Structural Contributions to Autocatalysis and Asymmetric Amplification in the Soai Reaction

Heterochiral coupling to bilateral β-turn structured azapeptides bearing two remote chiral centers

Enantioselective synthesis governed by chiral catalysts has been extensively developed, but that without any chiral auxiliaries or chiral catalysts is rare, particularly when remote stereogenic centers are involved. Here we report an enantioselectivity of heterochiral coupling in the one-pot reaction of racemic hydrazides with achiral 1,4-bis(isothiocyanine)benzene, yielding preferentially the heterochiral bilateral azapeptides over the homochiral ones. Despite bearing two hydrogen-bonded β-turn structures that allow intramolecular chiral transfer, the bilateral azapeptide products have two chiral centers separated by 14 atoms or 15 bonds, which prevent the direct intramolecular asymmetric communication between the two chiral centers. Interestingly, the heterochiral azapeptides feature intermolecular hydrogen bonding stacking between homochiral β-turns to form a superstructure of alternative M- and P-helices in the crystals. In contrast, the homochiral azapeptide counterparts adopt a β-sheet-like structure, which is less favorable compared to the helical-like superstructure from heterochiral azapeptides, accounting for the favored heterochiral coupling of the one-pot reaction. This work demonstrates enantioselective synthesis involving distant chiral centers through the formation of biomimetic superstructures, opening up new possibilities for the regulation of enantioselectivity.

Exploring Border Conditions for Spontaneous Emergence of Chirality in Allylboration of 1,2,3-Triazolic Aldehydes

A case of spontaneous chirality generation was observed during a synthetic project studying the allylboration of 1,2,3-triazolic aldehydes. Here, we present computational studies supported by experimental findings targeting the elucidation of border conditions required for the observation of spontaneous chirality generation in the reaction of 1-Ar-1H-1,2,3-triazole-4-carbaldehydes 1a,b with triallylborane. Three possible sources of symmetry breaking were found computationally. Thus, dimerization of the initial reaction products, alcoholates 4a,b, gives dimers 5a,b (homochiral) and 6a,b (heterochiral). The latter were computed to be more stable thermodynamically, which can lead to amplification of the initial stochastic imbalance of the enantiomers of 4a,b via the reservoir mechanism. Furthermore, enantiomeric excess can be increased during the transfer of the second allylic group in the reaction of optically active boronates 4a,b with 1a,b, which was computed to be enantioselective due to the strong activating and stereoregulating properties of the 1,2,3-triazole group. In addition, reactions of borinic esters 8a,b, products of the previous reaction with triallylborane, recovered in each case two molecules of 4a,b of the same handedness, which can lead to additional chirality amplification. Experimentally, reactions of optically active alcohols (+)-R-2a,b with triallylborane provided chiral alcoholates 4a,b, which were reacted with equivalent amounts of corresponding aldehydes 1a,b. Unexpectedly, in two series of 10 experiments each, preferential formation of both enantiomers of the newly formed product was observed: seven times S and three times R in the case of 1a and six times S and four times R in the case of 1b.

The Reduction of Carbonyl Compounds with Dicyclopentylzinc: A New Example of Asymmetric Amplifying Autocatalysis

A previously unknown reduction of carbonyl compounds with dicyclopentylzinc is reported. Aldehydes react in mild conditions yielding corresponding primary alcohols and cyclopentene. Although cyclohexanone and acetophenone are inert to dicyclopentylzinc, a variety of heterocyclic ketones reacted readily, yielding reasonable to high yields of corresponding secondary alcohols. When the reaction was catalyzed with (-)-(1R,2S)-ephedrine, 3-acetylpyridine (10) resulted in a high yield of (S)-1-(pyridin-3-yl)ethanol (19) with >99% ee. 5-Acetyl-2-bromopyridine (11) also provided the corresponding optically active alcohol 20, albeit with a much lower optical yield. When 10% of 19 with 92% ee was used as an autocatalyst, 55% yield of the same compound was obtained, with 95% ee and 96% ee in two independent experiments. A three-stage reaction sequence starting from "no chirality" reaction yielded 19 with 6% ee. Thus, amplifying autocatalysis was detected in the reaction of ketone 10 with dicylopentylzinc.

Fluorine-Assisted Rearrangement of Geminal Azidofluorides to Imidoyl Fluorides

Organoazide rearrangement constitutes versatile synthetic strategies but typically requires an extremely strong acid and/or a high reaction temperature. Our group recently discovered the remarkable accelerating effect of the geminal fluorine substituent that enables the facile rearrangement of azides into imidoyl fluorides without the aid of acid under much milder reaction conditions. The role of geminal fluorine was elucidated by both experimental and computational investigations. This new reactivity led to the development of a practical one-step tandem preparative method for potentially useful and bench-stable imidoyl fluorides from a wide range of structurally diverse geminal chlorofluorides. Our additional efforts to expand the reaction scope regarding the migrating group, halogen, and carbonyl function are described, and the synthetic utility of the imidoyl fluoride products was demonstrated in hopes of promoting the use of this under-appreciated functional group in the synthetic organic community.

Structural insights into the catalytic and inhibitory mechanisms of the flavin transferase FmnB in Listeria monocytogenes

Listeria monocytogenes, a food-borne Gram-positive pathogen, often causes diseases such as gastroenteritis, bacterial sepsis, and meningitis. Newly discovered extracellular electron transfer (EET) from L. monocytogenes plays critical roles in the generation of redox molecules as electron carriers in bacteria. A Mg2+-dependent protein flavin mononucleotide (FMN) transferase (FmnB; UniProt: LMRG_02181) in EET is responsible for the transfer of electrons from intracellular to extracellular by hydrolyzing cofactor flavin adenine dinucleotide (FAD) and transferring FMN. FmnB homologs have been investigated in Gram-negative bacteria but have been less well studied in Gram-positive bacteria. In particular, the catalytic and inhibitory mechanisms of FmnB homologs remain elusive. Here, we report a series of crystal structures of apo-FmnB and FmnB complexed with substrate FAD, three inhibitors AMP, ADP, and ATP, revealing the unusual catalytic triad center (Asp301-Ser257-His273) of FmnB. The three inhibitors indeed inhibited the activity of FmnB in varying degrees by occupying the binding site of the FAD substrate. The key residue Arg262 of FmnB was profoundly affected by ADP but not AMP or ATP. Overall, our studies not only provide insights into the promiscuous ligand recognition behavior of FmnB but also shed light on its catalytic and inhibitory mechanisms.

One Soai reaction, two mechanisms?

For over 25 years the chemistry community has puzzled over the mechanism of the Soai reaction, a fascinating chemical process which achieves chiral symmetry breaking by combining autocatalysis with asymmetric amplification. In 2020, the groups of Denmark and Trapp each made a proposal, based on extensive experimental work, on what is the catalytic species there: either a tetrameric product alkoxide aggregate ("SMS tetramer") or a product-substrate dimer ("hemiacetal"). These models seemingly oppose and exclude each other; however, they might also be both valid since the studies were conducted on different substrates which are not necessarily equivalent. This is shown in this Viewpoint by an in-depth comparison of the two studies and of data from earlier reports, which opens up to a discussion on this scenario's far-reaching implications on the fundamental understanding of asymmetry-amplifying autocatalysis.

Ultrasound-assisted synthesis of pyrimidines and their fused derivatives: A review

The pyrimidine scaffold is present in many bioactive drugs; therefore, efficient synthetic routes that provide shorter reaction times, higher yields, and site-selective reactions are constantly being sought. Ultrasound (US) irradiation has emerged as an alternative energy source in the synthesis of these heterocyclic scaffolds, and over the last ten years there has been a significant increase in the number of publications mentioning US in either the construction or derivatization of the pyrimidine core. This review presents a detailed summary (with 140 references) of the effects of US (synergic or not) on the construction and derivatization of the pyrimidine core through classical reactions (e.g., multicomponent, cyclocondensation, cycloaddition, and alkylation reactions). The main points that were taken into consideration are as follows: chemo- and regioselectivity issues, and the results of conventional heating methods compared to US and mechanistic insights that are also presented and discussed for key reactions.

Circular dichroism spectroscopy of catalyst preequilibrium in asymmetric autocatalysis of pyrimidyl alkanol

Mechanistic understanding of the asymmetric autocatalysis of pyrimidyl alkanol is a highly attractive and challenging topic due to its unique feature of amplification of enantiomeric excess. Circular dichroism spectroscopic analysis of this reaction allows monitoring of the structual changes of possible catalyst precursors in the solution state and shows characteristic temperature and solvent dependence. TD-DFT calculations suggest that these spectral changes are induced by a dimer-tetramer equilibrium of zinc alkoxides.

Absolute Asymmetric Catalysis, from Concept to Experiment: A Narrative

The generally accepted hypothesis to explain the origin of biological homochirality (that is to say, the fact that proteinogenic amino acids are left-handed, and carbohydrates right-handed, in all living beings) is to assume, in the course of prebiotic chemical evolution, the appearance of an initial enantiomeric excess in a set of chiral molecular entities by spontaneous mirror-symmetry breaking (SMSB), together with suitable amplification and replication mechanisms that overcome the thermodynamic drive to racemization. However, the achievement of SMSB in chemical reactions taking place in solution requires highly specific reaction networks showing nonlinear dynamics based on enantioselective autocatalysis, and examples of its experimental realization are very rare. On the other hand, emergence of net supramolecular chirality by SMSB in the self-assembly of achiral molecules has been seen to occur in several instances, and the chirality sign of the resulting supramolecular system can be controlled by the action of macroscopic chiral forces. These considerations led us to propose a new mechanism for the generation of net chirality in molecular systems, in which the SMSB takes place in the formation of chiral supramolecular dissipative structures from achiral monomers, leading to asymmetric imbalances in their composition that are subsequently transferred to a standard enantioselective catalytic reaction, dodging in this way the highly limiting requirement of finding suitable reactions in solution that show enantio­selective autocatalysis. We propose the name ‘absolute asymmetric catalysis’ for this approach, in which an achiral monomer is converted into a nonracemic chiral aggregate that is generated with SMSB and that is catalytically active.

Our aim in this Account is to present a step-by-step narrative of the conceptual and experimental development of this hitherto unregarded, but prebiotically plausible, mechanism for the emergence of net chirality in molecular reactions.

1 Introduction: The Origin of Biological Homochirality and Spontaneous Mirror-Symmetry Breaking

2 Experimental Chemical Models for Spontaneous Mirror-Symmetry Breaking: The Soai Reaction and Beyond

3 Spontaneous Mirror-Symmetry Breaking in Supramolecular Chemistry: Plenty of Room at the Top

4 Absolute Asymmetric Catalysis: An Alternative Mechanism for the Emergence of Net Chirality in Molecular Systems

5 Experimental Realization of Top-Down Chirality Transfer to the

Molecular Level

6 Conclusions and Outlook

Chiral Amplification of Phosphoramidates of Amines and Amino Acids in Water

The origin of biomolecular homochirality continues to be one of the most fascinating aspects of prebiotic chemistry. Various amplification strategies for chiral compounds to enhance a small chiral preference have been reported, but none of these involves phosphorylation, one of nature's essential chemical reactions. Here we present a simple and robust concept of phosphorylation-based chiral amplification of amines and amino acids in water. By exploiting the difference in solubility of a racemic phosphoramidate and its enantiopure form, we achieved enantioenrichment in solution. Starting with near racemic, phenylethylamine-based phosphoramidates, ee's of up to 95 % are reached in a single amplification step. Particularly noteworthy is the enantioenrichment of phosphorylated amino acids and their derivatives, which might point to a potential role of phosphorus en-route to prebiotic homochirality.