Catalytic enantioselective protonation of lithium enolates with chiral imides

The catalytic enantioselective protonation of simple enolates was achieved using a catalytic amount of chiral imides and stoichiometric amount of achiral proton sources. Among the achiral proton sources examined in the protonation of the lithium enolate of 2,2,6-trimethylcyclohexanone catalyzed by (S,S)-imide 1, 2, 6-di-tert-butyl-p-cresol (BHT) and its derivatives gave the highest enantiomeric excess. For example, 90% ee of (R)-enriched ketone was obtained when (S,S)-imide 1 (0.1 equiv) and BHT (1 equiv) were used. Use of 0.01 equiv of the chiral catalyst still caused a high level of asymmetric induction. For catalytic protonation of the lithium enolate of 2-methylcyclohexanone, chiral imide 6 possessing a chiral amide portion was superior to (S,S)-imide 1 as a chiral proton source and the enolate was effectively protonated with up to 82% ee.     

A ternary complex reagent for an asymmetric Michael reaction of lithium ester enolates with enoates

A lithium ester enolate was activated by both a chiral etheral ligand and a lithium amide to form a ternary complex reagent that reacted with enoates giving the corresponding Michael addition products in reasonably high enantioselectivity of up to 97% ee.     

A chiral ligand-mediated asymmetric addition of a lithium BHA ester enolate to an aldehyde

The asymmetric reaction of a lithium enolate generated from a BHA (2, 6-di-tert-buty-4-methoxyphenyl) propanoate was allowed to react with benzaldehyde in the presence of a diether-type chiral ligand affording the corresponding anti-aldol product in a moderate enantioselectivity. A tetradentate ligand induced better enantioselectivity albeit relative loss of anti-selectivity. A variation of lithiating amide agent affected the selectivity, indicating involvement of an amine as a component of the mixed aggregate. Absolute configuration of some of the aldol products was determined by standard transformations.     

Diastereo- and enantioselective aldol reaction of granatanone (pseudopelletierine)

Granatanone (granatan-3-one, 9-methyl-9-azabicyclo[3.3.1]nonan-3-one, pseudopelletierine or pseudopelletrierin) undergoes deprotonation with lithium amides giving a lithium enolate, which reacts with aldehydes diastereoselectively giving exclusively exo isomers and anti/syn selectivity up to 98:2. Granatanone can be enantioselectively lithiated by chiral lithium amides and the resulting non-racemic enolate can be reacted with aldehydes giving aldols with enantiomeric excess up to 93% (99% ee after recrystallization). The absolute and relative configuration of the aldol products was determined by NMR spectroscopy and X-ray analysis.Granatanone; aldol reaction; asymmetric synthesis; enantioselective deprotonation; chiral lithium amide.     

Characterization of a chiral enolate aggregate and observation of 6Li-1H scalar coupling

A chiral enolate aggregate 1 containing a lithium enolate and a chiral lithium amide was systematically investigated by various NMR techniques. (1)H and (13)C DOSY at 25 and -78 degrees C provide its solution structure, aggregation number, and formula weight. Multiple 2D (6)Li NMR techniques, such as (6)Li-(6)Li EXSY, (6)Li-(1)H HOESY, were utilized to investigate its stereochemical structure. The configuration of the enolate in complex 1 was confirmed by (6)Li-(1)H HOESY and trapping with TMS-Cl. A unique (6)Li-(1)H coupling through the Li-N-C-H network was observed. This scalar coupling was corroborated by (6)Li-(1)H HMQC, deuterium labeling experiments, and selective (1)H decoupling (6)Li NMR. The stereostructure of 1 provides a model for the origin of enantioselectivity of chiral lithium amide-induced enolate addition reactions.     

Enantioselective aldol reaction mediated by chiral lithium amide bases

Highly enantioselective aldol reaction mediated by chiral lithium amide bases was achieved between some methylketones and aldehydes     

Studies towards complex bridged alkaloids: regio- and stereocontrolled enolate chemistry of 2,5-diketopiperazines

The substitution of symmetrical N-protected diketopiperazines (DKPs) via enolate intermediates has been studied. The enolate reactions were highly diastereocontrolled, leading to enantiopure DKP products if chiral amino acid precursors were employed, and giving racemic products, starting with centrosymmetric DKPs, even when a chiral lithium amide base was used to generate the lithium enolate. With unsymmetrical DKPs derived from proline and either alanine, phenylalanine or valine, the enolate substitution occurred with high regio- and stereoselectivity on the proline residue. This enabled the synthesis of substituted DKPs that could be cyclised via cationic processes to give the bicyclo[2.2.2]diazaoctane core structure present in paraherquamide and stephacidin natural products.     

A novel asymmetric route to succinimides and derived compounds: synthesis of the lignan lactone (+)-hinokinin

A novel approach to chiral succinimides and derived compounds has been developed that involves chiral lithium amide desymmetrisation of an N-ortho-tert-butylphenyl succinimide to generate a putative atropisomeric intermediate enolate, alkylation of which enables access to the lignan lactone (+)-hinokinin.     

Stereoselective syntheses of substituted succinic acid derivatives of the iron chiral auxiliary [(η-C5H5)Fe(CO)(PPh3)]

A range of alkyl- or aryl-substituted iron succinoyl complexes, incorporating the iron chiral auxiliary [(η⁵-C₅H₅)Fe(CO)(PPh₃)], were prepared in high regio- and diastereoselectivities by employing four successful strategies: (i) the alkylation of chiral enolate equivalents with tert-butyl bromoacetate; (ii) the mutual kinetic resolution of tert-butyl α-bromoacetate with a chiral acetate enolate equivalent; (iii) the alkylation of chiral succinoyl enolate equivalents; (iv) the conjugate addition of organolithium reagents or lithium amide reagents to chiral fumaroyl derivatives. Oxidative cleavage of the iron chiral auxiliary was shown to occur without compromising the stereochemical integrity of the succinoyl fragments.     

Computational strategies for evaluating barrier heights for gas-phase reactions of lithium enolates

Gas-phase activation energies were calculated for three lithium enolate reactions by using several different ab initio and density functional theory (DFT) methods to determine which levels of theory generate acceptable results. The reactions included an aldol-type addition of an enolate to an aldehyde, a proton transfer from an alcohol to a lithium enolate, and an S(N)2 reaction of an enolate with chloromethane. For each reaction, the calculations were performed for both the monomeric and dimeric forms of the lithium enolate. It was found that transition state geometry optimization with B3LYP followed by single point MP2 calculations generally provided acceptable results compared to higher level ab initio methods.     

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.     

Homo- and heterocomplexes of sodium and lithium amides--structures in solution

Addition of the chiral amine (S)-methyl(1-phenyl-2-pyrrolidinoethyl)[15N]amine (1) to a large excess of nBuNa resulted in the formation of a mixed sodium amide/nBuNa complex. This is the first observation of such a complex. Addition of nBuLi to the chiral sodium amide dimer 3 gave a new mixed lithium/sodium amide 5. The use of 15N,6Li coupling constants showed that the lithium in 5 occupied the tetracoordinated site. The use of chiral sodium amide 3 in the desymmetrization of cyclohexene oxide gave a modest enantiomeric excess (ee) of 37%. The corresponding lithium amide gave an ee of 70% of the same enantiomer. This is the first example of the comparison of asymmetric induction by sodium as cation with that of lithium.     

Catalytic asymmetric chiral lithium amide-promoted epoxide rearrangement: a NMR spectroscopic and kinetic investigation

The lithium amide derived from the chiral diamine (1R,3S,4S)-3-(1-pyrrolidinyl)methyl-2-azabicyclo[2.2.1]heptane, has been reported to catalytically deprotonate cyclohexene oxide and other epoxides, yielding chiral allylic alcohols in excellent enantiomeric excess. In this work, ⁶Li, ¹H and ¹³C NMR spectroscopy have been used to study the aggregation of the chiral lithium amide in THF and the influence on the aggregation by the addition of additives, such as 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU). The activated complex under catalytic deprotonation of cyclohexene oxide, that is, with excess Li-DBU and free DBU, is built from one monomer of the chiral lithium amide, one molecule of epoxide and one additional molecule of DBU. The reaction order (?0.97) obtained for the bulk base Li-DBU shows an inverse dependence on the concentration, suggesting a deaggregation of the initial mixed dimer to a monomer-based transition state containing a monomer of the lithium amide.     

Bridgehead enolates: substitution and asymmetric desymmetrization of small bridged carbonyl compounds by lithium amide bases

[reaction: see text] Contrary to expectations, a number of bridged carbonyl compounds undergo facile bridgehead metalation with lithium amide bases. Diketone, lactone, lactam, and imide functions are all demonstrated to participate in this type of "bridgehead enolate" chemistry, leading to a range of substituted products. Meso compounds can also be desymmetrized in very high ee by asymmetric bridgehead metalation.     

Chiral ligand-controlled asymmetric conjugate addition of alpha-trimethylsilanylacetate to acyclic and cyclic enones

The reaction of lithium ester enolate with enones provides a challenge for chemoselectivity, that is, discrimination between a conjugate addition and a 1,2-addition. Asymmetric conjugate addition of a lithium enolate of alpha-trimethylsilanylacetate to acyclic and cyclic alpha,beta-unsaturated ketones was mediated by an external chiral ligand to give the corresponding 1,4-adducts in good enantioselectivity of 74% and good chemoselectivity.     

Mechanistic studies of the lithium enolate of 4-fluoroacetophenone: rapid-injection NMR study of enolate formation, dynamics, and aldol reactivity

Lithium enolates are widely used nucleophiles with a complicated and only partially understood solution chemistry. Deprotonation of 4-fluoroacetophenone in THF with lithium diisopropylamide occurs through direct reaction of the amide dimer to yield a mixed enolate-amide dimer (3), then an enolate homodimer (1-Li)(2), and finally an enolate tetramer (1-Li)(4), the equilibrium structure. Aldol reactions of both the metastable dimer and the stable tetramer of the enolate were investigated. Each reacted directly with the aldehyde to give a mixed enolate-aldolate aggregate, with the dimer only about 20 times as reactive as the tetramer at -120 °C.     

Stereoselective reactions. XXXII. Enantioselective deprotonation of 4-tert-butylcyclohexanone by fluorine-containing chiral lithium amides derived from 1-phenylethylamine and 1-(1-naphthyl)ethylamine

Enantioselective deprotonation of 4-tert-butylcyclohexanone was examined using 1-phenylethylamine- and 1-(1-naphthyl)ethylamine-derived chiral lithium amides having an alkyl or a fluoroalkyl substituent at the amide nitrogen. The lithium amides having a 2,2,2-trifluoroethyl group on the amide nitrogen are easily accessible in both enantiomeric forms, and were found to induce good enantioselectivity in the present reaction.     

Total synthesis of (-)-neplanocin a by using lithium thiolate-initiated Michael-aldol tandem cyclization reaction.

(-)-Neplanocin A (1), S-adenosylhomocystein hydrolase inhibitor, was synthesized. The characteristic of this synthesis is a stereoselective construction of five-membered ring of neplanocin A by intramolecular aldol reaction of the lithium enolate that was generated by conjugate addition of lithium thiolate. TBS-protected chiral omega-oxo-alpha,beta-unsaturated ester 16, which was prepared from D-mannitol, was treated with 1.2 equiv of lithium benzylthiolate in THF at -20 degrees C to give three separable cyclization products in good yields and stereoselectivity. After conversions of protective groups, the benzylsulfanyl part of 21 was removed by oxidation to sulfoxide and subsequent thermal elimination to give the requisite double bond. Through the functional group transformations of 30, total synthesis of (-)-neplanocin A (1) was accomplished.     

Enantioselective alkylation of lactams and lactones via lithium enolate formation using a chiral tetradentate lithium amide in the presence of lithium bromide

Enantioselective alkylation of lactams and lactones can be realized up to 98% ee by deprotonation with a chiral tetradentate lithium amide (4b) in the presence of lithium bromide, and subsequent alkylation with active alkylating agents in non-chelating solvents.     

MM3 force field prediction of the enantioselective preference in the asymmetric synthesis of a chiral 2-cyclohexen-1-ol using a chiral lithium amide reagent

Enantioselective preference in the asymmetric synthesis where cyclohexene oxide is transformed enantioselectively to chiral (S)- or (R)-2-cyclohexen-1-ol by the reaction with the appropriate chiral lithium amide reagent has been evaluated theoretically using the MM3 force field. The plausible possible structures for each precursor (reaction intermediate complex) leading to a (S)- or (R)-2-cyclohexen-1-ol have been optimized with the extended MM3 force field applicable to the lithium amide functional group, and the populations of their (S)- or (R)-reaction intermediate complexes at an ambient temperature (298 K) were calculated. The initial structure for evaluating the reaction intermediates of this asymmetric synthesis was constructed on the basis of the optimized ab initio transition state structure (MP2/6-31+G^∗) comprising lithium amide LiNH₂ and propene oxide. To the thus obtained transition state structure composed of LiNH₂ and propene oxide, the other remaining Cartesian coordinates for the actual reaction intermediates composed of the chiral lithium amides and cyclohexene oxide were added to make the reaction intermediate structure. The conformational search for the reaction intermediate has been carried out by using the Stochastic search Algorithm, and the optimized geometries and their conformational energies (steric energies) have been calculated by the MM3 force field. The populations calculated from the conformational energies of the reaction intermediate leading to the (S)- or (R)-2-cyclohexen-1-ol were shown to be linearly well correlated with the experimentally reported enantiomer excess (% ee) values. The critical factors to control the enantioselectivity were investigated on the basis of the optimized structures of the reaction intermediate complexes. The MM3 force field approach was shown to be applicable to the theoretical evaluation of the enantioselectivity and be useful for designing a new functional chiral lithium amide reagent for the asymmetric synthesis.