(2S, 3S)-3-Hydroxyleucine is an amino acid (Chapter 26) that is a key component in the structures of many "depsipeptide" antibiotics, such as sanjoining (margin),

(a) Find the part of the sanjoinine molecule that is derived structurally from (2S, 3S)-3-hydroxyleucine.
(b) Although many depsipeptide antibiotics occur in nature, the quantities available are too small to be useful pharmaceutically; thus these molecules must be synthesized. (2S, 3S)-3-Hydroxyleucine, which is also not available in quantity from nature, must be synthesized as well. Possible starting materials are the four diastereomers of 2-bromo-3-hydroxy-4-methylpentanoic acid (margin). Draw structural formulas for each of these diastereomers and identify which of the four should be the best starting material for a preparation of (2S,3S)-3-hydroxyleucine.

HO NH NH NH (CH3)2N Sanjoinine NH2 (2S,3S)-3-Hydroxyleucine 2-Bromo-3-hydroxy-4-methyl- pentanoic acid