The first total synthesis of actinomycins containing L-4-hydroxyproline (1) and the separation and NMR spectroscopic assignment of the regioisomers XOβ and iso-XOβ (1c, 1e) are described. The synthetic XOβ proves, that the oxidized proline ring of the natural actinomycins XOβ , XOδ and X2 (1c, 1d, 1b ) is situated in position 3' of the (β)-peptide chain. The new nonnatural variants iso-XOβ (1e ) and Bis(hyp)-X1 (1f) permitted interesting structure/activity studies. Furthermore, the amino acid sequence of the actinomycins X2 (1b) and X1a (1g) could be derived from NMR correlation spectra. A crystal structure analysis confirmed the complete structure of 1b (and thus that of 1c - e ) and showed the characteristical ,,A “-type conformation present in the depsipeptide rings. This corresponds to the solution conformation, which is clearly verified by typical NMR data.
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Lifferth et al. (1999) studied this question.