Tandem 1,3-dipolar cycloaddition and electrophilic cyclization reactions: Cyclic ether subunits of polyether antibiotics from unsaturated isoxazolines

A new strategy for the construction of polyether antibiotic substructural units [spiroketals (I), tetrahydrofurans (II), and tetrahydropyrans (III)] consisting of bis-addition of an oxygen nucleophile across an α,ω-diene moiety (i.e., IV) is described. The method exploits a tandem 1,3-dipolar cycloaddition/electrophilic cyclization sequence that proceeds via the intermediacy of an isoxazoline. Experiments presented illustrate the versatility of this strategy, which provides a unique exploit of the control elements operative in dipolar cycloaddition and electrophilic cyclization chemistry.