Total Syntheses of the Histone Deacetylase Inhibitors Largazole and 2-epi-Largazole: Application of N-Heterocyclic Carbene Mediated Acylations in Complex Molecule Synthesis

Details of the evolution of strategies toward convergent assembly of the histone deacetylase inhibiting natural product largazole exploiting gamma,delta-unsaturated-alpha,beta-epoxy-aldehydes and a thiazole-thiazoline containing omega-amino-acid are described. The initial N-heterocyclic carbene mediated redox amidation exploying these two types of building blocks representing largazole's structural domains of distinct biosynthetic origin directly afforded the seco-acid of largazole. This was accomplished without any protecting groups resident upon either thioester bearing epoxy-aldehyde or the tetrapeptide. However, the ineffective production of largazole via the final macrolactonization led to an alternative intramolecular esterification/macrolactamization strategy employing the established two building blocks. This provided largazole along with its C2-epimer via an unexpected inversion of the alpha-stereocenter at the valine residue. The biological evaluation demonstrated that both largazole and 2-epi-largazole led to dose-dependent increases of acetylation of histone H3, indicating their potencies as class I histone deacetylase selective inhibitiors. Enhanced p21 expression was also induced by largazole and its C2 epimer. In addition, 2-epi-largazole displayed more potent activity than largazole in cell viability assays against PC-3 and LNCaP prostate cancer cell lines.