The enantioselective synthesis of APTO and AETD: polyhydroxylated beta-amino acid constituents of the microsclerodermin cyclic peptides

The polyhydroxylated beta-amino acids (2S,3R,4S,5S,7E)-3-amino-8-phenyl-2,4,5-trihydroxyoct-7-enoic acid (APTO) and (2S,3R,4S,5S,7E,9E)-3-amino-10-(4-ethoxyphenyl)-2,4,5-trihydroxydeca-7,9-dienoic acid (AETD) are key components of the microsclerodermin family of anti-fungal cyclic peptides. They have been synthesised in protected form in twelve steps using a unified strategy, with the introduction of the unsaturated sidechain in the final step of the synthesis from a common aldehyde intermediate. The synthesis features the ordered application of asymmetric aminohydroxylation and dihydroxylation reactions to efficiently introduce the stereochemistry of the targets with high selectivity.