Ti-mediated addition of diethylzinc to benzaldehyde. The effect of chiral additives

In the presence of a chiral tridentate bissulfonamide, the titanium-mediated addition of diethylzinc to benzaldehyde gave alkylated products ranging from the (R)-enantiomer, formed with an e.e. of 26%, to the (S)-enantiomer, formed in 72% e.e. The enantioselectivity was also affected by the presence of additional chiral mono- and bidentate ligands, with the reactions proceeding via complexes containing the chiral sulfonamide and the additive. The addition of (1R,2R)-1,2-diphenyl-1,2-ethanediamine and (1S,2S)-1,2-diphenyl-1,2-ethanediamine gave the (S)-product with e.e. of 49% and the (R)-product with 16% e.e., respectively, whereas without additives the (R)-product was obtained in 26% e.e. In the presence of (1R,2R)-1,2-diphenyl-1,2-ethanediamine only (i.e. without the chiral sulfonamide), the (S)-product formed with a 3% e.e.     

Enantioselective addition of diethylzinc to aromatic aldehydes catalyzed by C2-symmetric chiral diols

Optically pure C₂-symmetric diols have been synthesized with moderate yields in a straightforward manner, and are used as catalysts in the enantioselective alkylation of aromatic aldehydes with diethylzinc. The addition of diethylzinc to benzaldehyde and sterically hindered 1-naphthaldehyde was achieved with excellent enantioselectivities (97–99% ee) under catalysis with (1R,2R)-1,2-bis(3,5-dibromophenyl)-ethane-1,2-diol and (1R,2R)-1,2-bis(3,5-diphenylphenyl)-ethane-1,2-diol.     

Chiral relay in NHC-mediated asymmetric β-lactam synthesis II; asymmetry from NHCs derived from acyclic 1,2-diamines

The synthesis of a range of imidazolinium salts derived from acyclic 1,2-diamines, and an evaluation of the reactivity and asymmetric induction of the corresponding NHCs as catalysts for the asymmetric synthesis of β-lactams, is reported. An N-methyl-substituted NHC derived from (1R,2R)-1,2-diphenylethanediamine shows optimal reactivity and enantioselectivity in this series, in contrast to that observed with NHCs derived from (1R,2R)-cyclohexane-1,2-diamine.     

Trichosporon cutaneum-promoted deracemization of (±)-2-hydroxindan-1-one: a mechanistic study

A mechanistic study of the deracemization of (±)-2-hydroxy-1-indanone mediated by the yeast Trichosporon cutaneum to afford pure (1S,2R)-1,2-indandiol is reported. The key aspect of the study was the use of pure (R)- and (S)-2-hydroxy-1-indanone enantiomers to ensure reliable conclusions. Experiments in the absence of yeast cells or using dead cells disclosed that the pure enantiomers were not racemized, which suggest that the whole dynamic kinetic resolution process is enzymatic in character. When living yeast cells were used the (R)-substrate was smoothly converted to (1S,2R)-1,2-indandiol, whilst the (S)-2-hydroxy-1-indanone was converted to the same diol through a more complex fashion, which requires a more lengthy oxidation–reduction pathway having the 1,2-indanedione as an achiral intermediate. An unexpected observation was that 1,2-indanone acts as a moderate inhibitor of the reductive enzymes acting in the conversion of (R)-2-hydroxy-1-indanone to (1S,2R)-1,2-indandiol.     

Enantiomerically pure piperazines via NaBH4/I2 reduction of cyclic amides

Enantiomerically pure (3S,7R,8aS)-3-phenyloctahydropyrrolo[1,2-a]pyrazine-7-ol, (3S,7R,8aS)-3-methyl octahydropyrrolo[1,2-a]pyrazine-7-ol, (3S,7R,8aS)-3-isopropyloctahydropyrrolo[1,2-a]pyrazine-7-ol and (3S,7R,8aS)-3-isobutyloctahydropyrrolo[1,2-a]pyrazine-7-ol 16d were synthesized via preparation of the corresponding cyclic amides from enantiomerically pure l-proline and hydroxyproline derivatives followed by reduction using sodium borohydride-iodine.     

Stereoselective carbon–carbon bond forming reactions of chiral cyclopent-2-enone and cyclopentene-1-methanol,both spiro-connecting a 1,2:5,6-di-O-isopropylidene-α-d-glucofuranosyl ring

The conjugate additions of a variety of organocopper reagents or dimethyl malonate anion to a spirocyclic cyclopent-2-enone connecting a 1,2:5,6-di-O-isopropylidene-α-d-glucofuranosyl ring as a constituent of the spiro structure, namely (1S,3R,4R,5R)-3,4-(isopropylidenedioxy)-1-[(1R)-1,2-(isopropylidenedioxy)ethyl]-2-oxa-spiro[4.4]non-6-en-8-one, proceeded stereoselectively in some cases affording a variety of β-functionalized cyclopentanone derivatives. The thermal treatment of (1S,3R,4R,5R)-7-(hydroxymethyl)-3,4-(isopropylidenedioxy)-1-[(1R)-1,2-(isopropylidenedioxy)ethyl]-2-oxaspiro[4.4]non-6-ene, another d-glucose-derived spirocyclic substrate, with triethyl orthoacetate in the presence of a catalytic amount of acid afforded the Claisen rearrangement product with a high level of diastereoselectivity.     

Enantiopure vic-amino alcohols and vic-diamines from (1R,2S)-1,2-dihydroxy-1,2-dihydronaphthalene

(1S,2S)-2-Hydroxy-1-amino-1,2,3,4-tetrahydronaphthalene, (1R,2R)-1-hydroxy-2-amino-1,2,3,4-tetrahydronaphthalene, (1S,2R)-1,2-diamino-1,2,3,4-tetrahydronaphthalene, (2R,3S)-2-hydroxy-3-amino-1,2,3,4-tetrahydronaphthalene and (2S,3S)-2,3-diammino-1,2,3,4-tetrahydronaphthalene have been synthesized from (1R,2S)-1,2-dihydroxy-1,2-dihydronaphthalene. The latter was obtained, using a protocol reported in a previous paper, from naphthalene using an Escherichia coli recombinant strain containing the naphthalene dioxygenase gene cloned from Pseudomonas fluorescens N3.     

Stereoselective synthesis of novel benzoins catalysed by benzaldehyde lyase in a gel-stabilised two-phase system

Asymmetric benzoin condensation was performed using recombinant benzaldehyde lyase (BAL) from Pseudomonas fluorescens Biovar I. To enable the conversion of hydrophobic substrates, the enzyme was entrapped in polyvinyl alcohol and suspended in hexane. Compared to the reported application of the biocatalyst in an aqueous phase containing 20% DMSO, the productivity of the resulting gel-stabilised two-phase system was 3-fold better. The entrapment process had an efficiency of >90%, no enzyme or cofactor was lost during reaction or storage. The entrapped enzyme was stable in hexane for 1 week at 4 °C and more than 1 month at −20 °C. Without preceding optimisation the novel benzoins (R)-1,2-di(3-furanyl)-2-hydroxyethanone, (R)-2-hydroxy-1,2-di(3-thienyl) ethanone, (R)-1,2-di(4-ethoxyphenyl)-2-hydroxyethanone, (R)-1,2-di(3-ethoxyphenyl)-2-hydroxyethanone, (R)-2-hydroxy-1,2-di(3-tolyl)ethanone, and (R)-1,2-di(benzofuran-2-yl)-2-hydroxyethanone were prepared with yields up to 31.8% and enantiomeric excess >99%.     

New non-C2-symmetric phosphine-phosphonites as ligands in asymmetric metal catalysis

Starting from 1,2-dibromobenzene, the synthesis of N,N,N′,N′-tetraethyl-[2-(diphenylphosphino)phenyl]phosphonous acid tetraamide is possible in two simple steps. This key compound reacts with a variety of chiral diols such as (R)- and (S)-binaphthol, 1,2:5,6-diisopropylidene-d-mannitol or (1R,2R)-1,2-diphenyl-1,2-ethane diol to form the corresponding non-C₂-symmetric phosphine-phosphonite compounds. These ligands react with Rh(COD)₂BF₄ to form bidentate Rh-complexes which serve as catalysts in the asymmetric hydrogenation of dimethyl itaconate with ee values of up to 88%.     

Acid-catalyzed enzymatic hydrolysis of 1-methylcyclohexene oxide

Limonene-1,2-epoxide hydrolase from Rhodococcus erythropolis DCL14, an enzyme involved in the limonene metabolism of this microorganism, catalyzes the enantioselective hydrolysis of 1-methylcyclohexene oxide. (1R,2S)-1-Methylcyclohexene oxide was the preferred substrate and it was mainly hydrolyzed to (1S,2S)-1-methylcyclohexane-1,2-diol, while (1S,2R)-1-methylcyclohexene oxide was converted more slowly and mainly yielded (1R,2R)-1-methylcyclohexane-1,2-diol. The reaction proceeded with a high regioselectivity (C1:C2, 85:15). H₂¹⁸O-labelling experiments confirmed that the nucleophile was mainly incorporated at the most substituted carbon atom, suggesting that limonene-1,2-epoxide hydrolase uses an acid-catalyzed enzyme mechanism.     

Two chiral mononuclear and one-dimensional cadmium(II) complexes constructed by (1R,2R)-N1,N2-bis(pyridinylmethyl)cyclohexane-1,2-diamine derivatives: Effect of positional isomerism

Reactions of (1R,2R)-N¹,N²-bis(pyridinylmethyl)cyclohexane-1,2-diamine derivatives, (1R,2R)-2-bpcd and (1R,2R)-3-bpcd [(1R,2R)-2-bpcd = (1R,2R)-N¹,N²-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine, (1R,2R)-3-bpcd = (1R,2R)-N¹,N²-bis(pyridin-3-ylmethyl)cyclohexane-1,2-diamine], with CdI₂ in an analogous way led to the formation of a chiral discrete mononuclear complex and a chiral one-dimensional polymeric chain, respectively, which may be attributed to the positional isomerism of the ligands. The chiral organic ligands and complexes display luminescent properties indicating that they may have a potential application as optical materials. Powder second-harmonic generation (SHG) efficiency measurement shows that the SHG efficiency of the complexes is approximately 0.3 and 0.45 times that of KDP, respectively.     

Synthesis of N,N′-disubstituted cyclic 1,2-diamines derived from (1R,2R)-1,2-diaminocyclohexane

The synthesis of N,N′-unsymmetrically tetrasubstituted cyclic 1,2-diamines derived from (1R,2R)-diaminocyclohexane is reported. We comment on the structural nature of these cyclic 1,2-diamines and discuss their characteristic features.     

Racemate resolution via diastereomeric helicates in hydrogen-bonded co-crystals: the case of BINOL–diamine complexes

The structures of a series of co-crystals formed by enantiomers of BINOL (2,2′-dihydroxy-1,1′-binaphthalene) with (R,R)-1,2-diaminocyclohexane (DACH) and (R,R)-1,2-diamino-1,2-diphenylethane have been determined by X-ray diffraction to investigate the factors responsible for diastereoselective co-crystallization, the process responsible for the highly effective resolution of rac-BINOL by DACH. The absolute configurations of the molecules and directional preferences of the hydrogen bonds in the supramolecular aggregates have been analyzed. The supramolecular aggregation of hydrogen-bonded BINOL and diamines shows considerable distortions from the helical shape, which indicates that cohesion forces other than hydrogen bonds play a significant role in the diastereoselective resolution.     

New analogues of fosfomycin—synthesis of diethyl (1R,2R)- and (1S,2R)-1,2-epoxy-3-hydroxypropylphosphonates

The trans-configured fosfomycin analogue, diethyl (1R,2R)-1,2-epoxy-3-hydroxypropylphosphonate, was synthesised via the intramolecular Williamson reaction from 3-O-protected (trityl or TBDMS) or even unprotected diethyl (1S,2R)-2,3-dihydroxy-1-mesyloxypropylphosphonate, which was obtained from the known diethyl (1S,2R)-2,3-O-cyclohexylidene-1,2,3-trihydroxypropylphosphonate. On the other hand, the cis-analogue, diethyl (1S,2R)-1,2-epoxy-3-hydroxypropylphosphonate, could only be prepared from diethyl (1R,2R)-2-hydroxy-1-mesyloxy-3-trityloxypropylphoshonate.     

Enantioselective synthesis of (−)-(1R,2R)-1,2-dihydrochrysene-1,2-diol

A general chiral building block containing the 1R,2R-trans-diol moiety was constructed utilizing the stereoselective Shi-epoxidation reaction on a tetralone scaffold assembled by a Negishi cross-coupling on N,N-diethylbenzamide. Further elaboration of this chiral building block into polycyclic aromatic compounds was demonstrated with the total synthesis of the precursor for the most carcinogenic metabolite of chrysene, (−)-(1R,2R)-1,2-dihydrochrysene-1,2-diol in 87% ee.     

Synthesis of Ni(II) complexes with chiral derivatives of cyclohexane-1,2-diamine,bycyclo[2.2.2]octane-2,3-diamine,and 1,2-diphenylethane-1,2-diamine

Chiral ligands—derivatives of (1R,2R)-cyclohexane-1,2-diamine, (1R,2R)-diphenylethane-1,2-diamine, and (2S,3S)-bicyclo[2.2.2]octane-2,3-diamine—and octahedral Ni(II) complexes on their basis have been synthesized.     

Chiral diphenylselenophosphoramides: a new class of chiral ligands for the titanium(IV) alkoxide-promoted addition of diethylzinc to aldehydes

Chiral C₂-symmetric diphenylselenophosphoramides 1 and 2 were prepared from the reaction of diphenylselenophosphinic chloride with (1R,2R)-(−)-1,2-diaminocyclohexane and (1R,2R)-(+)-1,2-diphenylethylenediamine, respectively, in high yields. Another novel chiral ligand 3 was prepared from the reaction of diphenylselenophosphinic chloride with (R)-(+)-1,1′-binaphthyl-2,2′-diamine using butyllithium as the base. The ligands were used as catalytic chiral ligands in the titanium(IV) alkoxide-promoted enantioselective addition reaction of diethylzinc to aldehydes.     

Chiral diphenylthiophosphoramides: a new class of chiral ligands for the silver(I)-promoted enantioselective allylation of aldehydes

Chiral C₂-symmetric diphenylthiophosphoramides 1 and 2 were prepared in high yields from the reaction of diphenylthiophosphinic chloride with (1R,2R)-(−)-1,2-diaminocyclohexane and (1R,2R)-(+)-1,2-diphenylethylenediamine, respectively. Another novel chiral ligand 4 was prepared from reaction of diphenylthiophosphinic chloride with (R)-(+)-1,1′-binaphthyl-2,2′-diamine using butyllithium as a base. They were used as catalytic chiral ligands in the silver(I)-promoted enantioselective allylation reaction of aldehydes with allyltributyltin.     

Studies towards the synthesis of (1R,2S)- and (1S,2S)-1,2-epoxy-3-hydroxypropylphosphonates and (1S,2S)- and (1R,2S)-2,3-epoxy-1-hydroxypropylphosphonates

The trans-configured fosfomycin analogue, diethyl (1S,2S)-1,2-epoxy-3-hydroxypropylphosphonate, was synthesised by the intramolecular Williamson reaction of diethyl (1S,2R)-1,3-dihydroxy-2-mesyloxypropylphosphonate. The cis-analogue was obtained as O-ethyl or O,O-diethyl (1R,2S)-1,2-epoxy-3-hydroxypropylphosphonates, when (1R,2R)-1,3-dihydroxy-2-mesyloxypropylphosphonate or its 3-O-trityl derivative were used as starting materials, respectively. The intramolecular Williamson cyclisations of diethyl (1S,2R)- and (1R,2S)-1-benzyloxy-3-hydroxy-2-mesyloxypropylphosphonates led to diethyl (1S,2S)- and (1R,2S)-2,3-epoxy-1-benzyloxypropylphosphonates, respectively, with the concomitant formation of diethyl (E)-1-benzyloxy-3-hydroxyprop-1-en-1-phosphonate. From diethyl (1S,2S)- and (1R,2S)-2,3-epoxy-1-benzyloxypropylphosphonates, enantiomerically pure diethyl (1S,2S)- and (1R,2S)-1,2-dihydroxypropylphosphonates were obtained by catalytic hydrogenation, while diethyl (1S,2S)- and (1R,2S)-3-acetamido-1,2-dihydroxypropylphosphonates were produced after epoxide ring opening with dibenzylamine, acetylation and hydrogenolysis.