A two step synthesis of a key unit B precursor of cryptophycins by asymmetric hydrogenation

Recent Advances in Asymmetric Hydrogenation Catalysis Utilizing Spiro and Other Rigid C-Stereogenic Phosphine Ligands

Transition-metal-catalyzed asymmetric reactions have been a powerful tool in organic synthesis for many years. The design of chiral ligands with the right configuration is fundamental to induce high regio- and stereoselectivity to catalytic reactions and to achieve high turnover numbers and high yields. A challenge is the control of prochiral centers with similar electronic properties in a similar steric environment within the same molecule. Over the last 10 years, a range of novel rigid C-stereogenic chiral phosphine ligands has been developed and successfully applied in various types of asymmetric transformations. Many of these ligands are of a di-, tri-, or multidentate nature. The purpose of this Perspective is to highlight recent synthetic achievements (since 2010) with spiro-phosphines and other rigid phosphines and discuss some mechanistic aspects of the catalytic reactions.

Novel unit B cryptophycin analogues as payloads for targeted therapy

Cryptophycins are naturally occurring cytotoxins with great potential for chemotherapy. Since targeted therapy provides new perspectives for treatment of cancer, new potent analogues of cytotoxic agents containing functional groups for conjugation to homing devices are required. We describe the design, synthesis and biological evaluation of three new unit B cryptophycin analogues. The O-methyl group of the unit B D-tyrosine analogue was replaced by an O-(allyloxyethyl) moiety, an O-(hydroxyethyl) group, or an O-(((azidoethoxy)ethoxy)ethoyxethyl) substituent. While the former two maintain cytotoxicity in the subnanomolar range, the attachment of the triethylene glycol spacer with a terminal azide results in a complete loss of activity. Docking studies of the novel cryptophycin analogues to β-tubulin provided a rationale for the observed cytotoxicities.

Total synthesis and biological evaluation of fluorinated cryptophycins

Cryptophycins are cytotoxic natural products that exhibit considerable activities even against multi-drug-resistant tumor cell lines. As fluorinated pharmaceuticals have become more and more important during the past decades, fluorine-functionalized cryptophycins were synthesized and evaluated in cell-based cytotoxicity assays. The unit A trifluoromethyl-modified cryptophycin proved to be highly active against KB-3-1 cells and exhibited an IC(50) value in the low picomolar range. However, the replacement of the 3-chloro-4-methoxyphenyl-substituent in unit B by a pentafluorophenyl moiety resulted in a significant loss of activity.