The leucines are primarily the four isomeric amino acids: leucine, isoleucine, tert-leucine (terleucine, pseudoleucine) and norleucine.^[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.

Isomers

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

Leucines
Name	L-Leucine	L-Isoleucine	L-tert-Leucine (terleucine, pseudoleucine)	L-Norleucine
Other names	2-Amino-4-methylpentanoic acid, Isobutylglycine	2-Amino-3-methylpentanoic acid, sec-Butylglycine	2-Amino-3,3-dimethylbutanoic acid, tert-Butylglycine	2-Amino-hexanoic acid, n-Butylglycine
Structure
CAS-number	61-90-5	73-32-5	20859-02-3	327-57-1
PubChem	CID 6106 from PubChem	CID 791 from PubChem	CID 164608 from PubChem	CID 21236 from PubChem
Molecular formula	C6H13NO2
Molar mass	131.18 g/mol

Derivatives

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.