Rhodium generated carbonyl ylides with p-quinones: synthesis of oxa-bridged polycyclic systems

A series of α-diazo carbonyl compounds tethered to substituted cyclopentanone and cyclohexanone units with different tether lengths have been synthesized using diazomethane solution or methanesulphonyl azide. The above synthesized α-diazo carbonyl compounds with rhodium(II) acetate dimer furnish cyclic five- or six-membered-ring carbonyl ylide dipoles, which undergo facile 1,3-dipolar cycloaddition with p-quinones to furnish various novel oxa-bridged polycyclic systems 10, 13 and 11, 14 as CC and CO addition products of p-quinones. Very interesting unusual cyclization product, tri-oxapolycyclic compounds 12 and 15 were also obtained. Single-crystal X-ray analyses of 11a, 12d and 14a are reported to firmly establish the structure and stereochemistry of the oxa-bridged polycyclic systems. The molecules 11a and 14a exhibit novel C–H⋯O hydrogen bonding motif in the crystal structure. On the other hand, the molecule 12d revealed both O–H⋯O and C–H⋯O motifs in the solid-state architecture.