For the first time the total synthesis of the peptaibol zervamicin IIB is described. Synthesis of this peptaibol was achieved by the Fmoc/tert-butyl strategy in solution using a fragment condensation approach. Three fragments of zervamicin IIB were obtained by stepwise elongation with Fmoc amino acids using BOP as a coupling reagent. For the introduction of the highly sterically hindered alpha-aminoisobutyric acid residues BOP/DMAP activation was applied. The fmoc group was removed by reaction with 0.1 M NaOH in dioxane/methanol/water (30/9/1, v/v/v). Peptide fragments were coupled by means of a new coupling reagent, CF3-PyBOP. Using the strategy developed, zervamicin IIB and two analogues specifically deuterium-labelled at different positions of the glutamine-11 residue have been synthesized in 40% overall yield based on the isotopically labelled amino acid and with 98 +/- 2% of isotope enrichment. FAB mass spectroscopy, 600 MHz 1H-NMR spectroscopy and high-performance liquid chromatography provided convincing evidence that the synthetic products, zervamicin IIB and its deuterium-labelled analogues, fully correspond to the naturally occurring zervamicin IIB.
[15N]‐D‐isovaline was prepared from DL‐[α‐15N]‐α‐aminoisovaleramide by enzymatic resolution with Mycobacterium neoaurum. The 15N‐isotope was introduced during the Strecker synthesis of its precursor, e.g. aminoisovaleronitrile. Attempts to prepare the amino nitrile precursor of [15N]‐α‐aminoisobutyric acid (Aib) led to a poor yield and loss of the label. Significantly, improved results were obtained when a cosolvent is present during formation of aminoisobutyronitrile.
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