Jesús Razkin

Asymmetric synthesis of propargylic alcohols via aldol reaction of aldehydes with ynals promoted by prolinol ether–transition metal–Brønsted acid cooperative catalysis

A catalytic and highly stereoselective entry to propargylic alcohols and products derived thereof is reported based on an unprecedented cross-aldol coupling between unmodified aldehydes and ynals. The method requires an amine–metal salt–Bronsted acid ternary catalyst system and implies synergistic activation of the donor aldehyde via enamine and of the acceptor carbonyl via unique and reversible metal–alkyne complexation. Specifically, by using a combined a,a-dialkylprolinol silyl ether–CuI–PhCO2H catalyst system, remarkably high levels of diastereo- and enantioselectivity (anti/syn up to >20 : 1, ee up to >99%) are achieved.

Downsizing of Enzymes by Chemical Methods: Arginine Mimics with Low pKa Values Increase the Rates of Hydrolysis of RNA Model Compounds

This thesis addresses structure and reactivity of polypeptide catalysts in reactions that mimic the hydrolysis of RNA and DNA. A designed helix-loop-helix motif was used as a scaffold where the amino acid residues were systematically varied. The reactivity of a previously reported catalyst, HNI, was evaluated and the catalytic residues of the active site, Arg and His, were replaced in a stepwise manner by the artificial amino acid Gcp. Gcp has a guanidinocarbonyl pyrrole side chain, i.e. a side chain that mimics that of Arg but with a lower pKa. Gcp was used to replace both histidine and arginine in the polypeptide catalysts and was able to bind phosphate as well as carry out general base catalysis. The parent polypeptide HNI was shown to catalyse phosphoryl transfer reactions of phosphodiesters in an active site with two His and two Arg residues. The performance of the active site was improved by the introduction of two Tyr residues to form the catalyst HJ1 designed to provide nucleophilic catalysis in the hydrolysis of DNA model substrates. To improve the catalytic activity beyond that of HJI, Gcp was introduced to replace Arg and His residues in the HN1 scaffold. The designed catalyst JL3 was capable of a 150-fold rate enhancement compared to HNI in the reaction of the substrate HPNP, representing the first step in RNA hydrolysis. Mechanistic studies of JL3 catalysis suggested that the pKa value of the Gcp residue in the folded polypeptides was around 5. In combination with the observation of a solvent kinetic isotope effect of 1.7 the Gcp residue was proposed to provide general base catalysis and transition state stabilisation in the reaction of uridine 3′-2,2,2-trichloroethylphosphate, a realistic RNA model with a leaving group pKa of 12.5. The JL3 polypeptide catalyst followed saturation kinetics with a kcat/KM of 1.08 x 10-3 M-1s-1. The introduction of a designed photoswitchable amino acid in a catalytic polypeptide allowed the activity of the polypeptide in the reaction with an activated ester to be under photochemical control. Photoisomerization of this switch altered the structure of the polypeptide and affected the catalytic activity of the polypeptide catalyst. The chemical synthesis of designed molecules expands the amino acid alphabet and makes it possible to downsize enzymatic functions. It opens up possibilities for the production of novel biocatalysts that can catalyse natural as well as non-natural reactions.

Stereoselective synthesis of sitophilate and sitophilure

Abstract Synthesis of sitophilate 6 , the male-produced aggregation pheromone of the granary weevil, has been achieved with an overall yield of 46% from 1 in high enantiomeric and diastereomeric purity. From sitophilate, sitophilure 8 , aggregation pheromone of the rice weevil and maize weevil, was prepared in a four-step synthesis without any racemization.

Stereoselective synthesis of dominicalure 1 and 2: Components of aggregation pheromone from male lesser grain borerRhyzopertha dominica (F.)

Dominicalure 1 (9a) and dominicalure 2 (9b), were synthesized by esterification of α,β-unsaturated acids4a and4b with (S)-(+)-2-pentanol (8). The key step was the asymmetric reduction of 3-penten-2-one (5) to give the chiral intermediate6, which, upon diimide reduction, DNB derivatization, recrystallization, and hydrolysis, yielded8 in 63% ee. Acids4a and4b were prepared in a simple and efficient three-step synthesis with an overall yield of 54% and 62%, respectively, in stereoisomerically pure form.

Combined Enzyme and Substrate Design: Grafting of a Cooperative Two-Histidine Catalytic Motif into a Protein Targeted at the Scissile Bond in a Designed Ester Substrate

A histidine‐based, two‐residue reactive site for the catalysis of hydrolysis of designed sulfonamide‐containing para‐nitrophenyl esters has been engineered into a scaffold protein. A matching substrate was designed to exploit the natural active site of human carbonic anhydrase II (HCAII) for well‐defined binding. In this we took advantage of the high affinity between the active site zinc atom and sulfonamides. The ester substrate was designed to position the scissile bond in close proximity to the His64 residue in the scaffold protein. Three potential sites for grafting the catalytic His–His pair were identified, and the corresponding N62H/H64, F131H/V135H and L198H/P202H mutants were constructed. The most efficient variant, F131H/V135H, has a maximum kcat/KM value of approximately 14 000 M−1 s−1, with a kcat value that is increased by a factor of 3 relative to that of the wild‐type HCAII, and by a factor of over 13 relative to the H64A mutant. The results show that an esterase can be designed in a stepwise way by a combination of substrate design and grafting of a designed catalytic motif into a well‐defined substrate binding site.

Catalytic Enantioselective Quick Route to Aldol‐Tethered 1,6‐ and 1,7‐Enynes from ω‐Unsaturated Aldehydes

An effective asymmetric route to functionalized 1,6- and 1,7-enynes has been developed based on a direct cross-aldol reaction between ω-unsaturated aldehydes and propargylic aldehydes (α,β-ynals) promoted by combined α,α-dialkylprolinol ether/Brønsted acid catalysis. This synergistic activation strategy is key to accessing the corresponding aldol adducts with high stereoselectivity, both enantio- and diastereoselectivity. The aldol reaction also proceeds well with propargylic ketones (α,β-ynones) thus enabling a stereocontrolled access to the corresponding tertiary alcohols. The utility of these adducts, which are difficult to prepare through standard methodology, is demonstrated by their transformation into trisubstituted bicyclic enones using standard Pauson-Khand conditions.