Zinc-Mediated Ring-Expansion and Chain-Extension Reactions of β-Keto Esters

Towards a qualitative understanding of the carbonyl reactivity of α-substituted ethyl 5-oxohomoadamantyl-4-carboxylates

Substituted ethyl 5-oxohomoadamantane-4-carboxylates were subjected to reactions with several nucleophiles to establish some aspects of the carbonyl reactivity. However, only one example of the desired Claisen retro-reaction was observed as 3,7-disubstituted bicyclo[3.3.1]nonane formation. Most of the reactions yielded α-substituted homoadamantan-5-ones or some products of their further transformations. Reductive amination of substituted homoadamantane-5-ones gave several homoadamantane-fused nitrogen heterocycles, which could be considered as GABA- and/or ∂-aminovaleric acid analogues. Reduction products of several substituted ketones were observed as single products when treated with organomagnesium reagents. These deviations from the general chemical reactivity patterns are associated with steric factors and the geometry of the cage unit and could be a particular case, which is reflecting the overall specificity of the chemistry of cage carbonyl compounds.

Copper-Catalyzed Three-Component Reactions of α-Ketoaldehyde, 1,3-Dicarbonyl Compound, and Organic Boronic Acid in Water: A Route to 1,4-Diketones

A novel three-component reaction of α-ketoaldehydes, 1,3-dicarbonyl compounds, and organic boronic acids catalyzed by CuO in water has been developed to give a wide range of products containing 1,3/1,4-diketones. The method has some advantages such as the use of readily available starting materials, wide substrate scopes, excellent yields, gram-scale synthesis, and mild reaction conditions.

Photochemical Synthesis of 1,4-Dicarbonyl Bifluorene Compounds via Oxidative Radical Coupling Using TEMPO as the Oxygen Atom Donor

A visible-light-induced metal-free synthesis of 1,4-dicarbonyl compounds from alkyne-containing aryl iodides via photochemical C-I bond cleavage, intramolecular cyclization, oxidation, and intermolecular radical coupling sequence is reported. TEMPO was employed as the oxygen atom donor in this transformation. This protocol provided a new strategy for the synthesis of 1,4-dicarbonyl bifluorene compounds.

Construction of Polycyclic β-Ketoesters Using a Homoconjugate Addition/Decarboxylative Dieckmann Annulation Strategy

The construction of arene-fused cyclic β-ketoesters from 2-iodoaryl esters and 1,1-cyclopropane diesters is detailed. The synthetic method takes advantage of a CuI·SMe₂-mediated homoconjugate addition followed by a decarboxylative Dieckmann cyclization to afford valuable polycyclic building blocks. Various iodoaryl esters and 1,1-cyclopropane diesters were evaluated, and the limitations of both reactions are discussed. Several mechanistic probes are detailed and synthetic applications are described.

Asymmetric Total Synthesis of Lancifodilactone G Acetate. 2. Final Phase and Completion of the Total Synthesis

The asymmetric total synthesis of lancifodilactone G acetate was accomplished in 28 steps. The key steps in this synthesis include (i) an asymmetric Diels-Alder reaction for formation of the scaffold of the BC ring; (ii) an intramolecular ring-closing metathesis reaction for the formation of the trisubstituted cyclooctene using a Hoveyda-Grubbs II catalyst; (iii) an intramolecular Pauson-Khand reaction for construction of the sterically congested F ring; (iv) sequential cross-metathesis, hydrogenation, and lactonization reactions for installation of the anomerically stabilized bis-spiro ketal fragment of lancifodilactone G; and (v) a Dieckmann-type condensation reaction for installation of the A ring. The strategy and chemistry developed for the total synthesis will be useful in the synthesis of other natural products and complex molecules.

Biomimetically inspired asymmetric total synthesis of (+)-19-dehydroxyl arisandilactone A

Complex natural products are a proven and rich source of disease-modulating drugs and of efficient tools for the study of chemical biology and drug discovery. The architectures of complex natural products are generally considered to represent significant barriers to efficient chemical synthesis. Here we describe a concise and efficient asymmetric synthesis of 19-dehydroxyl arisandilactone A—which belongs to a family of architecturally unique, highly oxygenated nortriterpenoids isolated from the medicinal plant Schisandra arisanensis. This synthesis takes place by means of a homo-Michael reaction, a tandem retro-Michael/Michael reaction, and Cu-catalysed intramolecular cyclopropanation as key steps. The proposed mechanisms for the homo-Michael and tandem retro-Michael/Michael reactions are supported by density functional theory (DFT) calculation. The developed chemistry may find application for the synthesis of its other family members of Schisandraceae nortriterpenoids.

Arisandilactone A is a natural product with a complex oxa-bridged tricyclic carbon core, making it a challenging target in total synthesis. Here the authors report an asymmetric total synthesis of its 19-dehydroxy derivative, with homo-Michael and tandem retro-Michael/Michael reactions as key steps.

Metal-free benzannulation of 1,7-diynes towards unexpected 1-aroyl-2-naphthaldehydes and their application in fused aza-heterocyclic synthesis

A novel I₂-mediated benzannulation of 1,7-diyne-involved 1,4-oxo-migration was established, providing unexpected 1-aroyl-2-naphthaldehydes. The resulting 1-aroyl-2-naphthaldehydes were successfully applied in the synthesis of polycyclic isoindoles.

Synthesis of γ-, δ-, and ε-lactams by asymmetric transfer hydrogenation of N-(tert-butylsulfinyl)iminoesters

Highly enantiomerically enriched γ- and δ-lactams have been prepared by a simple and very efficient procedure that involves the asymmetric transfer hydrogenation of N-(tert-butylsulfinyl)iminoesters followed by desulfinylation of the nitrogen atom and spontaneous cyclization to the desired lactams during the basic workup procedure. Five- and six-membered ring lactams bearing aromatic, heteroaromatic, and aliphatic substituents have been obtained in very high yields and ee's up to >99%. A slight modification of the procedure also allowed the preparation of ε-lactams in good yields and very high enantioselectivities. Both enantiomers of the final lactams could be prepared with equal efficiency by changing the absolute configuration of the sulfinyl chiral auxiliary.

A novel class of tunable cyclopropanation reagents (RXZnCH2Y) and their synthetic applications

The Simmons-Smith cyclopropanation is a widely used method to synthesize cyclopropanes from alkenes using methylene iodide and a zinc reagent. A novel class of organozinc species, RXZnCH(2)Y, has been found to efficiently cyclopropanate alkenes, including traditionally unreactive unfunctionalized alkenes. The reactivity and selectivity of this class of organozinc reagents can be regulated by tuning the electronic and/or steric nature of the RX group attached to Zn. During recent years, this class of organozinc reagent has been widely used in organic synthesis as a reagent for cyclopropanation and other useful synthetic transformations. Catalytic, asymmetric versions of this reaction have been developed providing high enantiomeric excess for unfunctionalized olefins.

A mechanistic investigation into the zinc carbenoid-mediated homologation reaction by DFT methods: is a classical donor-acceptor cyclopropane intermediate involved?

An extensive density functional theory (DFT, M05-2X) investigation has been performed on the zinc carbenoid-mediated homologation reaction of β-keto esters. The mechanistic existence of a classical donor-acceptor cyclopropane intermediate was probed to test the traditional school of thought regarding these systems. Calculations of the carbenoid insertion step, following enolate formation, unmasked two possible pathways. Pathway B was shown to explain the proposed, but spectroscopically unobservable donor-acceptor cyclopropane intermediate, while the second (pathway A) reveals an alternative to the classical intermediate in that a cyclopropane transition state leads to product.

Zinc-Mediated C−C Bond Sigmatropic Rearrangement: A New and Efficient Methodology for the Synthesis of β-Diketones

A new and efficient methodology has been developed for the synthesis of beta-diketones from aromatic alpha-bromo ketones in the presence of Furukawa reagent under mild conditions. The present transformation is proposed to proceed via a Reformatsky-type reaction of alpha-bromo ketones, followed by C-C bond sigmatropic rearrangement of the aldolate intermediate to give beta-diketones in moderate to good isolated yields, while aliphatic alpha-bromomethyl ketones resulted in the formation of 2,4-disubstituted furans or cis-1,2-disubstituted cyclopropanols in moderate yields. The scope of this process was investigated, and a tentative mechanism was proposed.

Zinc-Mediated Chain Extension Reaction of 1,3-Diketones to 1,4-Diketones and Diastereoselective Synthesis of trans-1,2-Disubstituted Cyclopropanols

[reaction: see text] A variety of 1,3-diketones can be efficiently converted into the corresponding 1,4-diketones and trans-1,2-disubstituted cyclopropanols by using organozinc species in one-pot reactions. It was found that 2.3 equiv of CF3CO2ZnCH2I was effective to give the corresponding chain-extended products in 44-85% yields, while a mixture of organozinc species formed from 4.0 equiv of Et2Zn, 2.0 equiv of CF3CO2H, and 4.0 equiv of CH2I2 resulted in the formation of trans-1,2-disubstituted cyclopropanols with quite good yields and diastereoselectivity.