Brønsted Acid-Catalyzed Cycloisomerization of But-2-yne-1,4-diols with or without 1,3-Dicarbonyl Compounds to Tri- and Tetrasubstituted Furans

A Brønsted acid-catalyzed method to prepare tri-
and tetrasubstituted furans efficiently from cycloisomerization of
but-2-yne-1,4-diols with or without 1,3-dicarbonyl compounds is described.
By taking advantage of the orthogonal modes of reactivity of the alcoholic
substrate through slight modification of the reaction conditions,
a divergence in product selectivity was observed. At room temperature, p-TsOH·H2O-mediated tandem alkylation/cycloisomerization
of the propargylic 1,4-diol with the β-dicarbonyl compound was
found to selectively occur to provide the tetrasubstituted furan product.
On the other hand, increasing the reaction temperature to 80 °C
was discovered to result in preferential p-TsOH·H2O-catalyzed dehydrative rearrangement of the unsaturated alcohol
and formation of the 2,3,5-trisubstituted furan adduct.