Bicyclic fused ring systems, conformation

Hall has drawn attention to the conformational problems associated with bicyclic fused ring systems, particularly where one or both of the rings is five-membered. Several n.m.r. studies of such systems (and similar systems with more rings) have been described subsequently, and these studies are discussed below. 

Table 1 Some bicyclic 5,5-fused ring systems whose conformations have been determined by...

Similarly, in a 1,3-dipolar cycloaddition of DMAD to the conformationally locked cyclic a-alkoxycarbonylnitrone (727), bicyclic ring systems, containing a nitrogen atom at the bridgehead position have been synthesized. A mechanistic interpretation of the origin of the fused pyrroles (729) includes the intermediate formation of the aziridine ring in (728) (Scheme 2.303) (820). 

The asymmetric synthesis of conformationally constrained a-amino acids and their incorporation into peptides are currently topics of great interest [47]. Much sought after are fused bicyclic a-amino acids having the N atom incorporated into a ring system [48]. Many of these amino acids display interesting pharmacological activities, and they have served as building blocks for the synthesis of pep-... 

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