N,O-isopropylidenated threonines as tools for peptide cyclization: application to the synthesis of mahafacyclin B.

[structure: see text] The influence of a single N,O-isopropylidenated threonine turn-inducer on the cyclization of a linear heptapeptide precursor to mahafacyclin B has been investigated. Incorporation of an N,O-isopropylidenated threonine more than doubles the head-to-tail cyclization yield. The N,O-isopropylidene grouping is then readily disassembled to give the antimalarial cyclic peptide in high yield.