Structure-Selectivity Relationship in Alkyllithium−Aldehyde Condensations Using 3-Aminopyrrolidine Lithium Amides as Chiral Auxiliaries

Chiral Lithium Amides: Tuning Asymmetric Synthesis on the Basis of Structural Parameters

An overview on the structural arrangements adopted by Chiral Lithium Amides (CLAs), alone or in mixed complexes, is presented. These species are important reagents for asymmetric synthesis and understanding their organization is essential to improve their design and the reaction conditions.

Structure-Reactivity Relationships in Lithiated Evans Enolates: Influence of Aggregation and Solvation on the Stereochemistry and Mechanism of Aldol Additions

Aldol additions to isobutyraldehyde and cyclohexanone with lithium enolates derived from acylated oxazolidinones (Evans enolates) are described. Previously characterized trisolvated dimeric enolates undergo rapid addition to isobutyraldehyde to give a 12:1 syn:syn selectivity in high yield along with small amounts of one anti isomer. The efficacy of the addition depends critically on aging effects and the reaction quench. Unsolvated tetrameric enolates that form on warming the solutions are unreactive toward isobutyraldehyde and undergo retroaldol reaction under forcing conditions. Additions to cyclohexanone are relatively slow but form a single isomeric adduct in >80% yield. The ketone-derived aldolates are robust. All attempts to control stereoselectivity by controlling aggregation failed. Rate studies of addition to cyclohexanone trace the lack of aggregation-dependent selectivities to a monomer-based mechanism. The synthetic implications and possible utility of lithium enolates in Evans aldol additions are discussed.

Probing solvent effects on mixed aggregates associating a chiral lithium amide and n-BuLi by NMR: from structure to reactivity

An NMR study of a 1 : 1 mixture of a chiral lithium amide (4a) and n-BuLi shows that depending on the solvent employed (Et2O or THF) a mixed aggregate can form in proportions that are directly related to the ees measured during the enantioselective alkylation of o-tolualdehyde by these same species.

Mixed aggregates of an alkyl lithium reagent and a chiral lithium amide derived from N-ethyl-O-triisopropylsilyl valinol

The crystal structure of a mixed aggregate containing lithiated (S)-N-ethyl-3-methyl-1-(triisopropylsilyloxy)butan-2-amine derived from (S)-valinol and cyclopentyllithium is determined by X-ray diffraction. The mixed aggregate adopts a ladder structure in the solid state. The ladder-type mixed aggregate is also the major species in a toluene-d8 solution containing an approximately 1:1 molar ratio of the lithiated chiral amide to cyclopentyllithium. A variety of NMR experiments including diffusion-ordered NMR spectroscopy (DOSY) with diffusion coefficient-formula (D-FW) weight correlation analyses and other one- and two-dimensional NMR techniques allowed us to characterize the complex in solution. Solution state structures of the mixed aggregates of n-butyl, sec-butyllithium, isopropyllithium with lithiated (S)-N-ethyl-3-methyl-1-(triisopropylsilyloxy)butan-2-amine are also reported. Identical dimeric, ladder-type, mixed aggregates are the major species at a stoichiometric ratio of 1:1 lithium chiral amide to alkyllithium in toluene-d8 solution for all of the different alkyllithium reagents.

Crystal structure and solution state characterization of lithium (S)-(1-(bis(2-methoxyethyl)amino)-3-methylbutan-2-yl)(methyl)amide

The solid state structure of lithiated (S)-N(1),N(1)-bis(2-methoxyethyl)-N(2),3-dimethylbutane-1,2-diamine, which is a chiral amide base synthesized from (S)-valine was determined by single-crystal X-ray diffraction. The complex in solution state is also characterized by a variety of NMR experiments including diffusion-ordered NMR spectroscopy (DOSY) with diffusion coefficient-formula weight correlation analyses and other one- and two-dimensional NMR techniques by dissolving the crystal in toluene-d8. The crystallography and NMR results suggest that the chiral amide is dimeric in both solid and solution states.

Characterization of dimeric chiral lithium amide structures derived from N-isopropyl-O- triisopropylsilyl valinol

The dimeric structure is characterized for a chiral amide base complex consisting of an (S)-N-isopropyl-O-triisopropylsilyl valinol ligand and lithium. The complex is characterized by a variety of NMR techniques, including multinuclear one- and two-dimensional NMR experiments and diffusion-ordered NMR spectroscopy (DOSY) as well as diffusion coefficient-formula weight (D-fw) correlation analyses. Spartan calculations are presented which support the structural assignment. This structural characterization leads to an explanation of the behavior and the reactivity of these complexes in solution.

Analysis of an asymmetric addition with a 2:1 mixed lithium amide/n-butyllithium aggregate

A 2:1 lithium amide/ n-butyllithium aggregate 1 is investigated as an asymmetric addition template in hydrocarbon solvents. Several different chiral lithium amides were synthesized from l-valine and tested in the asymmetric addition of n-BuLi to various aldehydes. Enantiomeric excesses up to 83% were obtained in the case of the addition of n-BuLi to pivaldehyde at -116 degrees C in pentane. (1)H and (13)C INEPT DOSY were utilized to characterize a new trimeric complex 12 between 2 equiv of lithium amide and 1 equiv of lithium alkoxide. This mixed aggregate strongly indicates the possibility of product-induced chirality inhibition that is detrimental to the enantioselectivity of asymmetric addition reaction.

Origin of the detrimental effect of lithium halides on an enantioselective nucleophilic alkylation of aldehydes

The effect of lithium halides on the enantioselectivity of the addition of methyllithium on o-tolualdehyde, in the presence of chiral lithium amides derived from chiral 3-aminopyrrolidines (3APLi), has been investigated. The enantiomeric excess of the resulting 1-o-tolylethanol was found to drop upon addition of significant amounts of LiCl, introduced before the aldehyde. The competitive affinity between the lithium amide, the methyllithium, and the lithium halides in THF was examined by multinuclear NMR spectroscopy and DFT calculations. The results showed that the original mixed aggregate of the chiral lithium amide and methyllithium is rapidly, totally, and irreversibly replaced by a similar 1:1 complex involving one lithium chloride or bromide and one lithium amide. While the MeLi/LiX substitution occurs with some degree of epimerization at the nitrogen for the endo-MeLi:3APLi complex, it is mostly stereospecific for the exo-type arrangements of the aggregate. The thermodynamic preference for mixed aggregates between 3APLi and LiX was confirmed by static DFT calculations: the data show that the LiCl and LiBr aggregates are more stable than their MeLi counterparts by more than 10 kcal.mol(-1) provided THF is explicitly taken into account. These results suggest that a sequestration of the source of chirality by the lithium halides is at the origin of the detrimental effect of these additives on the ee of the model reaction.

Improved and Efficient Synthesis of Chiral N,P-Ligands via Cyclic Sulfamidates for Asymmetric Addition of Butyllithium to Benzaldehyde

A robust and scaleable route to chiral 1-isopropylamino-2-(diphenylphosphino)ethanes is described via the ring-opening of chiral, cyclic sulfamidates with potassium diphenylphosphide (KPPh(2)). The novel protocol offers a robust access to gram quantities of chiral amino phosphinoethanes in high yields. The Li-amides of the chiral aminophosphines were evaluated as chiral ligands in the asymmetric addition of n-butyllithium (BuLi) to benzaldehyde, yielding 1-phenylpentanol up to 98% ee.

Enantioselective Conjugate Addition of a Lithium Ester Enolate Catalyzed by Chiral Lithium Amides

Mixed aggregates of chiral lithium amide and lithium ester enolate have been employed in the enantioselective conjugate addition on alpha,beta-unsaturated esters. Michael adducts were obtained in ee's up to 76% combining a lithium enolate and a chiral 3-aminopyrrolidine lithium amide. The sense of the induction was found to be determined by both the relative configuration of the stereogenic centers borne by the amide and the solvent in which the reaction was conducted. [reaction: see text]

Design, synthesis, and biological evaluation of cyclic and acyclic nitrobenzylphosphoramide mustards for E. coli nitroreductase activation

In efforts to obtain anticancer prodrugs for antibody-directed or gene-directed enzyme prodrug therapy using E. coli nitroreductase, a series of nitrobenzylphosphoramide mustards were designed and synthesized incorporating a strategically placed nitro group in a position para to the benzylic carbon for reductive activation. All analogues were good substrates of E. coli nitroreductase with half-lives between 2.9 and 11.9 min at pH 7.0 and 37 degrees C. Isomers of the 4-nitrophenylcyclophosphamide analogues 3 and 5 with a benzylic oxygen para to the nitro group showed potent selective cytotoxicity in nitroreductase (NTR) expressing cells, while analogues 4 and 6 with a benzylic nitrogen para to the nitro group showed little selective cytotoxicity despite their good substrate activity. These results suggest that good substrate activity and the benzylic oxygen are both required for reductive activation of 4-nitrophenylcyclophosphamide analogues by E. coli nitroreductase. Isomers of analogue 3 showed 23,000-29,000x selective cytotoxicity toward NTR-expressing V79 cells with an IC(50) as low as 27 nM. They are about as active as and 3-4x more selective than 5-aziridinyl-2,4-dinitrobenzamide (CB1954). The acyclic 4-nitrobenzylphosphoramide mustard ((+/-)-7) was found to be the most active and most selective compound for activation by NTR with 170,000x selective cytotoxicity toward NTR-expressing V79 cells and an IC(50) of 0.4 nM. Compound (+/-)-7also exhibited good bystander effect compared to 5-aziridinyl-2,4-dinitrobenzamide. The low IC(50), high selectivity, and good bystander effects of nitrobenzylphosphoramide mustards in NTR-expressing cells suggest that they could be used in combination with E. coli nitroreductase in enzyme prodrug therapy.

Correlation between the6Li,15N Coupling Constant and the Coordination Number at Lithium

The 6Li,15N coupling constants of lithium amide dimers and their mixed complexes with n-butyllithium, formed from five different chiral amines derived from (S)-[15N]phenylalanine, were determined in diethyl ether (Et2O), tetrahydrofuran (THF) and toluene. Results of NMR spectroscopy studies of these complexes show a clear difference in 6Li,15N coupling constants between di-, tri- and tetracoordinated lithium atoms. The lithium amide dimers with a chelating ether group exhibit 6Li,15N coupling constants of approximately 3.8 and approximately 5.5 Hz for the tetracoordinated and tricoordinated lithium atoms, respectively. The lithium amide dimers with a chelating thioether group show distinctly larger 6Li,15N coupling constants of approximately 4.4 Hz for the tetracoordinated lithium atoms, and the tricoordinated lithium atoms have smaller 6Li,15N coupling constants, approximately 4.9 Hz, than their ether analogues. In diethyl ether and tetrahydrofuran, mixed dimeric complexes between the lithium amides and n-butyllithium are formed. The tetracoordinated lithium atoms of these complexes have 6Li,15N coupling constants of approximately 4.0 Hz, and the 6Li,15N coupling constants of the tricoordinated lithium atoms differ somewhat, depending on whether the chelating group is an ether or a thioether; approximately 5.1 and approximately 4.6 Hz, respectively. In toluene, mixed trimeric complexes are formed from two lithium amide moieties and one n-butyllithium. In these trimers, two lithium atoms are tricoordinated with 6Li,15N coupling constants of approximately 4.6 Hz and one lithium is dicoordinated with 6Li,15N coupling constants of approximately 6.5 Hz.

3-Aminopyrrolidines via Ring Rearrangement of 2-Aminomethylazetidines. Synthesis of (−)-Absouline

[reaction: see text] A new entry to enantiopure 3-aminopyrrolidines was developed using a boron trifluoride-mediated rearrangement of 2-aminomethylazetidines. The method is quite general and produces rearranged products in good yield regardless of both substitution pattern and relative stereochemistry of the starting material. A concise stereocontrolled synthesis of (-)-absouline was achieved on the basis of this new method.

Reaction of Ketones with Lithium Hexamethyldisilazide: Competitive Enolizations and 1,2-Additions

Reaction of 2-methylcyclohexanone with lithium hexamethyldisilazide (LiHMDS, TMS(2)NLi) displays highly solvent-dependent chemoselectivity. LiHMDS in THF/toluene effect enolization. Rate studies using in situ IR spectroscopy are consistent with a THF concentration-dependent monomer-based pathway. LiHMDS in pyrrolidine/toluene affords exclusively 1,2-addition of the pyrrolidine fragment to form an alpha-amino alkoxide-LiHMDS mixed dimer shown to be a pair of conformers by using (6)Li, (15)N, and (13)C NMR spectroscopies. Rate studies are consistent with a monomer-based transition structure [(TMS(2)NLi)(ketone)(pyrrolidine)(3)](). The partitioning between enolization and 1,2-addition is kinetically controlled.

A NMR and theoretical study of the aggregates between alkyllithium and chiral lithium amides: control of the topology through a single asymmetric center

The complexes between methyllithium and chiral 3-aminopyrrolidine (3-AP) lithium amides bearing a second asymmetric center on their lateral amino group were studied using multinuclear ((1)H, (6)Li, (13)C, (15)N) low-temperature NMR spectroscopies in tetrahydrofuran-d(8). The results indicate that lithium chelation forces the pyrrolidine ring of the 3-AP to adopt a norbornyl-like conformation and that robust 1:1 noncovalent complexes between methyllithium and 3-AP lithium amides form in the medium. A set of (1)H-(1)H and (1)H-(6)Li NMR cross-coupling correlations shows that the binding of methyllithium can take place along the "exo" or the "endo" face of this puckered structure, depending on the relative configuration of the lateral chiral group. This aggregation step renders the nitrogen of the 3-amino group chiral, the "exo" and "endo" topologies corresponding to the (S) and (R) configurations, respectively, of this atom. Density functional theory calculations show that the "exo" and "endo" arrangements are, for both diastereomers, almost isoenergetic even when solvent is taken into account. This result suggests that the formation of the mixed aggregates is under strict kinetic control. A relationship between the topology of these complexes and the sense of induction in the enantioselective alkylation of aromatic aldehydes by alkyllithiums is proposed.

A DFT theoretical analysis of aldehyde condensation pathways onto methyllithium, lithium dimethylamide, and their aggregates

A DFT analysis of the condensation of monomeric methyllithium and lithium dimethylamide (LMA), as well as their homo and hetero dimers, on formaldehyde and acetaldehyde is reported. A stable complex, exhibiting a directional interaction between a lone pair of the oxygen on the aldehyde and a lithium, is first found. At this stage, the aldehyde carbonyl and the Li-X (X = C or N) bonds lie in the same plane. To proceed, the condensation reaction has to go through a transition state that mainly consists of a rotation of the aldehyde plane, placing it perpendicular to the C-C or C-N forming bond. The reaction then leads, in a strongly exothermic final step, to the addition product that is a lithium alcoholate or alpha-amino alcoholate, associating into an hetero-aggregate with the remaining moiety of the initial dimer. From the relative heights of the activation barriers, it appears that, for the heterodimer MeLi-LMA, the formation of the C-N bond should be kinetically favored over the C-C one, while the lithium ethylate resulting from the C-C binding is the thermodynamic product. A decomposition of the activation energy barriers has been carried out in order to determine the physicochemical forces responsible for the variation of the condensation activation barriers with the structure of the final species formed. The results obtained are discussed in relation with corresponding experimental data.