Furan 2-acetyl-, conformation

Computer simulation of lanthanide-induced shifts in the 2-formyl and 2-acetyl derivatives of furan, thiophene, selenophene, and tellurophene108 indicate a nearly equipopulated mixture of trans and cis conformers of the furan, and a preponderance of the cis for the thiophene, selenophene, and tellurophene derivatives. This difference is due to an interaction between the ring heteroatom and the carbonyl oxygen lone pair electrons. 

The trifluoroacetyl and acetyl groups show the same qualitative conformational preference, at least when bonded to N-alkylpyrrole (80JCR(S)42) and benzo[h]furan (840MR197). 

In acetyl and higher COAlk derivatives of furan (76ZN(A)1217 84JST(116)377 85JCS(P2)1839), for example, in the 2-formyl- and 7-formylbenzo[h]furans (84JCS(P2)1479), the conformational equilibrium was found to be solvent dependent. This behavior should characterize all situations in which the energy difference between cis- and trans-conformers is small (<8 kJ mol" ) and the more stable conformer in nonpolar media is the one with lower polarity. 

Relatively few simple benzo[6]furan derivatives have been studied by X-ray or electron diffraction techniques. (Their hydro derivatives are better known.) An aldehyde prepared by formylating 2,4,7,-trimethylbenzo[fr]furan could not be orientated by spectroscopic methods so was converted into the derivative (36) and found to have the dimensions shown (81CSC83). In the dibenzofuran (37) (78CSC629) the effect of an acetyl group on the bond lengths and angles can be assessed by internal reference and seems to be very small, yet the molecule is nearly planar and is the s-syn rotamer in conformity with the later discussion. 

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