Highly diastereoselective N-acyliminium ion cyclization reactions of a tethered furan

The acid catalysed cyclization reactions of tethered furan-4,5- dihydroxypyrrolid-2-ones and furan-4,5-diacetoxypyrrolid-2-ones, via their corresponding N-acyliminium ion intermediates, have been studied. In the case of the tethered furan-3,4-dihydroxypyrrolid-2-one 16, having a two carbon tether, the linearly fused tricyclic compound 18 was formed with high cis selectivity (cis/trans=84:16) when BF₃·OEt₂ was used as a catalyst. However, when the cyclization reaction was carried out on the related tethered furan-4,5-diacetoxypyrrolid-2-one 17, the linearly fused tricyclic compound 20 was formed as a single diastereoisomer with trans selectivity. In contrast, the attempted cyclization of the tethered furan-4,5-dihydroxypyrrolid- 2-one 26, having a one carbon tether, did not result in formation of the corresponding linearly fused tricyclic system, instead it formed the dimer 29 as a single diastereoisomer. Cyclization reactions of the related tethered furan-4,5-diacetoxypyrrolid-2-one 27 also failed to give the corresponding linearly fused tricyclic system or macrocycle.