Leucines

[1] Being compared with the four butanols, they could be classified as butyl-substituted glycines; they represent all four possible variations.

Leucine and isoleucine belong to the proteinogenic amino acids; the others are non-natural.

Including the stereoisomers, six further isomers could be added: D-leucine, D-isoleucine, L-alloisoleucine, D-alloisoleucine, D-tert-leucine and D-norleucine.

Cycloleucine could be classified as a cyclic derivative of norleucine.

With a cyclopentane-ring, it has two hydrogen atoms fewer and thus is not an isomer.