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. The α-carbon atom is not a stereocenter.
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