientations of the S—O bond with respect to the heterocyclic oxygen (labeled 0,0-cis and 0,0-... 

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