Replication of α-amino acids via Strecker synthesis with amplification and multiplication of chiral intermediate aminonitriles

Primary α-Phosphino- and α-Arseno-Nitriles, Analogues of α-Aminonitriles

More than 170 years after the synthesis of α-aminonitriles by the reaction of Strecker, α-phosphinonitriles, and α-arsenonitriles have been prepared and characterized by NMR and IR spectroscopy. For the simplest derivatives, the reaction was carried out by reaction of cyanomethyltributylstannane with phosphorus or arsenic trichloride, followed by a chemoselective reduction of the dichlorophosphine and dichloroarsine formed to the corresponding primary phosphine and arsine. The phosphinoacetonitrile can be stored at -80 °C for months in its pure state, but arsenoacetonitrile decomposes at this temperature. Chiral compounds can be synthesized from C-substituted cyano-1-alkyltributylstannanes. In 1H NMR spectroscopy, these chiral phosphines and arsines display diastereotopic protons for the PH2 and AsH2 groups, a property never observed for the NH2 group of α-aminonitriles. This work paves the way for further studies on the chemistry of these compounds, including a comparative chemistry of these phosphorus and arsenic derivatives with the well-known α-aminonitriles.

Asymmetric Strecker reaction at the solid/solid interface

Related to absolute asymmetric synthesis, a stereospecific reaction at the solid/solid interface arising from crystal chirality of the achiral or racemic substrates has not yet been reported. Here, we demonstrate the asymmetric Strecker-type solid/solid reaction between the chiral crystal of a racemic cyanohydrin (kryptoracemate) and the achiral crystal of an ammonium salt to afford highly enantioenriched α-aminonitrile in combination with amplification of chirality. rac-Cyanohydrin provides its chiral surface as a reactive site and the reaction proceeds with dissociation of cyanohydrin; thus, an asymmetric Strecker-type reaction takes place at the interface of the substrate crystals. Strecker synthesis coupled with cyanohydrin synthesis offers a credible abiotic synthesis mechanism of α-amino acids and α-hydroxy acids. For the first time, stereochemical relationship has been found between the two chiral intermediates, aminonitrile and cyanohydrin, which are in equilibrium in the synthesis mechanism.

Possible chemical and physical scenarios towards biological homochirality

The single chirality of biological molecules in terrestrial biology raises more questions than certitudes about its origin. The emergence of biological homochirality (BH) and its connection with the appearance of life have elicited a large number of theories related to the generation, amplification and preservation of a chiral bias in molecules of life under prebiotically relevant conditions. However, a global scenario is still lacking. Here, the possibility of inducing a significant chiral bias "from scratch", i.e. in the absence of pre-existing enantiomerically-enriched chemical species, will be considered first. It includes phenomena that are inherent to the nature of matter itself, such as the infinitesimal energy difference between enantiomers as a result of violation of parity in certain fundamental interactions, and physicochemical processes related to interactions between chiral organic molecules and physical fields, polarized particles, polarized spins and chiral surfaces. The spontaneous emergence of chirality in the absence of detectable chiral physical and chemical sources has recently undergone significant advances thanks to the deracemization of conglomerates through Viedma ripening and asymmetric auto-catalysis with the Soai reaction. All these phenomena are commonly discussed as plausible sources of asymmetry under prebiotic conditions and are potentially accountable for the primeval chiral bias in molecules of life. Then, several scenarios will be discussed that are aimed to reflect the different debates about the emergence of BH: extra-terrestrial or terrestrial origin (where?), nature of the mechanisms leading to the propagation and enhancement of the primeval chiral bias (how?) and temporal sequence between chemical homochirality, BH and life emergence (when?). Intense and ongoing theories regarding the emergence of optically pure molecules at different moments of the evolution process towards life, i.e. at the levels of building blocks of Life, of the instructed or functional polymers, or even later at the stage of more elaborated chemical systems, will be critically discussed. The underlying principles and the experimental evidence will be commented for each scenario with particular attention on those leading to the induction and enhancement of enantiomeric excesses in proteinogenic amino acids, natural sugars, and their intermediates or derivatives. The aim of this review is to propose an updated and timely synopsis in order to stimulate new efforts in this interdisciplinary field.

Chirality in Organic and Mineral Systems: A Review of Reactivity and Alteration Processes Relevant to Prebiotic Chemistry and Life Detection Missions

Chirality is a central feature in the evolution of biological systems, but the reason for biology’s strong preference for specific chiralities of amino acids, sugars, and other molecules remains a controversial and unanswered question in origins of life research. Biological polymers tend toward homochiral systems, which favor the incorporation of a single enantiomer (molecules with a specific chiral configuration) over the other. There have been numerous investigations into the processes that preferentially enrich one enantiomer to understand the evolution of an early, racemic, prebiotic organic world. Chirality can also be a property of minerals; their interaction with chiral organics is important for assessing how post-depositional alteration processes could affect the stereochemical configuration of simple and complex organic molecules. In this paper, we review the properties of organic compounds and minerals as well as the physical, chemical, and geological processes that affect organic and mineral chirality during the preservation and detection of organic compounds. We provide perspectives and discussions on the reactions and analytical techniques that can be performed in the laboratory, and comment on the state of knowledge of flight-capable technologies in current and future planetary missions, with a focus on organics analysis and life detection.

A Four-Step Enzymatic Cascade for Efficient Production of L-Phenylglycine from Biobased L-Phenylalanine

Enantiopure amino acids are of particular interest in the agrochemical and pharmaceutical industries. Here, we reported a multi-enzyme cascade for efficient production of L-phenylglycine (L-Phg) from biobased L-phenylalanine (L-Phe). We first attempted to engineer Escherichia coli for expressing L-amino acid deaminase (LAAD) from Proteus mirabilis, hydroxymandelate synthase (HmaS) from Amycolatopsis orientalis, (S)-mandelate dehydrogenase (SMDH) from Pseudomonas putida, the endogenous aminotransferase (AT) encoded by ilvE and L-glutamate dehydrogenase (GluDH) from E. coli. However, 10 mM L-Phe only afforded the synthesis of 7.21 ± 0.15 mM L-Phg. The accumulation of benzoylformic acid suggested that the transamination step might be rate-limiting. We next used leucine dehydrogenase (LeuDH) from Bacillus cereus to bypass the use of L-glutamate as amine donor, and 40 mM L-Phe gave 39.97 ± 3.84 mM (6.04 ± 0.58 g/L) L-Phg, reaching 99.9% conversion. In summary, this work demonstrated a concise four-step enzymatic cascade for the L-Phg synthesis from biobased L-Phe, with a potential for future industrial applications.

Quantitative Difference in Solubility of Diastereomeric (2H/1H)-Isotopomers

Many achiral organic compounds become chiral by an isotopic substitution of one of the enantiotopic moieties in their structures. Although spectroscopic methods can recognize the molecular chirality due to an isotopic substitution, the effects of isotopically chiral compounds in enantioselective reactions have remained unsolved because the small chirality arises only from the difference between the number of neutrons in the atomic nuclei. The difference between the diastereomeric isotopomers of reactive sources should be the key to these effects. However, the energy difference between them is difficult to calculate, even using present computational methods, and differences in physical properties have not yet been reported. Here, we demonstrate that the small energy difference between the diastereomeric isotopomers at the molecular level can be enhanced to appear as a solubility difference between the diastereomeric (²H/¹H) isotopomers of α-aminonitriles, synthesized from an isotopically chiral amine, achiral aldehyde, and HCN. This small, but measurable, difference induces the chiral (d/l) imbalance in the suspended α-aminonitrile; therefore, a second enhancement in the solid-state chirality proceeds to afford a highly stereoimproved aminonitrile (>99% selectivity) whose handedness arises completely from the excess enantiomer of isotopically chiral amine, even in a low enantiomeric excess and low deuterium-labeling ratio. Because α-aminonitriles can be hydrolyzed to chiral α-amino acids with the removal of an isotope-labeling moiety, the current sequence of reactions represents a highly enantioselective Strecker amino acid synthesis induced by the chiral hydrogen (²H/¹H) isotopomer. Thus, hydrogen isotopic chirality links directly with the homochirality of α-amino acids via a double enhancement of α-aminonitrile, the chiral intermediate of a proposed prebiotic mechanism.

Absolute asymmetric Strecker synthesis in a mixed aqueous medium: reliable access to enantioenriched α-aminonitrile

Without using chiral sources, the Strecker reaction of achiral hydrogen cyanide, p-tolualdehyde and benzhydrylamine gave enantioenriched l- or d-N-benzhydryl-α-(p-tolyl)glycine nitriles with up to >99% ee in a mixed solvent of water and methanol. Therefore, total spontaneous resolution of α-aminonitriles could occur through a prebiotic mechanism of α-amino acid synthesis. Moreover, it was demonstrated that the repetition of partial dissolution and crystallization of a suspended conglomerate of aminonitrile under solution-phase racemization could generate the enantiomeric imbalance to afford, in combination with the amplification of chirality, an enantioenriched product in every case. Among the 73 experiments that were carried out, d- and l-enriched isomers occurred 36 and 37 times, respectively. This stochastic behavior, under achiral or racemic starting conditions, meets the requirements of the spontaneous absolute asymmetric Strecker synthesis. The implications of the present results for the origin of chirality of α-amino acids are discussed.

The Strecker reaction coupled to Viedma ripening: a simple route to highly hindered enantiomerically pure amino acids

The Strecker reaction is broadly used for the preparation of α-amino acids. However, control of enantioselectivity remains challenging. We here couple the Strecker reaction to Viedma ripening for the absolute asymmetric synthesis of highly sterically hindered α-amino acids. As proof-of-principle, the enantiomerically pure α-amino acids tert-leucine and α-(1-adamantyl)glycine were obtained.

Photoredox-Catalyzed Cα-H Cyanation of Unactivated Secondary and Tertiary Aliphatic Amines: Late-Stage Functionalization and Mechanistic Studies

This paper describes the development and mechanistic studies of a general, high-yielding amine C_α-H cyanation protocol via photoredox catalysis. Inexpensive NaCN is employed as the cyanide source and air is the external oxidant, resulting in mild and highly functional group tolerant conditions. Notably, efficient C_α-H cyanations of secondary and tertiary aliphatic amines and of complex, biologically active compounds (drugs) can be performed using the established methodology. Mechanistic studies suggest that the carboxylic acid additive has three effects: formation of a stabilizing hemiaminal intermediate, prevention of catalyst decomposition by protonating the substrate, and modulation of fluorescence quenching of the photoexcited catalyst species.

The self-disproportionation of enantiomers (SDE): a menace or an opportunity?

Herein we report on the well-documented, yet not widely known, phenomenon of the self-disproportionation of enantiomers (SDE): the spontaneous fractionation of scalemic material into enantioenriched and -depleted fractions when any physicochemical process is applied.

Herein we report on the well-documented, yet not widely known, phenomenon of the self-disproportionation of enantiomers (SDE): the spontaneous fractionation of scalemic material into enantioenriched and -depleted fractions when any physicochemical process is applied. The SDE has implications ranging from the origins of prebiotic homochirality to unconventional enantiopurification methods, though the risks of altering the enantiomeric excess (ee) unintentionally, regrettably, remain greatly unappreciated. While recrystallization is well known as an SDE process, occurrences of the SDE in other processes are much less recognized, e.g. sublimation and even distillation. But the most common process that many workers seem to be completely ignorant of is SDE via chromatography and reports have included all manner of structures, all types of interactions, and all forms of chromatography, including GC. The SDE can be either a blessing – as a means to obtain enantiopure samples from scalemates – or a curse, as unwitting alteration of the ee leads to errors in the reporting of results and/or misinterpretation of the system under study. Thus the ramifications of the SDE are relevant to any area involving chirality – natural products, asymmetric synthesis, etc. Moreover, there is grave concern regarding errors in the literature, in addition to the possible occurrence of valid results which may have been overlooked and thus remain unreported, as well as the potential for the SDE to alter the ee, particularly via chromatography, and the following concepts will be conveyed: (1) the SDE occurs under totally achiral conditions of (a) precipitation, (b) centrifugation, (c) evaporation, (d) distillation, (e) crystallization, (f) sublimation, and (g) achiral chromatography (e.g. column, flash, MPLC, HPLC, SEC, GC, etc.). (2) The SDE cannot be controlled simply by experimental accuracy and ignorance of the SDE unavoidably leads to mistakes in the recorded and reported stereochemical outcome of enantioselective transformations. (3) The magnitude of the SDE (the difference between the extremes of enantioenrichment and -depletion) can be controlled and used to: (a) minimize mistakes in the recorded experimental values and (b) to develop unconventional and preparatively superior methods for enantiopurification. (4) The magnitude of the SDE cannot be predicted but can be expected for compounds possessing SDE-phoric groups or which have a general tendency for strong hydrogen or halogen bonds or dipole–dipole or aromatic π–π interactions. (5) An SDE test and the rigorous reporting and description of applied physicochemical processes should become part of standard experimental practice to prevent the erroneous reporting of the stereochemical outcome of enantioselective catalytic reactions and the chirooptical properties of scalemates. New directions in the study of the SDE, including halogen bonding-based interactions and novel, unconventional enantiopurification methods such as pseudo-SDE (chiral selector-assisted SDE resolution of racemates), are also reported.

Asymmetric Strecker Reaction Arising from the Molecular Orientation of an Achiral Imine at the Single-Crystal Face: Enantioenriched l- and d-Amino Acids

Strecker synthesis has long been considered one of the prebiotic reactions for the synthesis of α-amino acids. However, the correlation between the origin of chirality and highly enantioenriched α-amino acids through this method remains a puzzle. In the reaction, it may be conceivable that the handedness of amino acids has been determined at the formation stage of the chiral intermediate α-aminonitrile, that is, the enantioselective addition of hydrogen cyanide to an imine. Herein, an enantiotopic crystal surface of an achiral imine acted as an origin of chirality for the enantioselective formation of α-aminonitriles by the addition of HCN. In conjunction with the amplification of the enantiomeric excess and multiplication of enantioenriched aminonitrile, a large amount of near enantiopure α-amino acids, with the l- and d-handedness corresponding to the molecular orientation of the imine, is reported.