100 Furans and Thiophenes
Vilsmeier formylation and Friedel–Crafts acylations proceed effi ciently with thiophene and furan, using SnCl 4 as the
catalyst for thiophenes and BF 3 for furans.
Though -selectivity is the rule, -substitution also occurs easily when the 2- and 5-positions cannot be accessed. This
can be illustrated by the synthesis of 4-oxo-4,5,6,7-tetrahydrobenzo[b]thiophene from thiophene, involving two acyla-
tions, the fi rst at an -position and the second at a -position.
The Mannich dimethylaminomethylation of thiophene and furan is conducted using the preformed iminium
salt. This is especially important in the case of furan as this heterocycle is sensitive to acid; indeed, under the
appropriate conditions, furans can be converted, using acid, into useful non-cyclic derivatives of 1,4-dicarbonyl
compounds. It is not possible to be sure of the exact sequence of events, but a series of protonations and additions
of methanol are involved.