Studies on the synthesis of orthogonally protected azalanthionines,and of routes towards β-methyl azalanthionines,by ring opening of N-activated aziridine-2-carboxylates

Orthogonally protected azalanthionines were successfully synthesised by the ring-opening of N-activated aziridine-2-carboxylates with protected diaminopropanoic acids (DAPs). The required DAPs were also prepared by ring-opening of N-activated aziridine-2-carboxylates with para-methoxybenzylamine, but it was found that the choice of aziridine protecting groups dictated both the success of the reaction as well as the regioselectivity of the isolated products. Attempts to extend the methodology to the preparation of the more sterically demanding β-methyl azalanthionines have, so far, been unsuccessful.     

Synthesis of the four enantiomers of diethyl 1,2-di(N-Boc-amino)propylphosphonates

A simple and efficient synthetic strategy to all four enantiomerically pure diethyl 1,2-di(N-Boc-amino)propylphosphonates has been elaborated starting from the corresponding N-[(R)-(1-phenylethyl)]aziridine-(2S)- and N-[(S)-(1-phenylethyl)]aziridine-(2R)-carboxaldehydes, employing a one-pot three-components Kabachnik-Fields reaction followed by the hydrogenolytic removal of the chiral auxiliary and aziridine ring opening with simultaneous protection of the amino groups as the N-Boc derivatives.     

An efficient synthesis of chiral terminal 1,2-diamines using an enantiomerically pure [1-(1′R)-methylbenzyl]aziridine-2-yl]methanol

Enantiomerically pure terminal 1,2-diamines, which can serve as precursors for the synthesis of many biologically important compounds, were synthesized efficiently from a commercially available chiral [1-(1′R)-methylbenzyl]aziridine-2-yl]methanol. Various enantiomerically pure 2-vinylaziridines were prepared by Wittig reactions from aziridine-2-carboxaldehyde and the corresponding phosphonium salts. The C(2)-N bond of the vinyl substituted aziridine ring was regioselectively cleaved by azidotrimethylsilane (TMSN₃). The azido group and the double bond were reduced successively to give the target compounds in high yields.     

Asymmetric nitrogen

The possibility of tranferring chirality from nitrogen to carbon by a three-step transformation of racemic dimethyl 1-methoxyaziridine-2,2-dicarboxylate into 3-amino-2-chloromethyl-2-methoxyaminopropan-1-ol was demonstrated. The first representative of 1-silyloxyaziridines,viz., dimethyl 1-(tert-butyldimethylsilyloxy)aziridine-2,2-dicarboxylate, was synthesized by acidolysis or thermolysis of triazoline, obtained by the reaction oftert-butyldimethylsilyloxime of dimethy mesoxalate with CH₂N₂     

Synthesis of four enantiomers of 2-acetamido-1-hydroxypropylphosphonates

Enantiomerically pure diethyl 2-acetamido-1-hydroxypropylphosphonates were synthesised from N-[(R)-(1-phenylethyl)]aziridine-(2S)- and N-[(S)-(1-phenylethyl)]aziridine-(2R)-carboxaldehydes via phosphonylation followed by acetylation of the hydroxy groups and the simultaneous hydrogenolytic cleavage of the aziridine rings and the removal of the chiral auxiliaries. In addition, enantiomerically pure diethyl (1S,2R)- and (1R,2S)-2-amino-1-hydroxypropylphosphonates (the phosphonate analogues of isothreonine) were separated.     

N-Methylative aziridine ring opening and asymmetric synthesis of MeBmt

Asymmetric synthesis of MeBmt, an unusual amino acid constituent of cyclosporine A, was achieved from aziridine-(2R)-carboxaldehyde through the highly stereoselective addition of (E)-crotylboronate and the subsequent N-methylative aziridine ring opening as key steps.     

Preparation of the Key Dolutegravir Intermediate via MgBr2-Promoted Cyclization

A novel approach for synthesizing the key dolutegravir intermediate is described via MgBr₂-promoted intramolecular cyclization. Condensation of commercially available methyl oxalyl chloride and ethyl 3-(N,N-dimethylamino)acrylate afforded the vinylogous amide in an excellent yield. Subsequent substitution by aminoacetaldehyde dimethyl acetal and methyl bromoacetate gave rise to the expected precursor for cyclization, which was promoted by MgBr₂ to highly selectively convert into pyridinone diester. The key dolutegravir intermediate was finally prepared by the selective hydrolysis of the corresponding diester via LiOH.     

Aziridin-2-yl methanols as organocatalysts in Diels–Alder reactions and Friedel–Crafts alkylations of N-methyl-pyrrole and N-methyl-indole

A series of enantiomerically pure aziridin-2-yl methanols have been synthesized from aziridine-2-carboxylic esters and have been tested as organocatalysts in Diels–Alder reactions and Friedel–Crafts alkylations of N-methyl-pyrrole and N-methyl-indole using α,β-unsaturated aldehydes. Moderate to good ee’s have been obtained. The coupling of N-methyl-pyrrole with crotonaldehyde and cinnamaldehyde using (2S,3S)-3-methylazirin-2-yl(diphenyl)methanol-TFA salt as the catalyst gave the best results (ee 75%).     

Synthesis of Rh(I) and Ir(I) complexes with chiral C2-multitopic ligands: Structural and catalytic properties

Four multitopic ligands, N,N′-bis[(S)-prolyl)phenylenediamine, N,N′-bis{[(S)-pyrrolidin-2-yl]methyl}phenylenediamine, N,N′-bis[(S)-N-benzylprolyl]phenylenediamine, N,N′-bis{[(S)-N-benzyl-pyrrolidin-2-yl]methyl}phenylenediamine, were synthesised and their co-ordination properties with Rh(I) and Ir(I) studied. The complexes were prepared by the reaction of [MCl(cod)]₂ with AgPF₆ and further treatment with the ligand. All ligands form one to one [ML] species with the above metal ions. The structures of these complexes were elucidated by analytical and spectroscopic data (elemental analysis, mass spectroscopy, IR, ¹H- and ¹³C-NMR). Complexes show excellent activities and enantioselectivities up to 30% for the hydrogenation of prochiral olefins under mild reaction conditions.     

Synthesis of four stereoisomers of protected 1,2-epiimino-3-hydroxypropylphosphonates

Enantiomerically pure protected 1,2-epiimino-3-hydroxypropylphosphonates were synthesised from hydroxy-1-{[(R)- or (S)-1-phenylethyl]aziridin-2-yl}methylphosphonates via regioselective ring opening with acetic acid followed by a stereospecific intramolecular cyclisation of 3-acetoxy-1-mesyloxy-2-(1-phenylethyl)aminopropylphosphonates and hydrogenolytic removal of the 1-phenylethyl group in the presence of Boc₂O. The trans-isomers of 3-acetoxy-[N-(1-phenylethyl)-1,2-epiimino]propylphosphonates exist as a 2:1 mixture of invertomers, which were fully structurally characterised based on their ¹H and ¹³C NMR spectroscopic data. Large differences in the ¹J_C-P values in N-(1-phenylethyl)aziridine-2-phosphonates were noticed depending on the spatial arrangement of the nitrogen lone pair and the phosphorus atom (syn-periplanar—ca. 215 Hz; anti-periplanar—182 Hz).     

Synthesis and structure of silicon-containing <Emphasis Type="Italic">N</Emphasis>-methyland <Emphasis Type="Italic">N</Emphasis>-benzoylamides of diisopropylphosphoric acid

Diisopropyl N-benzoyl-N-(trimethylsilyl)phosphoramidate reacts with ClCH₂SiMe₂Cl under mild conditions to form diisopropyl N-benzoyl-N-[(chlorodimethylsilyl)methyl]phosphoramidate (III). Diisopropyl N-methyl-N-(trimethylsilyl)phosphoramidate with ClCH₂SiMe₂Cl affords an N-transsilylation product which does not rearrange into diisopropyl N-[(chlorodimethylsilyl)methyl]-N-methylphosphoramidate (XV) even under severe conditions (4 h, 130°C). Compound XV was prepared by the reaction of diisopropyl phosphorochloridate with N-[(methoxydimethylsilyl)methyl]-N-methylamine followed by treatment of diisopropyl N-[(methoxydimethylsilyl)methyl]-N-methylphosphoramidate with boron trichloride. Analysis of experimental and calculated ²⁹Si chemical shifts points to a five-coordinate silicon atom in compound III and a fourcoordinate silicon atom in compound XV. According to B3LYP calculations with due regard to solvent effects, compound III is an isomer with a C=O→Si bond. By variation of substituents at silicon, phosphorus, and carbonyl carbon atoms, chelate structures with either C=O→Si or P=O→Si dative bonds can be obtained.     

Synthesis,characterization, and catalytic application of a zinc(II) complex bearing a pyrazole-based ligand

The reaction between ZnCl₂ and N,N-bis[(3,5-dimethyl-1H-pyrazol-1-yl)methyl]-1-phenylethylamine (bdmppea) affords [(bdmppea)ZnCl₂], whose structure has been determined by X-ray crystallography. The [(bdmppea)ZnEt₂] complex in situ prepared by the reaction between [bdmppea] and ZnEt₂ exhibited high activity toward the polymerization reaction of rac-lactide at room temperature. However, its activity decreased sharply with decreasing temperature. Stereospecificity of this catalyst characterized by heterotacticity (P_r) was determined by homonuclear decoupled NMR spectroscopy, which value was ∼0.58.     

Stereoselective synthesis of enantiomerically pure d-threo-PDMP; manipulation of a core 2,3-diamino alcohol unit

The C(3)N bond of various non-activated enantiomerically pure aziridine-2-methanols was regioselectively cleaved by nitrogen nucleophiles to provide a variety of β-aminoalanine derivatives in high yields.     

Novel enantiopure non-C2-symmetric NCN-pincer palladium complexes with l-proline chiral auxiliaries: mer η3-N,C,N versus square planar η4-N,C,N,O coordination

New chiral NCN-pincer palladium complexes containing proline ester moieties as chiral auxiliaries have been synthesized. The parent ligands 2,6-bis{[(S)-2-(methoxycarbonyl)-1-pyrrolidinyl]methyl}-1-bromobenzene L^Me and 2,6-bis{[(S)-2-(benzoxycarbonyl)-1-pyrrolidinyl]methyl}-1-bromobenzene L^Bn were prepared in a single synthetic step and were obtained enantiomerically pure. Neutral arylpalladium bromide complexes 1a and 1b, formed upon treatment of the respective ligands L^Me and L^Bn with [Pd₂(dba)₃]·CHCl₃, were isolated as mixtures of three stereoisomers (S_NS_NS_CS_C, R_NS_NS_CS_C and R_NR_NS_CS_C). The ratio of stereoisomers is approximately 1:1:0.6 in the case of methyl ester derivative 1a, whereas the bulkier benzyl ester derivative 1b predominantly forms the (S_NS_NS_CS_C)-stereoisomer. Upon abstraction of the bromide ion from unresolved mixtures of 1a and 1b, cationic complexes 2 and 3, respectively, form as single diastereoisomers in which one of the ester prolinate carbonyl groups is coordinated to palladium according to X-ray crystal structure determination. This coordination of a carbonyl group to the metal has a substantial influence on the stereochemistry and results in the formation of a single diastereoisomer, having the (R_NR_NS_CS_C)-configuration, regardless of the stereochemistry or ratio of stereoisomers of the starting bromide compound. The structures of compounds 2 and 3 were somewhat unexpected since formation of the corresponding cationic [Pd(NCN)(OH₂)]⁺ complexes was anticipated. In preliminary tests of these cationic complexes as catalysts in the enantioselective aldol condensation of benzaldehyde with methyl isocyanoacetate, modest selectivities were observed.     

A stereochemical study of optically active thiazolidines

We report a stereochemical study of a series of free N-H and N-methylated 1,3-thiazolidines bearing H or CH₃ at C-(2). These compounds were readily prepared from ephedrine and pseudoephedrines. The stereochemistry of the compounds under study was deduced using ¹H and ¹³C NMR spectroscopy. Two isomers were found for compounds having a methyl group at C-(2) (i.e. C-(2)HCH₃); interconversion of these isomers, presumably via a non-cyclic zwitterionic intermediate, was observed.     

Preparation of 1,3-dienyl organotrifluoroborates and their Diels-Alder/cross-coupling reactions

2-BF₃-substituted 1,3-butadienes with potassium and tetrabutyl ammonium counterions have been prepared in gram quantities from chloroprene via a simple synthetic procedure. The potassium salt of this new main group element substituted diene has been characterized by ¹H, ¹³C, ¹¹B, and ¹⁹F NMR and the tetra n-butyl ammonium salt was also characterized by X-ray crystallography. Diels-Alder reactions of these dienes with dienophiles such as ethyl acrylate, methyl vinyl ketone, and N-phenylmaleimide are reported as well as subsequent Pd-catalyzed cross-coupling reactions of those Diels-Alder adducts. 4-Phenyl-2-BF₃-substituted 1,3-diene was prepared by magnesium-halogen exchange from the corresponding 2-bromo and iodo dienes. The 4-phenyl-2-bromo-1,3-butadiene was also characterized by X-ray crystallography. 4-Phenyl-2-BF₃-1,3-butadiene was used in Diels-Alder/cross-coupling reactions and the product of a Diels-Alder reaction with N-phenylmaleimide followed by cross-coupling with 4-bromo-benzonitrile was also characterized by X-ray crystallography.     

The effect of ligand basicity on the thermal stability of heterodinuclear NiII–ZnII complexes

Four complexes of the nuclear structure Ni^II–Zn^II were prepared with bis-N,N′-(salicylidene)-1,3-propanediamine (LH₂), bis-N,N′-(salicylidene)-2,2′-dimethyl-1,3-propanediamine (LDMH₂) and the reduced derivatives of these Schiff bases, bis-N,N′-(2-hydroxybenzyl)-1,3-propanediamine (L^HH₂), bis-N,N′-(2-hydroxybenzyl)-2,2′-dimethyl-1,3-propanediamine (LDM^HH₂). The complexes were characterized using IR spectroscopy, elemental analysis and thermogravimetric methods. The stoichiometry of the complex molecules were found to be NiL·ZnCl₂·(DMF)₂, NiLDM·ZnCl₂·(DMF)₂, NiL^H·ZnCl₂·(DMF)₂ and NiLDM^H·ZnCl₂·(DMF)₂. The molecular models of the complexes prepared with the reduced Schiff bases were determined according to the X-ray diffraction method. It is seen that in these complexes Ni(II) is in octahedral and Zn(II) is in tetrahedral coordination sphere. Ni(II) ion is coordinated between two nitrogen and two oxygen donors of the ligand and oxygen donors of the two DMF molecules. Zn(II) ion on the other hand is coordinated between two oxygen of the organic ligand forming two μ bonds. It also coordinates two Cl ions. The thermogravimetric analysis showed that the complex NiLDM^H·ZnCl₂·(DMF)₂ containing methyl groups is more stable than the other complex NiL^H·ZnCl₂·(DMF)₂ containing reduced Schiff base. The coordinative DMF molecules in NiLDM^H·ZnCl₂·(DMF)₂ were thermally cleaved. However, the cleavage of DMF molecules NiL^H·ZnCl₂·(DMF)₂ resulted in the thermal degradation of the complex. In order to explain the TG data of the ligands were titrated in non-aqueous medium and their basicity strengths were determined. It was found that the basicity of the ligands containing two methyl groups were stronger. It is understood that the two methyl groups increase the negative charge density on nitrogen causing an increase in complex stability.     

Synthesis of methyleneaminodipeptides via ring opening of a 2-(t-butoxycarbonylmethyl)aziridine derivative

The reactivity of 2-(t-butoxycarbonylmethyl)aziridine-1-carboxylic acid benzyl ester has been studied with various N-nucleophiles. The ring-opening reaction was always regioselective, the nucleophile attacking preferentially the less hindered carbon of the aziridine. The reaction was used to prepare a methyleneamino pseudodipeptide using the α-amine of a lysine ester.     

Synthesis and structure of stable cis-dimethyl complex of oxotungsten(VI)

Oxotungsten(VI) complex cis-[WO(L^tBu)Me₂] (L^tBu = methylamino-N,N-bis(2-methylene-4-methyl-6-tert-butylphenolate) dianion) was prepared by the transmetallation reaction of [WO(L^tBu)Cl₂] (either cis or trans isomer) with methyl magnesium iodide. This unexpectedly stable dialkyl complex can be activated by Et₂AlCl to catalyze the ring-opening metathesis polymerization of norbornene.     

Seeded dispersion polymerization of MMA using submicron PMMA particles as seed: a mechanistic study

Micron-size poly(methyl methacrylate) (PMMA) particles having a narrow particle size distribution were prepared by seeded dispersion polymerization of methyl methacrylate (MMA) using submicron PMMA particles as seed. The processes of particle aggregation and nucleation were controlled by the initial seed size, initial seed number, and initiator concentration, determining the formation of the mature particles and the number (N _(final)) and size of the final particles. It was found that N _(final) was equal to the number of particles produced in the absence of seed (N _{(ab initio)}) when the initial number of seed particles (N _(initial)) was less than N _{(ab initio)}. When N _(initial) was greater than N _{(ab initio)}, N _(final) was equal to k?×?N _(initial), where the value of k was a function of seed size and initiator concentration. k increased with seed size and was less than 1 at high initiator concentrations (0.52 and 1.00 %), while at low initiator concentrations (0.23 and 0.30 %), a maximum value of k was found for a 198 nm seed size. k could be greater than unity in some cases.