Photochemical Synthesis of Prochiral Dialkyl 3,3-Dialkylcyclopropene-1,2-dicarboxylates with Facial Shielding Substituents and Related Substrates

Different types of cyclopropene-1,2-dicarboxylates 1 have been obtained by photochemical methods from the corresponding pyrazoles 11, 12, or 13. These pyrazoles were synthesized by 1,3-dipolar cycloadditions of alkynes 8 or 9 with either preformed diazoalkanes or diazoalkanes generated photochemically in situ, by use of oxadiazolines as diazoalkane precursors. The numerous substrates have clearly established the scope and limitations of the syntheses of the precursors and the cyclopropenes by the different routes; even prochiral and enantiomerically pure chiral derivatives could be synthesized. Numerous precursors and cyclopropenes could be characterized by X-ray crystal structure analyses, which revealed interesting structural features and allowed unequivocal assignment of different diastereomers or constitutional isomers. Some of the photochemical reactions produced unique side-products; the crystal structure analyses were absolutely crucial for unambiguous structural assignment here.