Benzo furan, orientation

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. 

These differences in orientation between benzofuran and benzo-thiophene have long puzzled chemists however, as will be discussed below, the effect of the annelation on the reactivities of the a and /3 positions of furan and thiophene rings is substantially similar, and the apparent difference in behavior originates in the different a 1(8 reactivity ratios in the two monocyclic systems. 

Practical and diversity-oriented synthesis of multi-substituted benzo[lr] furans was accomplished from simple phenols through a Pummerer annula-tion/cross-coupling sequence. Operationally simple and rapid reactions of phenols with ketene dithioacetal monoxides with the aid of trifluoroacetic anhydride provided the corresponding 2-methylsulfanylbenzo[ ]furans (14BCJ1349). 

Electrophilic Substitution.—The isomer distributions and the rates relative to those of the parent heterocyclic compounds for acetylation, benzoylation, chlorination, and bromination of benzo[Z>]thiophen and benzo[6]furan have been determined. It was found that although the orientation of substitution in the two bicyclic systems is different, the effect caused by annelation on the reactivity of the a- and jS-positions is substantially the same in the two rings the reactivity of the a-position is always decreased by a similar factor, and the reactivity of the j8-position is increased (with some exceptions) in both systems. The different orientation observed in the... 

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