posted on 2002-01-30, 00:00authored byMatthew H. Todd, Chudi Ndubaku, Paul A. Bartlett
Bicyclization of peptide acetals via nucleophilic attack of a phenyl group on an endocyclic
acyliminium ion 4 was explored as a route to novel amino acid derived heterocycles and
peptidomimetic scaffolds. In the presence of protic acid, bridged structures such as 6 are formed
readily from phenylalanine derivatives, but the fused-ring analogues 5 could not be obtained in
good yield. In contrast, radical cyclization of the bromophenyl dihydropyrazinone 7 provides an
effective alternative for the synthesis of 5 (n = 0, 1, 2). Additional versatility in this process was
demonstrated by efficient synthesis of a different fused ring system, represented by the antihelmintic
praziquantel, 8.