Organic & Biomolecular Chemistry Blog

Catalysis is a fundamental concept in chemistry, allowing chemists to effectively carry out difficult transformations to access valuable materials with precision and control. For the most part, research has focused on the development of new catalysts for optimized performance to achieve high conversion and selectivity. However, special attention is now being paid to engineering catalysts whose activity can be tuned through external stimuli. This concept is ubiquitous in nature and its implementation into artificial systems offers unique opportunities and promising future applications.

A recent OBC publication by Prof. Ben Feringa of the University of Groningen Nijenborgh discusses his group’s success in developing two novel bisthiourea catalysts which display dynamic control over activity and stereoselectivity in the Henry reaction using light and heat as external stimuli.

This catalyst design is based on a molecular motor previously reported by Feringa and inspired by nature where control over function, activity and selectivity can be attained through conformational changes within the catalyst’s active site induced by external stimuli.

In the current publication, upon irradiation of the catalyst in its stable trans state, (R,R)-(P,P)-trans (see Scheme), an unstable cis state is obtained, (R,R)-(M,M)-cis, in which the catalytic groups A and B are brought into proximity to carry out the desired enantioselective transformation. The catalyst can then be converted through heating to a stable cis state, the (R,R)-(M,M)-cis isomer, via a thermal helix inversion. In this conformation, the two active groups A and B remain within reaction proximity, however, as the helicity of the motor core is inverted, a pseudoenantiomeric catalytic environment is produced which results in the formation of the opposite enantiomer.

This is the first example of a tunable bisthiourea catalyst and represents an important advancement in the field of dynamic stimuli responsive catalysis. This new area of research offers great potential for advanced materials and solving long-standing challenges that have thus far been impossible to overcome using conventional methodologies.

To find out more see:
Dynamic control over catalytic function using responsive bisthiourea catalysts
M. Vlatković,  J. Volarić,  B. S. L. Collins, L. Bernardi and  B. L. Feringa*
DOI:10.1039/C7OB01851G

Victoria Corless is currently completing her Ph.D. in organic chemistry with Prof. Andrei Yudin at The University of Toronto. Her research is centred on the synthesis of kinetically amphoteric molecules which offer a versatile platform for the development of chemoselective transformations with particular emphasis on creating novel biologically active molecules.

Synthetic oligosaccharides and glycoconjugates are being increasingly used to solve important problems within biological research for vaccine development and drug discovery purposes. However, due to their complicated isolation and characterization from biological fluids, as well as a lack of general and efficient protocols for their preparation, progress within the field has been limited.

A recent OBC publication from Prof. Carmen Galan of the University of Bristol seeks to overcome this longstanding challenge using imidazolium-labeled (ITag) glycosides. Ionic liquid-based labels are emerging as a new class of covalent chemical labels that provide a unique handle for facile identification and purification of substrates from complex reaction mixtures using mass spectroscopy.

It was hypothesised that the ITag-glycan probes could be used to harness the natural biosynthetic machinery present in human breastmilk (i.e. glycosyltransferases) to access biologically important oligosaccharides. This study circumvents lengthy and costly chemical syntheses as well as the need to isolate a specific enzyme which can be challenging to express and applicable only to small-scale production.

In a matter of days, the group was able to access a series of highly desirable oligosaccharides, including LacNAc-ITag, ITag-Lewisx and ITAg-Lewisa through incubation of a labelled glycoside in readily sourced human breast milk. The capability of ITag to aid in the synthesis of oligosaccharides from a complex, multi-enzyme environment is pivotal. This technology could revolutionize the way in which synthetic, enzymatic transformations are carried out and has the potential to expand outside of carbohydrate chemistry.

To find out more see:

Imidazolium-labeled glycosides as probes to harness glycosyltransferase activity in human breast milk
I. Sittel and M. C. Galan
DOI: 10.1039/C7OB00550D

Victoria Corless is currently completing her Ph.D. in organic chemistry with Prof. Andrei Yudin at The University of Toronto. Her research is centred on the synthesis of kinetically amphoteric molecules which offer a versatile platform for the development of chemoselective transformations with particular emphasis on creating novel biologically active molecules.

The rare phenomenon of enantioselectivity reversal, using a simple change in reaction temperature control, is presented in the latest HOT article published in OBC.

The ability to mimic nature’s stereochemical control in the production of complex molecules has been a longstanding challenge in chemistry. There are numerous strategies chemists have implemented to generate stereochemically complex structures, however, with the advent of asymmetric catalysis, highly stereoselective reactions can be achieved using chiral reagents and catalysts.

In a recent OBC publication by Prof. Kenso Soai of Tokyo University and researchers from Merck, one of the very few examples in which the enantioselective outcome of a reaction is controlled through temperature was presented. Gaining enantioselective control by changing simple reaction parameters has been an attractive and long sought after advancement within the field of asymmetric catalysis. While there are examples of enantioselective control using solvent, metals and additives, very few examples exist that use temperature alone.

The study outlines the effect of temperature on the asymmetric autocatalysis of pyrimidal alkanol in the addition reaction of diisopropyl zinc to the pyrimidine-5-carbaldehyde. After reaction initiation using a chiral initiator, the product alkanol behaves as an asymmetric catalyst for its own formation and infers its chirality to the product in an autocatalytic cycle. When the reaction was performed at 0 ºC in the presence of (S)-1-phenyl-ethyl alcohol, as expected, (S)-pyrimidal alkanol was afforded in high enantiomeric excess. Interestingly, when the reaction was cooled to -44 ºC, the opposite enantioselectivity was observed though with a slightly lower enantiomeric excess of the desired alkanol.

The exact mechanism through which this reversal happens is still unclear however, it’s speculated that the relationship between temperature and the relative enthalpic vs. entropic contributions to free energy may play a part or the temperature dependent aggregation of zinc alkoxide may also be involved.

It’s important to remember that the temperature effect on reactions involving organozinc reagents is not always straight forward and may not always lead to the best outcome.

Regardless, this study provides interesting insight into temperature controlled enantioselectivity that may lead to a more detailed understanding of such processes and how they can be synthetically exploited.

To find out more see:

Unusual reversal of enantioselectivity in the asymmetric autocatalysis of pyrimidyl alkanol triggered by chiral aromatic alkanols and amines
Arimasa Matusmoto, Satoshi Fujiwara, Yui Hiyoshi,aKerstin Zawatzky, Alexey A. Makarov, Christopher J. Welch and Kenso Soai
DOI: 10.1039/C6OB02415G

Victoria Corless is currently completing her Ph.D. in organic chemistry with Prof. Andrei Yudin at The University of Toronto. Her research is centred on the synthesis of kinetically amphoteric molecules, which offer a versatile platform for the development of chemoselective transformations with particular emphasis on creating novel biologically active molecules.