Chemical Dynamic Thermodynamic Resolution and S/R Interconversion of Unprotected Unnatural Tailor-made α-Amino Acids.

Described here is an advanced, general method for purely chemical dynamic thermodynamic resolution and S/R interconversion of unprotected tailor-made α-amino acids (α-AAs) through intermediate formation of the corresponding nickel(II)-chelated Schiff bases. The method features virtually complete stereochemical outcome, broad substrate generality (35 examples), and operationally convenient conditions allowing for large-scale preparation of the target α-AAs in enantiomerically pure form. Furthermore, the new type of nonracemizable axially chiral ligands can be quantitatively recycled and reused, rendering the whole process economically and synthetically attractive.

Biochemical quantitation of the eIF5A hypusination in Arabidopsis thaliana uncovers ABA-dependent regulation.

The eukaryotic translation elongation factor eIF5A is the only protein known to contain the unusual amino acid hypusine which is essential for its biological activity. This post-translational modification is achieved by the sequential action of the enzymes deoxyhypusine synthase (DHS) and deoxyhypusine hydroxylase (DOHH). The crucial molecular function of eIF5A during translation has been recently elucidated in yeast and it is expected to be fully conserved in every eukaryotic cell, however the functional description of this pathway in plants is still sparse. The genetic approaches with transgenic plants for either eIF5A overexpression or antisense have revealed some activities related to the control of cell death processes but the molecular details remain to be characterized. One important aspect of fully understanding this pathway is the biochemical description of the hypusine modification system. Here we have used recombinant eIF5A proteins either modified by hypusination or non-modified to establish a bi-dimensional electrophoresis (2D-E) profile for the three eIF5A protein isoforms and their hypusinated or unmodified proteoforms present in Arabidopsis thaliana. The combined use of the recombinant 2D-E profile together with 2D-E/western blot analysis from whole plant extracts has provided a quantitative approach to measure the hypusination status of eIF5A. We have used this information to demonstrate that treatment with the hormone abscisic acid produces an alteration of the hypusine modification system in Arabidopsis thaliana. Overall this study presents the first biochemical description of the post-translational modification of eIF5A by hypusination which will be functionally relevant for future studies related to the characterization of this pathway in Arabidopsis thaliana.

Assignment of absolute configuration on the basis of the conformational effects induced by chiral derivatizing agents: the 2-arylpyrrolidine case.

A novel approach for determining the absolute configuration of a chiral compound is proposed. The methodology is based on the distinct conformational effects imposed on a chiral substrate by each enantiomer of a chiral derivatizing agent. As a proof of concept, it is shown that the absolute configuration of 2-arylpyrrolidines can easily be determined by inspection of the multiplicity of the NMR signal of the methine proton of the pyrrolidine ring in the corresponding Mosher's amides.

The stereocontrolled total synthesis of altohyrtin A/spongistatin 1: the southern hemisphere EF segment.

The fully functionalised C29-C51 southern hemisphere of altohyrtin A/spongistatin 1 , incorporating the E- and F-ring tetrahydropyran rings and the unsaturated side chain, has been synthesised in a highly convergent and stereocontrolled manner. Key steps in the synthesis of this phosphonium salt include four highly diastereoselective, substrate-controlled, boron aldol reactions to establish key C-C bonds and accompanying stereocentres, where the introduction of the chlorodiene side chain and the C47 hydroxyl-bearing centre were realised by exploiting remote stereoinduction from the F-ring tetrahydropyran.

The stereocontrolled total synthesis of altohyrtin A/spongistatin 1: fragment couplings, completion of the synthesis, analogue generation and biological evaluation.

The antimitotic marine macrolide altohyrtin A/spongistatin 1 has been synthesised in a highly convergent and stereocontrolled manner, thus contributing to the replenishment of the largely exhausted material from the initial isolation work. Coupling of the AB- and CD-spiroacetal subunits by a stereoselective aldol reaction was achieved by using either a lithium (67 : 33 dr) or boron enolate (90 : 10 dr). A highly (Z)-selective Wittig coupling was used to unite the northern hemisphere aldehyde with the southern hemisphere phosphonium salt . Deprotection and subsequent regioselective macrolactonisation on a triol seco-acid completed the synthesis of altohyrtin A. Two structural analogues were also prepared and evaluated as growth inhibitory agents against a range of human tumour cell lines, including Taxol-resistant strains, alongside altohyrtin A and paclitaxel (Taxol), revealing that dehydration in the E-ring is tolerated and results in enhanced cytotoxicity (at the low picomolar level), whereas the presence of the full C44-C51 side-chain appears to be crucial for biological activity.

Synthesis and biological evaluation of spongistatin/altohyrtin analogues: E-ring dehydration and C46 side-chain truncation.

Simplified analogues of the potent antimitotic marine macrolide spongistatin 1/altohyrtin A were synthesised and evaluated as growth inhibitory agents against a range of human tumour cell lines, including Taxol-resistant strains, revealing that E-ring dehydration leads to enhanced cytotoxicity at the low picomolar level while truncation of the side-chain at C46 results in a drastic decrease in activity.

Stereocontrolled Total Synthesis of (+)-Altohyrtin A/Spongistatin 1.

As an exceptionally potent antimitotic macrolide, altohyrtin A/spongistatin 1 shows great promise in cancer chemotherapy but its extreme scarcity in the natural sponges has halted its further preclinical development. A highly stereocontrolled total synthesis, which exploits boron-mediated aldol bond constructions, has been realized to provide, for the first time, a useful amount of synthetic material.