From stereodynamics to high-throughput screening of catalysed reactions.

Characterising chemical reactions by kinetic analysis is of fundamental importance to experimentally obtain insights into reaction mechanisms. Based on such investigations reactions can be optimised and improved catalysts designed. Enhanced reaction conditions may drastically increase the performance of the reaction in terms of yield and (enantio-) selectivity. Understanding reaction kinetics in more complex systems involving adaptive chemical and dynamic systems on a molecular level as shown here is even more challenging. Here we review recent developments in monitoring reactions including the dynamic interconversion of stereoisomers by integrating (catalysed) reactions and chemical analysis in on-column reaction chromatographic devices. These recent developments allow rapid screening of reactions in great detail and are a central tool in determining reaction pathways and to understand how to control the stereodynamics of chiral molecules.

Modular total synthesis of rhizopodin: a highly potent G-actin dimerizing macrolide.

A highly convergent total synthesis of the potent polyketide macrolide rhizopodin has been achieved in 29 steps by employing a concise strategy that exploits the molecule's C2 symmetry. Notable features of this convergent approach include a rapid assembly of the macrocycle through a site-directed sequential cross-coupling strategy and the bidirectional attachment of the side chains by means of Horner-Wadsworth-Emmons (HWE) coupling reactions. During the course of this endeavor, scalable routes for synthesis of three main building blocks of similar complexity were developed that allowed for their stereocontrolled construction. This modular route will be amenable to the development of syntheses of other analogues of rhizopodin.

Integration of catalysis and analysis is the key: rapid and precise investigation of the catalytic asymmetric Gosteli-Claisen rearrangement.

The kinetics of the Cu(II)(bisoxazoline)-catalyzed diastereo- and enantioselective Gosteli-Claisen rearrangement of 2-alkoxycarbonyl-substituted allyl vinyl ethers has been investigated by enantioselective on-column reaction gas chromatography (ocRGC). Enantioselective ocRGC integrates (stereoselective) catalysis and enantioselective chromatography in a single microcapillary, which is installed in a GC-MS for direct analysis of conversion and selectivity. Thus, this technique allows direct differentiation of thermal and stereoselectively catalyzed reaction pathways and determination of activation parameters and selectivities of the individual reaction pathways starting from stereoisomeric reactants with high precision. Two modes of operation of enantioselective ocRGC are presented to investigate noncatalyzed, i.e., conversion of isopropyl-2-(allyloxy)but-2Z-enoate 1 to isopropyl-3R,S-methyl-2-oxy-hex-5-enoate (±)-2 and the [Cu{(R,R)-Ph-box}](SbF(6))(2)-catalyzed Gosteli-Claisen rearrangement, i.e., conversion of isopropyl-2-(but-2'E-en-1-yloxy)but-2Z-enoate (E,Z)-3 to isopropyl-3S,4S-dimethyl-2-oxy-hex-5-enoate 4b. Eyring activation parameters have been determined by temperature-dependent measurements: Uncatalyzed rearrangement of 1 to (±)-2 gives ΔG(‡) (298 K) = 114.1 ± 0.2 kJ·mol(-1), ΔH(‡) = 101.1 ± 1.9 kJ·mol(-1), and ΔS(‡) = -44 ± 5 J·(K·mol)(-1), and catalyzed rearrangement of (E,Z)-3 to 4b gives ΔG(‡)(298 K) = 101.1 ± 0.3 kJ·mol(-1), ΔH(‡) = 106.1 ± 6.6 kJ·mol(-1), and ΔS(‡) = 17 ± 19 J·(K·mol)(-1).