Beta-substituted non-canonical amino acids

A β-substituted amino acid according to Formula II:

• or a salt or ester thereof, wherein:
• R1 is C2-8 alkyl, which is optionally substituted with one or more R1a;
• each R1a is independently selected from the group consisting of halogen, —OH, —CN, —N3, —NO2, C6-14 aryl, C2-12 alkenyl, C1-12 alkynyl, C1-12 alkoxy, C1-12 thioalkoxy, —N(R1b)a, —C(O)R1c, —C(O)N(R1b)2, —NR1bC(O)R1cc, and —OC(O)R1c;
• each R1b is independently selected from the group consisting of H and C1-6 alkyl;
• each R1c is independently selected from the group consisting of H, —OH, halogen, C1-6 alkyl, C1-6 alkoxy;
• Y and Z are independently selected from the group consisting of CH, CR2, and N;
• each R2 is independently selected from the group consisting of halogen, —OH, —CN, —N3, —NO2, C1-12 alkyl, C6-14 aryl, C2-12 alkenyl, C1-12 alkynyl, C1-12 alkoxy, C1-12 thioalkoxy, —N(R2a)2, —C(O)R2b, —C(O)N(R2a)2, —NR2aC(O)R2b, and —OC(O)R2b;
• each R2a is independently selected from the group consisting of H and C1-6 alkyl;
• each R2b is independently selected from the group consisting of H, —OH, halogen, C1-6 alkyl, C1-6 alkoxy;
• R3 and R4 are independently selected from the group consisting of H and an amine protecting group;
• and
• subscript n is 0, 1, 2, or 3;
• provided that:
• if Y is CH, Z is CH, and subscript n is 0, then R1 is other than unsubstituted isopropyl, unsubstituted n-butyl, unsubstituted n-pentyl, unsubstituted n-hexyl, (2-acetoxy)ethyl, (1-ethyl)propyl, and 3-methylbut-1-en-3-yl;
• and
• if Y is CH, Z is CH, subscript n is 1 or 2, and R2 is C1-12 alkoxy, then R1 is other than unsubstituted ethyl.