Ring-constrained amino acid surrogates of formula I: where R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6a, R.sup.6b, R.sup.7, and y are as defined in the specification, methods for synthesizing ring-constrained amino acid surrogates of formula I, methods of use of ring-constrained amino acid surrogates of formula I, including use in linear or cyclic compounds which include a plurality of amino acid residues and one or more ring-constrained amino acid surrogates of formula I and linear or cyclic compounds which include a plurality of amino acid residues and one or more ring-constrained amino acid surrogates of formula I.