Mohamed Amedjkouh

Aminophosphonates as organocatalysts in the direct asymmetric aldol reaction : towards syn selectivity in the presence of Lewis bases

Chiral alpha-aminophosphonates have been synthesized and their performance was evaluated as organocatalysts in the direct asymmetric aldol reaction. High enantioselectivities (up to 99% ee) were achieved for a range of substituted cyclohexanones and benzaldehydes. Several organic bases, such as DBU, DBN, and TMG, were used together with the alpha-aminophosphonates in the aldol reactions and were found to favor syn-selectivity.

Asymmetric autocatalytic Mannich reaction in the presence of water and its implication in prebiotic chemistry

An enantioselective process in which the chiral Mannich product acts as a catalyst for its own replication was observed to occur under various conditions in the presence of water.

Synthesis of chiral diamines using novel 2-trichloromethyloxazolidin-4-one precursors derived from 5-oxo-proline and proline

Abstract Efficient syntheses of chiral vicinal diamines derived from (S)-oxo-proline and (S)-proline are described. The novel diastereomerically pure precursor (2R,5S)-2-trichloromethyl-1-aza-3-oxabicyclo-[3.3.0]-octan-4,8-dione 3 and its enantiomer are readily available by reaction of the inexpensive enantiomers of 5-oxo-proline with chloral. Compound 3 reacts with primary and secondary amines to afford the 5-oxo-prolylamides 4 in quantitative yield. In contrast, the (S)-proline-derived precursor (2R,5S)-2-trichloromethyl-1-aza-3-oxabicyclo[3.3.0]octan-4-one 6 gave (S)-N-formylprolylamides 9 and/or (S)-prolylamides 8 depending on the reaction conditions. Upon reduction with LiAlH4, amides 4 and 9 afforded the proline-derived (S)-2-(alkylaminomethyl)pyrrolidines 1 and (S)-N-methyl-2-(alkylaminomethyl)-pyrrolidines 5 in 70–90% yields.

A novel mixed dimer of a norephedrine-derived chiral lithium amide and 2-lithium-1-methylimidazole, and catalytic enantioselective deprotonation of cyclohexene oxide.

Improved stereoselectivity has been obtained by using 2-lithium-1-methylimidazole, 2, as a replacement for lithium diisopropylamide (LDA) as a bulk base in catalytic deprotonations. The chiral lithium amide 6 of (1R,2S)-N-methyl-1-phenyl-2-pyrrolidinylpropanamine, 5, has been found to deprotonate cyclohexene oxide 3 in the presence of compound 2 to yield (S)-cyclohex-2-en-1-ol, 4, in 96% ee. Compound 2 is a carbenoid species conveniently generated from nBuLi and 1-methylimidazole, 1. The base 2 has also been found to play a more intimate role in the deprotonation. Investigations by 1H, 6Li and 13C NMR of the 6Li/15N isotopologue 8 of 6 have shown that 6 is homodimeric in THF and that, in the presence of 2, it forms a novel heterodimer 10. This heterodimer is found to be the dominant reagent in the initial state, rather than the homodimer of 6. Computational investigations with PM3 and B3LYP/6-311 + G(d,p) have shown possible structures of the heterodimers, as well as the role of THF and I in the solvation of the dimers. The results are in line with the NMR results. Favoured complexes in the equilibria between homo- and heterocomplexes are also reported.

Improved enantioselectivity by using novel bulk bases in chiral lithium amide catalysed deprotonations: mixed dimers as reagents and catalysts

Abstract Novel bulk bases have been developed yielding improved enantioselectivity of chiral lithium amide catalysed deprotonations as compared to using the bulk base lithium diisopropylamide (LDA). The new bulk bases are 2-lithio-1-methylimidazole, 2-(lithiomethyl)-1-methylimidazole, 2-lithio-furan and 1,8-diazabicyclo-6-lithio[5.4.0]undec-7-ene which have been used together with chiral lithium amides in deprotonations of cyclohexene oxide. Using the chiral lithium amides enhanced stereoselectivities (96% ee) have been reached. The reactivity change has been traced to the formation of novel reagents—mixed dimers—formed from a bulk base molecule and a molecule of a chiral lithium amide. The results also show that DBU, which has commonly been used as an additive to alter reactivity and enantioselectivity in deprotonations, has another important role. DBU is lithiated under the conditions used and becomes a bulk base, which forms catalytic mixed dimers with the chiral lithium amides.

Stereoselective reduction and reductive dephosphonylation of β-iminophosphonates

Abstract Proline-like 2,4-dialkyl-5-phosphonylpyrrolidines 2 were obtained stereoselectively by reduction of the corresponding β-iminophosphonates 1 with NaBH 4 . The detailed characterization of compounds 2 was accomplished by 1 H and 13 C NMR as well as by crystal structure analysis. When 1 was reduced with LiAlH 4 the reaction gave dephosphonylated pyrrolidines 3 . The latter method is suitable for providing substituted pyrrolidines regioselectively. A possible mechanism for the dephosphonylation is proposed.

Enamine-Imine Tautomerism in Cyclic (5-Alkyl-2,3,3-Trimethylpyrrolin-2-YL)Diethylphosphonate: A New Insight into Cyclization of β-Allenylaminophosphonates

(5-alkyliden-2,3,3-trimethylpyrrolin-2-yl)diethylphosphonate 3 were obtained by rearrangement of (5-alkyl-2,3,3-trimethylpyrrolin-2-yl)diethylphosphonate 2 . Rearrangement of 2 was induced at high temperature under an inert atmosphere. This air sensitive transformation requires a methyl group on the f position adjacent to the phosphonyl group. These results give new insight into cyclization of g -allenylaminophosphonates 1 .

Development of chiral catalysts for stereoselective synthesis by deprotonations - Experimentation in interplay with computational chemistry

Results are presented advancing the application of quantum chemistry in the field of organic synthesis. Computational chemistry in interplay with experimental chemistry has been given a key role in the development of stereoselective synthesis. Novel molecular systems are being created for catalytic stereoselective deprotonations, a reaction type useful for synthesizing many new compounds, e.g., some having important biological activities. Problems met in this approach to design catalysts and their solutions are presented.

On the novel function of the additive DBU. Catalytic stereoselective deprotonation by a mixed dimer of lithiated DBU and a chiral lithium amide

The additive DBU is used to increase the selectivity and reactivity of e.g. chiral lithium amides in both catalysed and non-catalysed asymmetric syntheses. This has been attributed to the coordinating ability of DBU favoring more reactive aggregates. However, we have found that LDA in THF deprotonates DBU to yield lithiated DBU (1) as shown by multinuclear NMR studies. Furthermore, compound 1 is found to form a mixed dimer (5) with e.g. the norephedrine-derived chiral lithium amide 2. Results of an investigation of the stereoselectivity of this novel reagent in the epoxide deprotonation are also reported. Computational studies using PM3 and DFT show possible structures of 1 and 5 in line with the NMR results. In addition, the role of THF and DBU in the solvation of the aggregates has been investigated by computational modelling and favoured complexes in the equilibria between homo- and heterocomplexes are also reported.

Kinetic and computational studies of the composition and structure of activated complexes in the asymmetric deprotonation of cyclohexene oxide by a norephedrine-derived chiral lithium amide

Rational design of efficient chiral lithium amides for enantioselective deprotonations demands understanding of the origin of the selectivity. The mechanism of deprotonation of cyclohexene oxide 1 by lithium (1R,2S)-N-methyl-1-phenyl-2-pyrrolidinylpropanamide 3, which yields (S)-cyclohex-2-en-1-ol (S)-5 in 93% enantiomeric excess in tetrahydrofuran (THF), has been investigated. Kinetics have been used to show that the reaction is first order with respect to the reagents 1 and 3, respectively. NMR investigations of a 6Li and 15N labelled isotopologue of 3 have previously shown that 3 is mainly a dimer of the lithium amide monomer in THF in the initial state. On the basis of these results it is concluded that the rate-limiting activated complexes for the epoxide deprotonation are composed of two molecules of monomer of lithium amide 3 and one molecule of epoxide. Structures and energies of unsolvated and specific THF-solvated reagents and activated complexes have been calculated using PM3 and B3LYP/6-31+G(d). The results are currently being explored for the rational design of chiral lithium amides with improved stereoselectivities.