One Heteroatom Adjacent to the Ring Junction

Treatment of L-Ser-OMe with 3-bromocyclopentene after the hydroxy group was replaced with iodide gave in the presence of 2,2 -azobisisobutyro-nitrile (AIBN) and tri-w-butyltin hydride a mixture of stereoisomers. These stereoisomers were separated and transformed into (lS,35,5S)-54 and (lR,35,5R)-isomer 55, which are precursors of the potent angiotensinconverting enzyme inhibitors [89H(28)957], Isomer 54 was also obtained from N-acetylserine methyl ester which was converted into its chloro analog and reacted with cyclopentenepyrrolidine (84TL4479). 

Oxidation of 4-methoxycatechol with Ag20 in the presence of /3-alanine methyl ester resulted in the formation of a complex mixture from which compound 57 was isolated in 0.4% yield (82HCA1279). 

When treated with formaldehyde, esters of AAs form immonium ions which undergo reaction with cyclopentadiene to give 2-azanorbornenes 58. Decomposition of 58 afforded esters of N-methylamino acids without racemization (87JOC5746). 

A heterocyclic amino acid, 3-amino-3-(3 -thienyl)propionic acid, when treated with TFA and TFAA, is transformed under severe reaction conditions into compound 59 (86TL2607). 

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