Unprecedented Mechanism of an Organocatalytic Route to Conjugated Enynes with a Junction to Cyclic Nitronates

Conjugated enynes as well as cyclic nitronates are crucial building blocks for numerous natural products and pharmaceuticals. However, so far, no common and metal-free synthetic route to both conjugated enynes and cyclic nitronates has been reported. Herein, in situ NMR, labelling studies and theoretical calculations were combined to investigate the mechanism of the unusual triple bond formation towards conjugated enynes. Starting from nitroalkene dimers, first an isoxazolidine-2,5-diol derivative is formed as central intermediate. From this, enynes were generated by a combination of oxidation, dehydration, and retro 1,3-dipolar cycloaddition, whereas for nitronates a base induced intramolecular reorganization is proposed. While the product distribution could be controlled and high yields of nitronate were achieved, only medium to good yields for enynes were obtained due to polymerization losses. Nevertheless, we hope that these mechanistic investigations may provide a basis for further developments of organocatalytic or metal-free preparations of conjugated enynes and nitronates.