Asymmetric Synthesis of 1,2-Limonene Epoxides by Jacobsen Epoxidation

This study reported an asymmetric synthesis of 1,2-limonene epoxides. The absolute stereochemistry was controlled by a Jacobsen epoxidation of cis-1,2-limonene epoxide (with diastereomeric excess of 98%) and trans-1,2-limonene epoxide (with diastereomeric excess of 94%), which could be used as important raw materials for the preparation of related cannabinoid drugs.

A Theoretical Stereoselectivity Model of Photochemical Denitrogenations of Diazoalkanes Towards Strained 1,3-Dihalogenated Bicyclobutanes

<p>Photochemical reactions exemplify ‘green’ chemistry and are essential for synthesizing highly strained molecules with mild conditions with light. The light-promoted denitrogenation of bicyclic azoalkanes affords functionalized, stereoenriched bicyclo[1.1.0]butanes. We revisited these reactions with multireference calculations and non-adiabatic molecular dynamics (NAMD) simulations for a series of diazabicyclo[2.1.1]hexenes to predict the photophysics, reactivities, and stereoselectivities. We used complete active space self-consistent field (CASSCF) calculations with an (8,8) active space and ANO-S-VDZP basis set; the CASSCF energies were corrected with CASPT2(8,8)/ANO-S-VDZP. The excitation is consistently n→π* and ranges from 3.77–3.91 eV for the diazabicyclo[2.1.1]hexenes. Minimum energy path calculations showed stepwise C–N bond breaking and led to a minimum energy crossing point, which favors the stereochemical ‘double inversion’ bicyclobutane product. Wigner sampling of <b>1</b> provided Franck-Condon points for 692 NAMD trajectories. We identified competing complete stereoselective and stereochemical scrambling pathways. The stereoselective pathways feature concerted bicyclobutane inversion and N₂ extrusion. The stereochemical scrambling pathways involve N₂ extrusion followed by bicyclobutane planarization, leading to non-stereoselective outcomes. The predicted diastereomeric excess almost exactly match experiment (calc<i>.d.e.</i>=46% <i>vs.</i> exp<i>.d.e.</i>=47%). Our NAMD simulations with 672, 568, and 596 trajectories for <b>1-F</b>, <b>1-Cl</b>, and <b>1-Br</b> predicted diastereomeric excess (<i>d.e.</i>) of 94–97% for the double inversion products. Halogenation significantly perturbs the potential energy surface (PES) towards the retention products because of powerful hyperconjugative interactions. The n_C→σ^*_C–X,X = F, Cl, Br hyperconjugative interaction leads to a broadened shoulder region on the PES for double inversion.</p>

A Theoretical Stereoselectivity Model of Photochemical Denitrogenations of Diazoalkanes Towards Strained 1,3-Dihalogenated Bicyclobutanes

<p>Photochemical reactions exemplify ‘green’ chemistry and are essential for synthesizing highly strained molecules with mild conditions with light. The light-promoted denitrogenation of bicyclic azoalkanes affords functionalized, stereoenriched bicyclo[1.1.0]butanes. We revisited these reactions with multireference calculations and non-adiabatic molecular dynamics (NAMD) simulations for a series of diazabicyclo[2.1.1]hexenes to predict the photophysics, reactivities, and stereoselectivities. We used complete active space self-consistent field (CASSCF) calculations with an (8,8) active space and ANO-S-VDZP basis set; the CASSCF energies were corrected with CASPT2(8,8)/ANO-S-VDZP. The excitation is consistently n→π* and ranges from 3.77–3.91 eV for the diazabicyclo[2.1.1]hexenes. Minimum energy path calculations showed stepwise C–N bond breaking and led to a minimum energy crossing point, which favors the stereochemical ‘double inversion’ bicyclobutane product. Wigner sampling of <b>1</b> provided Franck-Condon points for 692 NAMD trajectories. We identified competing complete stereoselective and stereochemical scrambling pathways. The stereoselective pathways feature concerted bicyclobutane inversion and N₂ extrusion. The stereochemical scrambling pathways involve N₂ extrusion followed by bicyclobutane planarization, leading to non-stereoselective outcomes. The predicted diastereomeric excess almost exactly match experiment (calc<i>.d.e.</i>=46% <i>vs.</i> exp<i>.d.e.</i>=47%). Our NAMD simulations with 672, 568, and 596 trajectories for <b>1-F</b>, <b>1-Cl</b>, and <b>1-Br</b> predicted diastereomeric excess (<i>d.e.</i>) of 94–97% for the double inversion products. Halogenation significantly perturbs the potential energy surface (PES) towards the retention products because of powerful hyperconjugative interactions. The n_C→σ^*_C–X,X = F, Cl, Br hyperconjugative interaction leads to a broadened shoulder region on the PES for double inversion.</p>

Remote-controlled regio- and diastereodifferentiating photodimerization of a dynamic helical peptide-bound 2-substituted anthracene

Photodimerization of a right-handed 310-helical nonapeptide-bound 2-substituted anthracene produced the chiral head-to-head anti-photodimer with up to 97% diastereomeric excess.

Achiral Zeolites as Reaction Media for Chiral Photochemistry

Obtaining enantiomerically-enriched photoproducts from achiral reactants has been a long-sought goal. The various methods developed to achieve chiral induction in photoproducts during the last fifty years still suffer from a lack of predictability, generality, and simplicity. With the current emphasis on green chemistry, obtaining enantiomerically enriched products via photochemistry is a likely viable alternative for the future. Of the various approaches developed during the last three decades, the one pioneered in the author’s laboratory involved the use of commercially-available and inexpensive achiral zeolites as the media. This approach does not use any solvent for the reaction. Examples from these studies are highlighted in this article. Since no chiral zeolites were available, when the work was initiated in the author’s laboratory, commercially-available zeolites X and Y were modified with chiral inductors so that the reaction space becomes chiral. The results obtained established the value of chirally-modified, commercial zeolites as media for achieving chiral induction in photochemical reactions. A recent report of the synthesis of a chiral zeolite is likely to stimulate zeolite-based chiral photochemistry in synthesizing enantiomerically-pure organic molecules. The availability of chiral zeolites in future is likely to energize research in this area. Our earlier observations on this topic, we believe, would be valuable for progress of the field. Keeping this in mind, I have summarized the work carried out in our laboratory on chiral photochemistry on chirally-modified zeolites. This review does not include examples where high chiral induction has been obtained via a strategy that examines molecules appended with chiral auxiliary within achiral and chirally-modified zeolites. The latter approach yields products with diastereomeric excess >80%.

New Discoveries in Enantiomeric Separation of Racemic Tofisopam

Resolution process of tofisopam has been re-evaluated now based on our new investigations. Originally, it was carried out in the water-chloroform system, where the intermediate salt of high diastereomeric excess was described as (R)-TOF·(R,R)-DBTA·(H2O)3. Opposed to previous assumptions, we have actually found that a different solvate composition, (R)-TOF‐(R,R)-DBTA-CHCl3, forms with chloroform, in which molecules of CHCl3 are captured and held with different strengths. Moreover, resolution of TOF with (R,R)-DBTA is possible (and favourable) in water-free solvent and solvent mixture. However, presence of chloroform is essential, and thus, chloroform is also a suitable solvent alone. Among the tested solvents, toluene-chloroform mixture results in the highest resolution efficiency, while the highest enantiomeric purity was achieved when acetonitrile was in the system too. Resolution efficiency can be also increased by using the quasi-racemic resolving agent and thermodynamic control. Purification of enantiomeric mixtures was examined, and recrystallization of the diastereomeric salt was found to be the most efficient solution. Instructive behaviour of the complex enantiomer-conformer system of tofisopam is emphasized.

Plant-Mediated Biotransformations of S(+)- and R(–)-Carvones

The enzymatic system of vegetables is well known as an efficient biocatalyst in the stereoselective reduction of ketones. Therefore, we decided to use the comminuted material of several plants including five vegetables (Apium graveolens L., Beta vulgaris L., Daucus carota L., Petroselinum crispum L., and Solanum tuberosum L.) and three fruits (Malus pumila L. “Golden” and “Kortland” as well as Pyrus communis L. “Konferencja”) to obtain enantiomerically pure carveol, which is commercially unavailable. Unexpectedly, all of the used biocatalysts not only reduced the carbonyl group of (4R)-(–)-carvone and (4S)-(+)-carvone, but also reduced the double bond in the cyclohexene ring. The results revealed that (4R)-(–)-carvone was transformed into (1R, 4R)- and (1S, 4R)-dihydrocarvones, and (1R,2R,4R)-dihydrocarveol. Although the enzymatic system of the potato transformed the substrate almost completely, the %de was not the highest. Potato yielded 92%; however, when carrot was used as the biocatalyst, it was possible to obtain 17% of (1R, 4R)-(+)-dihydrocarvone with 100% diastereomeric excess. In turn, the (4S)-(+)-carvone was transformed, using the biocatalysts, into (1R, 4S)- and (1S, 4S)-dihydrocarvones and dihydrocarveols. Complete substrate conversion was observed in biotransformation when potato was used. In the experiments using apple, (1R, 4S)-dihydrocarvone with 100% diastereomeric excess was obtained. Using NMR spectroscopy, we confirmed both diastereoisomers of 4(R)-1,2-dihydrocarveols, which were unseparated in the GC condition. Finally, we proved the high usefulness of vegetables for the biotransformation of both enantiomers of carvone as well as dihydrocarvone.

Yeast-Mediated Stereoselective Reduction of α-Acetylbutyrolactone

α’-1’-Hydroxyethyl-γ-butyrolactone—a product of reduction of α-acetylbutyrolactone possesses two stereogenic centres and two reactive functionalities (an alcohol and an ester group). Additionally, this compound has a similar structure to γ-butyrolactone (GBL) which is psychoactive. In the present work, biotransformation using seven yeast strains was used to obtain anti stereoisomers of α’-1’-hydroxyethyl-γ-butyrolactone. The process was carried out in both growing and resting culture. The effect of media composition and organic solvent addition on stereoselectivity and effectiveness of biotransformation was also studied. After one day of transformation, optically pure (3R,1’R)-hydroxylactone was obtained by means of Yarrowia lipolytica P26A in YPG medium (yeast extract (1%), peptone (2%) and glucose (2%)). In turn, the use of resting cells culture of Candida viswanathi AM120 in the presence of 10% DES (deep eutectic solvent) allowed us to obtain a (3S,1’S)-enantiomer with de = 85% (diastereomeric excess) and ee 76% (enantiomeric excess).