Photoredox/palladium-cocatalyzed enantioselective alkylation of secondary benzyl carbonates with 4-alkyl-1,4-dihydropyridines

A photoredox/palladium-cocatalyzed enantioselective alkylation of racemic secondary carbonates with 4-alkyl-1,4-dihydropyridines under visible light irradiation has been developed. The present study provides a method for the preparation of optically active diarylalkanes from racemic diarylmethyl carbonates by a dynamic kinetic asymmetric transformation(DYKAT).This photoredox/palladium dual catalysis strategy expands the scope of the asymmetric Pd-catalyzed benzylic substitution reaction and serves as its potential alternative and complement.     

[N,N′‐Bis(2,6‐diisopropylphenyl)acenaphthene‐1,2‐diimine‐1κ2N,N′]heptacarbonyl‐1κC,2κ3C,3κ3C‐di‐μ3‐tellurido‐triiiron(II)(2 Fe—Fe)

The sterically hindered title complex, [Fe₃Te₂(C₃₆H₄₀N₂)(CO)₇], was obtained by substitution of two carbonyl groups in the [Fe₃(μ₃‐Te)₂(CO)₉] cluster by the bulky redox‐active N,N′‐bis(2,6‐diisopropylphenyl)acenaphthene‐1,2‐diimine (dpp‐BIAN) ligand. The asymmetric unit contains two molecules of the same geometry. The C=N bond lengths in dpp‐BIAN indicate a rather low level of electron transfer from the cluster core to the dpp‐BIAN ligand.     

Catalytic Asymmetric Construction of α‐Quaternary Cyclopentanones and Its Application to the Syntheses of (−)‐1,14‐Herbertenediol and (−)‐Aphanorphine

A novel and efficient strategy to build α‐benzylic quaternary cyclopentanones with excellent enantioselectivities (up to 96 % ee) and high yields (up to 99 % yield) has been developed, and its application demonstrated by the first catalytic asymmetric total synthesis of (?)‐1,14‐herbertenediol and the formal synthesis of (?)‐aphanorphine.     

Stereodivergent Quaternization of 2‐Alkyl‐2‐p‐tolylsulfinylacetonitriles: NMR Spectroscopic Evidence of Planar and Pyramidal Benzylic Carbanions

Enantiomerically enriched α,α‐disubstituted phenylacetonitriles have been readily prepared by stereoselective quaternization of 2‐alkyl‐2‐[2‐(p‐tolylsulfinyl)phenyl]acetonitriles with different alkylating electrophiles in the presence of bases. The use of potassium hexamethyldisilazane (KHMDS)/[18]crown‐6 ether and NHMDS with alkyl halides afforded S,S_S and R,S_S diastereoisomers, respectively, in high enantiomeric purities, thus providing stereodivergent processes for synthesizing both isomers. The dependence of the stereochemical course of the reactions on the experimental conditions (mainly on the counterion) has been rationalized by assuming a planar or pyramidal structure for the benzylic carbanions. This hypothesis has been supported by NMR spectroscopic studies, which permit one to assign a chelated pyramidal structure to the sodium benzylic carbanions and an almost planar naked carbanionic structure to the potassium benzylic carbanions generated in the presence of [18]crown‐6 ether.     

Conformational study of the 3,6‐dihydro‐2H‐1,4‐oxazin‐2‐one fragment in 8‐tert‐butyl‐7‐methoxy‐8‐methyl‐9‐oxa‐6‐azaspiro[4.5]decane‐2,10‐dione stereoisomers

Unnatural cyclic α‐amino acids play an important role in the search for biologically active compounds and macromolecules. Enantiomers of natural amino acids with a d configuration are not naturally encoded, but can be chemically synthesized. The crystal structures of two enantiomers obtained by a method of stereoselective synthesis, namely (5R ,8S )‐8‐tert‐butyl‐7‐methoxy‐8‐methyl‐9‐oxa‐6‐azaspiro[4.5]decane‐2,10‐dione, (1), and (5S ,8R )‐8‐tert‐butyl‐7‐methoxy‐8‐methyl‐9‐oxa‐6‐azaspiro[4.5]decane‐2,10‐dione, (2), both C₁₄H₂₁NO₄, were determined by X‐ray diffraction. Both enantiomers crystallize isostructurally in the space group P 2₁, with one molecule in the asymmetric unit and with the same packing motif. The crystal structures are stabilized by C—H…O hydrogen bonds, resulting in the formation of chains along the [100] and [010] directions. The conformation of the 3,6‐dihydro‐2H‐1,4‐oxazin‐2‐one fragment was compared with other crystal structures possessing this heterocyclic moiety. The comparison showed that the title compounds are not exceptional among structures containing the 3,6‐dihydro‐2H‐1,4‐oxazin‐2‐one fragment. The planar moiety was more frequently observed in derivatives in which this fragment was not condensed with other rings.     

Asymmetric Photochemical Synthesis of Chiral 5‐(R)‐(l)‐Menthyloxy‐4‐Cycloaminobutyrolactones

Through photocatalysed regiospecific and stereoselective additions of cycloamines to 5‐(R)‐(l)‐menthyloxy‐2 (5H)‐furanone (3), chiral 5‐(R)‐(l)‐menthyloxy‐4‐cycloaminobutyrolactones were synthesized. In the new asymmetric photoaddition of compound 3, the N‐methyl cyclic amines (4) gave novel chiral C? C photoadducts (5) in 24–50% isolated yields with d. e. ≥ 98%. However, the secondary cyclic amines (6) afforded optically active N? C photoadducts (7) in 34–58% isolated yields with d. e. ≥ 98% under the same condition. All the synthesized optically active compounds were identified on the basis of their analytical data and spectroscopic data, such as [α]₅₈₉²⁰, IR, ¹H NMR, ¹³C NMR, MS and elementary analysis. The photosynthesis of chiral butyrolactones and its mechanism were discussed in detail.     

(2S)‐2‐[1‐(2,4‐Di­fluoro‐5‐iodo­phenyl)‐2‐deoxy‐β‐d‐ribo­furanos‐5‐yloxy]‐8‐methyl‐4H‐1,3,2‐benzodioxa­phosphole 2‐oxide

The title compound, C₁₉H₁₈F₂IO₆P, prepared as a potential antiviral and anticancer agent from 3‐methyl­salicyl­chloro­phosphane and 1‐(2,4‐di­fluoro‐5‐iodo­phenyl)‐2‐deoxy‐β‐d ‐ribo­furan­ose, is one of a 1:1 mixture of two diastereomers. The diastereomers differ in their configuration, S or R, at the asymmetric phosphorus center. X‐Ray crystallographic analysis of the title compound has determined the absolute configuration at the asymmetric P center to be S.     

Reversal of the Stereochemical Course of 1‐Methyl‐1H‐indole Addition to Cinnamaldehyde with cis‐5‐Benzyl‐(2‐fluoromethyl)‐2,3‐dimethylimidazolidin‐4‐ones as Catalysts – a Puzzling ‘Fluorine Effect’

Replacement of the cis‐Me group by CH₂F in the imidazolidinone organocatalyst specified in the title (so‐called McMillan generation‐I catalyst) leads to reversal of the product configuration in the title reaction. The topicity reversal in the nucleophilic addition step must arise either from cis‐addition with respect to the benzylic substituent of an (E)‐iminium ion intermediate or from trans‐addition to the corresponding (Z)‐iminium ion. Mechanistic investigations have not provided evidence for either one of these two possibilities, so far.     

Tandem Insertion–[1,3]‐Rearrangement: Highly Enantioselective Construction of α‐Aminoketones

An enantioselective synthesis of α‐aminoketone derivatives were readily available through a tandem insertion–[1,3] O‐to‐C rearrangement reaction. The rhodium salt and chiral N,N′‐dioxide‐indium(III) complex make up relay catalysis, which enables the O?H insertion of benzylic alcohols to N‐sulfonyl‐1,2,3‐triazoles, and asymmetric [1,3]‐rearrangement of amino enol ether intermediates, subsequently. Preliminary mechanistic studies suggested that the [1,3] O‐to‐C rearrangement step proceeded through an ion pair pathway.     

Methyl (±)‐1‐ethyl‐2‐hydroxy‐4‐(4‐methoxybenzoyl)‐5‐(4‐methoxyphenyl)‐3‐oxo‐2,3‐di­hydro‐1H‐pyrrole‐2‐acetate

The title compound, C₂₄H₂₅NO₇, is a racemic mixture of 2,3‐di­hydro‐1H‐pyrrol‐3‐ones. It crystallizes in the triclinic system, space group P1, with Z = 2. The asymmetric unit contains two enantiomorphic mol­ecules and the structure is stabilized by hydrogen‐bond contacts.     

Synthesis of Derivatives of the Optically Active β‐Amino Acids from (+)‐Car‐2‐ene

The reaction of (+)‐car‐2‐ene ( 4 ) with chlorosulfonyl isocyanate (=sulfuryl chloride isocyanate; ClSO₂NCO) led to the tricyclic lactams 6 and 8 corresponding to the initial formation both of the tertiary carbenium and α‐cyclopropylcarbenium ions (Scheme 2). A number of optically active derivatives of β‐amino acids which are promising compounds for further use in asymmetric synthesis were synthesized from the lactams (see 16, 17 , and 19 – 21 in Scheme 3).     

Packing considerations of 10‐oxa‐9‐boraphenanthrene derivatives

The crystal structures of three products of the reaction of 2‐phenylphenol and BCl₃ have been determined. The structures show intriguing packing patterns and an interesting case of pseudosymmetry. In addition, one of the two polymorphs has a primitive monoclinic crystal system, but it is twinned and emulates an orthorhombic C‐centred structure. Tris(biphenyl‐2‐yl) borate, C₃₆H₂₇BO₃, ( III ), crystallizes with only one molecule in the asymmetric unit. The dihedral angles between the planes of the aromatic rings in the biphenyl moieties are 50.47 (13), 44.95 (13) and 42.60 (13)°. The boron centre is in a trigonal planar coordination with two of the biphenyl residues on one side of the BO₃ plane and the remaining biphenyl residue on the other side. One polymorph of 10‐oxa‐9‐boraphenanthren‐9‐ol, C₁₂H₉BO₂, ( V a ), crystallizes with two almost identical molecules (r.m.s. deviation of all non‐H atoms = 0.039 Å) in the asymmetric unit. All non‐H atoms lie in a common plane (r.m.s. deviation = 0.015 Å for both molecules in the asymmetric unit). The two molecules in the asymmetric unit are connected into dimers via O—H...O hydrogen bonds. A second polymorph of 10‐oxa‐9‐boraphenanthren‐9‐ol, ( V b ), crystallizes as a pseudo‐merohedral twin with two almost identical molecules (r.m.s. deviation of all non‐H atoms = 0.035 Å) in the asymmetric unit. All non‐H atoms lie in a common plane (r.m.s. deviation = 0.012 Å for molecule 1 and 0.014 Å for molecule A). Each of the two molecules in the asymmetric unit is connected into a centrosymmetric dimer via O—H...O hydrogen bonds. The main difference between the two polymorphic structures is that in ( V a ) the two molecules in the asymmetric unit are hydrogen bonded to each other, whereas in ( V b ), each molecule in the asymmetric unit forms a hydrogen‐bonded dimer with its centrosymmetric equivalent. 9‐[(Biphenyl‐2‐yl)oxy]‐10‐oxa‐9‐boraphenanthrene, C₂₄H₁₇BO₂, ( VI ), crystallizes with four molecules in the asymmetric unit. The main differences between them are the dihedral angles between the ring planes. Apart from the biphenyl moiety, all non‐H atoms lie in a common plane (r.m.s. deviations = 0.026, 0.0231, 0.019 and 0.033 Å for molecules 1, A, B and C, respectively). This structure shows pseudosymmetry; molecules 1 and A, as well as molecules B and C, are related by a pseudo‐translation of about in the direction of the b axis. Molecules 1 and B, as well as molecules A and C, are related by a pseudo‐inversion centre at ,,. Neither between molecules 1 and C nor between molecules A and B can pseudosymmetry be found.     

Comparison of racemic epi‐inosose and (−)‐epi‐inosose

The conversion of myo‐inositol to epi‐inositol can be achieved by the hydride reduction of an intermediate epi‐inosose derived from myo‐inositol. (−)‐epi‐Inosose, (I), crystallized in the monoclinic space group P2₁, with two independent molecules in the asymmetric unit [Hosomi et al. (2000). Acta Cryst. C 56 , e584–e585]. On the other hand, (2RS,3SR,5SR,6SR)‐epi‐inosose, C₆H₁₀O₆, (II), crystallized in the orthorhombic space group Pca2₁. Interestingly, the conformation of the molecules in the two structures is nearly the same, the only difference being the orientation of the C‐3 and C‐4 hydroxy H atoms. As a result, the molecular organization achieved mainly through strong O—H...O hydrogen bonding in the racemic and homochiral lattices is similar. The compound also follows Wallach's rule, in that the racemic crystals are denser than the optically active form.     

Enantiomerically pure (1S,5R) and racemic 3‐(1‐benzothiophen‐2‐yl)‐8‐azoniabicyclo[3.2.1]oct‐2‐ene acetate

The title compound, C₁₅H₁₆NS⁺·C₂H₃O₂⁻, has been crystallized as both a pure enantiomer (1S,5R) and a racemate. The racemate crystallizes in the space group Cc, with molecules of opposite handedness related to each other by the action of the c‐glide. The enantiomer is essentially isostructural with the racemate, except that the glide symmetry is violated by interchange of CH and CH₂ groups within the seven‐membered ring. The space‐group symmetry is reduced to P1 with two molecules in the asymmetric unit. The enantiomer structure shows disorder of the thiophene ring for one of the molecules in the asymmetric unit. The major component of the disorder has the thiophene ring in the same position as in the racemate, but generates a higher‐energy molecular conformation. The minor disorder component has different intermolecular interactions but retains a more stable molecular conformation.     

Preparation and structural analysis of (±)‐cis‐ethyl 2‐sulfanylidenedecahydro‐1,6‐naphthyridine‐6‐carboxylate and (±)‐trans‐ethyl 2‐oxooctahydro‐1H‐pyrrolo[3,2‐c]pyridine‐5‐carboxylate

Bicycle ring closure on a mixture of (4aS,8aR)‐ and (4aR,8aS)‐ethyl 2‐oxodecahydro‐1,6‐naphthyridine‐6‐carboxylate, followed by conversion of the separated cis and trans isomers to the corresponding thioamide derivatives, gave (4aSR,8aRS)‐ethyl 2‐sulfanylidenedecahydro‐1,6‐naphthyridine‐6‐carboxylate, C₁₁H₁₈N₂O₂S. Structural analysis of this thioamide revealed a structure with two crystallographically independent conformers per asymmetric unit (Z′ = 2). The reciprocal bicycle ring closure on (3aRS,7aRS)‐ethyl 2‐oxooctahydro‐1H‐pyrrolo[3,2‐c]pyridine‐5‐carboxylate, C₁₀H₁₆N₂O₃, was also accomplished in good overall yield. Here the five‐membered ring is disordered over two positions, so that both enantiomers are represented in the asymmetric unit. The compounds act as key intermediates towards the synthesis of potential new polycyclic medicinal chemical structures.     

Asymmetric Hydroformylation of 4‐Vinyl‐1,3‐dioxolan‐2‐one

A chiral cyclic carbonate, 4‐vinyl‐1,3‐dioxolan‐2‐one was used as racemic substrate in asymmetric hydroformylation. The catalysts were formed in situ from “pre‐formed” PtCl₂(diphosphine) and tin(II) chloride. (2S,4S )‐2,4‐Bis(diphenylphosphinopentane ((S,S )‐BDPP)), (S,S )‐2,3‐O‐izopropylidine‐2,3‐dihydroxy‐1,4‐bis(diphenylphosphino)butane ((S,S )‐DIOP)), and (R )‐2,2′‐bis(diphenylphosphino)‐1,1′‐binaphthyl ((R )‐BINAP)) were used as optically active diphosphine ligands. The platinum‐containing catalytic systems provided surprisingly high activity. The hydroformylation selectivities of up to 97% were accompanied by perfect regioselectivity towards the dioxolane‐based linear aldehyde. The enantiomeric composition of all components in the reaction mixture was determined and followed throughout the reaction. The unreacted 4‐vinyl‐1,3‐dioxolan‐2‐one was recovered in optically active form. The kinetic resolution was rationalized using the enantiomeric composition of the substrate and the products.     

Tandem Benzylic Oxidation/Dihydroxylation of α‐Vinyl‐ and α‐Alkenylbenzyl Alcohols

A de novo tandem benzylic oxidative dihydroxylation of α‐vinyl‐ and α‐alkenylbenzyl alcohols has been developed to give α,β‐dihydroxypropiophenones (=2,3‐dihydroxy‐1‐phenylpropan‐1‐ones) and α,β‐dihydroxyalkyl phenones. This method was shown to be substrate‐selective and specific for the oxidation of benzylic alcohols.     

A Highly cis‐Selective and Enantioselective Metal‐Free Hydrogenation of 2,3‐Disubstituted Quinoxalines

A wide range of 2,3‐disubstituted quinoxalines have been successfully hydrogenated with H₂ using borane catalysts to produce the desired tetrahydroquinoxalines in 80–99 % yields with excellent cis selectivity. Significantly, the asymmetric reaction employing chiral borane catalysts generated by the in situ hydroboration of chiral dienes with HB(C₆F₅)₂ under mild reaction conditions has also been achieved with up to 96 % ee, and represents the first catalytic asymmetric system to furnish optically active cis‐2,3‐disubstituted 1,2,3,4‐tetrahydroquinoxalines.     

A pair of epimeric 13‐hydroxy‐2,3,6,7‐tetra­methoxy‐8b,11,12,12a,13,13a‐hexa­hydro‐9H‐9a‐aza­cyclo­penta­[b]tri­phenyl­ene‐10‐ones

The structures of two compounds which are intermediates in the synthesis of phenanthroindolizidine alkaloids have been determined. (8bS,13aS,14R,14aR)‐8b,9,11,12,13,13a,14,14a‐Octa­hydro‐14‐hydroxy‐2,3,6,7‐tetra­methoxy­dibenzo­[f,h]pyrrolo[1,2‐b]­isoquinolin‐11‐one acetone solvate, C₂₄H₂₇NO₆·C₃H₆O, (II), crystallizes in a chiral space group with one solvent mol­ecule (acetone) present in the asymmetric unit. On the other hand, (8bS,13aS,14S,14aR)‐8b,9,11,12,13,13a,14,14a‐octa­hydro‐14‐hydroxy‐2,3,6,7‐tetra­methoxy­dibenzo­[f,h]pyrrolo[1,2‐b]­isoquinolin‐11‐one, C₂₄H₂₇NO₆, (III), crystallizes in a centrosymmetric space group with two mol­ecules in the asymmetric unit and with no solvent present. The two mol­ecules in the asymmetric unit of (III) are structurally the same. Compounds (II) and (III) are epimers at the C atom carrying the OH group; otherwise they are very similar in structure.     

New Chiral N,S—Ligands with Thiophenyl at Benzylic Position.Palladium（Ⅱ）—catalyzed Enantioselective Allylic Alkylation

New chiral N,S-ligands with oxazoline and thiophenyl sub-stituents at benzene ring and benzylic postition have been pre-pared and applied in palladium-catalyzed asymmetric allylic alkylation reaction to provide the product with high yield and entantioselectivity (82%-93%)ee.