Abstract
Starting from cinnamates 9, obtained by Wittig reaction or Heck coupling, the diols 17 were prepared by asymmetric dihydroxylation. This was followed by a regioselective substitution of the 3-OH group with hydrazoic acid under Mitsunobu conditions. Methylation of the 2-OH group and reduction of the azide group led to the β-tyrosine derivatives 8. Condensation with the dipeptide acid 6 furnished the tripeptide part of the chondramides. The derived acids 21 were combined with the hydroxy ester 7 to the esters 22. Cleavage of the tert-butyl groups and intramolecular lactam formation gave rise to the chondramide A analogues 2 b-k. Growth inhibition assays showed most of the analogues to be biologically active. Some of them even reach the activity of jasplakinolide. It can be concluded that the 4-position of the aryl ring in the β-tyrosine of chondramide A tolerates structural modifications quite well.
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