Catalytic enantioselective protonation of lithium enolates with chiral imides

Hydrodealkenylative C(sp3)-C(sp2) bond fragmentation

Chemical synthesis typically relies on reactions that generate complexity through elaboration of simple starting materials. Less common are deconstructive strategies toward complexity-particularly those involving carbon-carbon bond scission. Here, we introduce one such transformation: the hydrodealkenylative cleavage of C(sp³)-C(sp²) bonds, conducted below room temperature, using ozone, an iron salt, and a hydrogen atom donor. These reactions are performed in nonanhydrous solvents and open to the air; reach completion within 30 minutes; and deliver their products in high yields, even on decagram scales. We have used this broadly functionality tolerant transformation to produce desirable synthetic intermediates, many of which are optically active, from abundantly available terpenes and terpenoid-derived precursors. We have also applied it in the formal total syntheses of complex molecules.

Synthesis and characterization of two new chiral Kemp’s acid derivatives: structures fixed by a peculiar system of N–H···O, C–H···O and C–H···N hydrogen bonds

New chiral imide-oxazoline derivatives of Kemp’s acid were synthesized with the aim to produce new ligands suitable for catalytic asymmetric reactions. Compound 1 is C18H28N2O3, systematic name (1R, 5S, 7R)-7-[(S)-4-tert-butyl-4,5-dihydrooxazole-2-yl]-1,5,7-trimethyl-3-azabicyclo-[3.3.1]-nonane-2,4-dione. Compound 2 is C21H26N2O3, systematic name (1R,5S,7R)-7-[(S)-4-benzyl-4,5-dihydrooxazole-2-yl]-1,5,7-trimethyl-3-azabicyclo-[3.3.1]-nonane-2,4-dione. Both compounds contain two molecules with very similar conformations in the asymmetric unit. The two structures were characterized by single crystal X-ray diffraction. DFT calculations of two possible distinct conformations were performed to elucidate the differences between the preferred conformation in vacuo and the one observed in the single crystal. Also, charges on the key atoms of the compounds were compared with a common ligand used for asymmetric transfer hydrogenation.

A protecting group free synthesis of (+/-)-neostenine via the [5 + 2] photocycloaddition of maleimides

A concise, linear synthesis of the Stemona alkaloid (+/-)-neostenine is reported. Key features include an organocopper-mediated bislactone C2-desymmetrization for the stereoselective construction of the cyclohexane-lactone C,D-rings. The assembly of the fused pyrrolo[1,2-a]azepine core was achieved by application of a [5 + 2] maleimide photocycloaddition. A custom FEP flow reactor was used to successfully overcome the scale limitations imposed by a classical immersion well batch reactor. The synthesis was completed in 14 steps from furan, in 9.5% overall yield, without the use of any protecting groups.

Catalytic Asymmetric Protonation of Lithium Enolates Using Amino Acid Derivatives as Chiral Proton Sources

[reaction: see text] Asymmetric protonation of lithium enolates was examined using commercially available amino acid derivatives as chiral proton sources. Among the amino acid derivatives tested, Nbeta-l-aspartyl-l-phenylalanine methyl ester was found to cause significant asymmetric induction in the protonation of lithium enolates. The enantiomeric excess (up to 88% ee) of the products obtained in the presence of a catalytic amount of the chiral proton source was higher than those obtained in the stoichiometric reaction.

Enolate Structure and Electron Affinity

Photodetachment cross sections for a series of cyclic enolates were measured using a continuous wave (CW) ion cyclotron resonance instrument to generate and detect the ions. We report electron affinities for the radicals corresponding to the removal of the extra electron from the following anions: 2-methylcyclopent-1-enolate, 3-methylcyclopent-1-enolate, 4-methylcyclopent-1-enolate, 5-methylcyclopent-1-enolate, 2-methylcyclohex-1-enolate, 3-methylcyclohex-1-enolate, 4-methylcyclohex-1-enolate, 4-ethylcyclohex-1-enolate, 5-methylcyclohex-1-enolate, and 6-methylcyclohex-1-enolate. Some of these anions are mixed with their tautomers, derived from deprotonation of the parent ketone; the consequences of this are analyzed. The effect of alkylation on the electron affinities is discussed. The effect of vibrational modes on the lifetimes of the dipole-bound states of 4-methylcyclohex-1-enolate and 4-ethylcyclohex-1-enolate is discussed.

Asymmetric protonation of lithium enolates of alpha-amino acid derivatives with alpha-amino acid-based chiral Brønsted acids

The reaction of lithium enolates of alpha-amino acid derivatives with chiral amides, easily synthesized from L-tert-leucine, gives corresponding optically active unnatural alpha-amino acid derivatives with up to 87% ee.

Enantioselective Organocatalysis

The last few years have witnessed a spectacular advancement in new catalytic methods based on metal-free organic molecules. In many cases, these small compounds give rise to extremely high enantioselectivities. Preparative advantages are notable: usually the reactions can be performed under an aerobic atmosphere with wet solvents. The catalysts are inexpensive and they are often more stable than enzymes or other bioorganic catalysts. Also, these small organic molecules can be anchored to a solid support and reused more conveniently than organometallic/bioorganic analogues, and show promising adaptability to high-throughput screening and process chemistry. Herein we focus on four different domains in which organocatalysis has made major advances: 1) The activation of the reaction based on the nucleophilic/electrophilic properties of the catalysts. This type of catalysis has much in common with conventional Lewis acid/base activation by metal complexes. 2) Transformations in which the organic catalyst forms a reactive intermediate: the chiral catalyst is consumed in the reaction and requires regeneration in a parallel catalytic cycle. 3) Phase-transfer reactions: The chiral catalyst forms a host-guest complex with the substrate and shuttles between the standard organic solvent and the second phase (i.e. a solid, aqueous, or fluorous phase in which the organic transformation takes place). 4) Molecular-cavity-accelerated asymmetric transformations: the catalyst can select between competing substrates, depending on size and structure criteria. The rate acceleration of a given reaction is similar to the Lewis acid/base activation and is the consequence of the simultaneous action of different polar functions. Herein it is shown that organocatalysis complements rather than competes with current methods. It offers something conceptually novel and opens new horizons in synthesis.