Microwave-Assisted Paal−Knorr Reaction. A Rapid Approach to Substituted Pyrroles and Furans
摘要:
An array of tetrasubstituted pyrroles (and trisubstituted furans) was obtained using a simple three-step procedure. Functional homologation of beta-ketoester with an aldehyde followed by oxidation gave a series of differently substituted 1,4-dicarbonyl compounds that can be rapidly cyclized with the Paal-Knorr procedure carried out under microwave irradiation.
Reaction of methyl 2-siloxycyclopropanecarboxylate 4 with TiCl4 provides the unique titanoxycyclpropane whose structure could be elucidated by means of NMR-spectroscopy. Its additions to aldehydes and acetophenone occur with high stereoselectivity to afford homoaldol products like . These intermediates can further be transformed to highlysubstituted tetrahydrofuran derivatives by activation with BF3-OEt2