Synthesis of Spirocyclic and Fused Isoxazoline Building Blocks

A highly efficient multigram synthesis of spirocyclic and fused isoxazoline building blocks is described. Isoxazoline-3-carboxylates were synthesized via a regioselective 1,3-dipolar cycloaddition reaction of 2-chloro-2-(hydroxyimino)acetate and carbo- or heterocyclic alkenes bearing endo- or exocyclic C=C double bonds, resulting in fused or spirocyclic isoxazolines, respectively. The preparation of up to 300 g of these compounds was achieved in a single run. The ester group of isoxazolines was then subjected to common synthetic transformations for the synthesis of corresponding building blocks, including alcohols, chlorides, azides, amines, aldehydes, carboxylic acids, amino acids and their derivatives, difluoromethyl-substituted compounds, and bicyclic gamma-lactones. Additionally, a direct cycloaddition-based approach to the synthesis of aminoalkyl- and chloromethyl-substituted isoxazolines was proposed to improve their preparation. The described isoxazoline building blocks are expected to be valuable tools for drug discovery.