Total Synthesis of the Bicyclic Depsipeptide HDAC Inhibitors Spiruchostatins A and B, 5 ''-epi-Spiruchostatin B, FK228 (FR901228) and Preliminary Evaluation of Their Biological Activity

The bicyclic depsipeptide histone deacetylase (HDAC) inhibitors spiruchostatins A and B, 5 ''-epi-spiru-chostatin B and FK228 were efficiently synthesized in a convergent and unified manner. The synthetic method involved the following crucial steps: i) a Julia-Kocienski olefination of a 1,3-propanediol-derived sulfone and a L- or D-malic acid-derived aldehyde to access the most synthetically challenging unit, (3S or 3R,4E)-3-hydroxy-7-mercaptohept-4-enoic acid, present in a D-alanine- or D-valine-containing segment; ii) a condensation of a D-valine-D-cysteine- or D-allo-isoleucine-D-cysteine-containing segment with a D-alanine-or D-valine-containing segment to directly assemble the corresponding seco-acids; and iii) a macrocyclization of a seco-acid using the Shiina method or the Mitsunobu method to construct the requisite 15- or 16-membered macrolactone. The present synthesis has established the C5 '' stereochemistry of spiruchostatin B. In addition, HDAC inhibitory assay and the cell-growth inhibition analysis of the synthesized depsipeptides determined the order of their potency and revealed some novel aspects of structure-activity relationships. It was also found that unnatural 5 ''-epi-spiruchostatin B shows extremely high selectivity (ca. 1600-fold) for class I HDAC1 (IC50=2.4 nM) over class II HDAC6 (IC50=3900 nM) with potent cell-growth-inhibitory activity at nanomolar levels of IC50 values.