LINEAR AND CYCLIC BETA(3)-OLIGOPEPTIDES WITH FUNCTIONALIZED SIDE-CHAINS (-CH2OBN, -CO2BN, -CH(2)CH(2)CO2BN) DERIVED FROM SERINE AND FROM ASPARTIC AND GLUTAMIC-ACID

The natural beta-amino acid derivative Boc-Asp(beta-OH)-OBn, as well a s Boc-beta-HGlu(OBn)-OH and Boc-beta-HSer(OBn)-OH (prepared from appro priately protected glutamic acid and serine, respectively, by Arndt-Ei stert homologation), were employed as building blocks for the synthesi s of linear (11-20) and cyclic (21-23) beta-oligopeptides consisting o f two to six beta beta-amino acids [using trichloroethyl (TCE) ester g roups for C-terminal protection and pentafluorophenyl-ester activation for macrocyclisation]. While the linear derivatives are soluble enoug h for reactions and structural investigations in solution, the cyclo-b eta-tri- and -hexapeptides are not (according to FT-IR measurements th ey form networks of hydrogen bonds, perhaps consisting of so-called na notubes). The CD spectra of the Boc-OTCE-protected (19) and of the unp rotected (20) beta-hexapeptides [beta-Asp(OBn)-beta-HGlu(OBn)-beta-HSe r(OBn)](2) differ drastically, and only the unprotected form shows the familiar pattern of a negative Cotton effect between 210 and 220 nm ( indicative of a 3(14) helix). An NMR analysis in methanol of the beta- hexapeptide 20 with free termini reveals the presence of a single, cen tral, left-handed helix turn (14-membered hydrogen-bonded ring). The r esults are discussed and compared with those obtained previously for a nalogous beta-peptides carrying non-functionalised side chains.