Leucines

The leucines are primarily the four isomeric amino acids: leucine, isoleucine, tert-leucine and norleucine. 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-allo-isoleucine, D-allo-isoleucine, 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. The α-carbon atom is not a stereocenter.

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