Synthesis of Bridged Heterocyclic Systems

94JOC5347). When p-benzoquinone is used as a dipolarophile, the initially formed cycloadducts from 683 (R = MeO, R = H) can be treated with acetic anhydride to give compound 687, which contains the basic core dibenzo[a,d]cyclohepten-5,10-imine skeleton found in MK-801. However, it should be emphasized that cyclization of the Rh-carbenoids does not occur with weakly basic imines such as pyridines and the aromatic isoxazole system. For the synthesis of a nitrogen-bridged system, Padwa and coworkers have recently reported the successful employment of a domino transformation with displacement of a carbene center, followed by a tandem 

The transformation of penicillin-derived diazo ketones 696 into compounds 697a and b with the nitrogen atom in the ring fusion position on treatment by Cu(acac)2 proceeds via a sulfonium ylide with subsequent rearrangement (77T547 80TL2451 82H1647). 

A successful example of insertion reactions in the synthesis of N-bridged compounds is the Rh2(OAc)4-catalyzed decomposition of /3-lactams 705, which was employed as a key step in the synthesis of several anti-Bredt /3-lactams with nitrogen in the bridgehead (86JA6431 89JA1073). 

The use of diazocarbonyl compounds in the synthesis of O-bridged systems is the approach of choice. There are a few main synthetic strategies of bridged framework building based on this approach tandem cyclization with carbonyl ylide formation/cycloaddition, tandem cyclization with 0x0-nium ylide formation/sigmatropic shift, intramolecular cyclopropanation, and cyclization with single bond insertion. 

Carbonyl Ylide Formation Followed by Intermolecular Cycloaddition 

---