Synthesis of highly substituted oxetanes via [2+2] cycloaddition reactions of allenoates catalyzed by a guanidine Lewis base

摘要

The first synthesis of highly substituted 3-alkyl-oxetan-2-ylidenes from allenoates was developed by using the bicyclic guanidine 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as an exceptionally active nitrogen Lewis base catalyst. Oxetanes are important structural motifs occurring in a number of natural products, like anti-tumor taxanes. Additionally, oxetanes play an increasing role as lead-structures as well as versatile building blocks in medicinal chemistry. Synthetically, oxetanes are accessible through a number of intramolecular cyclization reactions, such as the classical Williamson ether synthesis, intramolecular alkylations and metal-catalyzed cyclizations, while the only practical intermolecular approach to oxetanes has long been the [2+2] photocycloaddition between olefins and carbonyl compounds (Paterno-Buchi reaction).