Palladium-catalyzed regioselective hydrodebromination of dibromoindoles: application to the enantioselective synthesis of indolodioxane U86192A

P4O10/TfOH mediated domino condensation-cyclization of amines with diacids: a route to indolizidine alkaloids under catalyst- and solvent-free conditions

A domino condensation–cyclization method is developed to synthesize indolizidine alkaloids using a P₄O₁₀/TfOH reagent system without the employment of either a catalyst or solvent. The use of a few aliphatic and aromatic dicarboxylic acids is shown along with various primary amines. This method is suitable for synthesizing pyrrolo[2,1-a]isoquinolines, pyrido[2,1-a]isoquinolines, and isoindolo[1,2-a]isoquinolinones in excellent yields. When phthalic acid is used, a workup with either NaBH₄ or a saturated NaHCO₃ solution provided 12b-H or 12b-OH isoindolo[1,2-a]isoquinolinones, respectively.

A highly efficient and direct methodology for the construction of pyrrolo[2,1-a]isoquinoline, pyrido[2,1-a]isoquinoline, and 12b-H and 12b-OH isoindolo[2,1-a]isoquinolinone alkaloids from commercially available synthons is devised.

Catalytic Enantioselective Synthesis of N-C Axially Chiral Mebroqualone and Its Derivatives through Reductive Asymmetric Desymmetrization

In the presence of (R)-DTBM-SEGPHOS-Pd(OAc)₂ catalyst, treatment of various 3-(2,6-dibromophenyl)quinazolin-4-ones with NaBH₄ gave optically active N-C axially chiral quinazolinone (mebroqualone) derivatives through reductive asymmetric desymmetrization (enantioselective monohydrodebromination) followed by kinetic resolution of the resulting monobromophenyl products (up to 99% ee). The enantioselectivity strongly depended on the substituent (R²) at the C4'position, amount of NaBH₄, and reaction temperature.

Enantioselective synthesis of 3-arylquinazolin-4(3H)-ones via peptide-catalyzed atroposelective bromination

We report the development of a tertiary amine-containing β-turn peptide that catalyzes the atroposelective bromination of pharmaceutically relevant 3-arylquinazolin-4(3H)-ones (quinazolinones) with high levels of enantioinduction over a broad substrate scope. The structure of the free catalyst and the peptide-substrate complex were explored using X-ray crystallography and 2D-NOESY experiments. Quinazolinone rotational barriers about the chiral anilide axis were also studied using density functional theory calculations and are discussed in light of the high enantioselectivities observed. Mechanistic studies also suggest that the initial bromination event is stereodetermining, and the major monobromide intermediate is an atropisomerically stable, mono-ortho-substituted isomer. The observation of stereoisomerically stable monobromides stimulated the conversion of the tribromide products to other atropisomerically defined products of interest. For example, (1) a dehalogenation Suzuki-Miyaura cross-coupling sequence delivers ortho-arylated derivatives, and (2) a regioselective Buchwald-Hartwig amination procedure installs para-amine functionality. Stereochemical information was retained during these subsequent transformations.

Regioselective derivatizations of a tribrominated atropisomeric benzamide scaffold

The enantioselective synthesis of atropisomeric, tribrominated benzamides and subsequent regioselective transformations to afford derivatized, axially chiral molecules is reported. The enantioenriched tribromides were carried through sequential Pd-catalyzed cross-coupling and lithium-halogen exchange with high regioselectivity and enantioretention. A variety of complexity-generation functional group installations were performed to create a library of homochiral benzamides. The potential utility of these molecules is demonstrated by using a phosphino benzamide derivative as an asymmetric ligand in a Pd-catalyzed allylic alkylation.

Synthesis of indoles, benzofurans, and related heterocycles via an acetylene-activated SNAr/intramolecular cyclization cascade sequence in water or DMSO

The synthesis of 2-substituted indoles and benzofurans was achieved by nucleophilic aromatic substitution, followed by subsequent 5-endo-dig cyclization between the nucleophile and an ortho acetylene.

Uncatalysed diaryldiazo cyclopropanations on bicyclic lactams: access to annulated prolines

The uncatalysed cycloaddition of substituted diaryldiazo compounds onto bicyclic unsaturated lactams derived from pyroglutamic acid efficiently leads to highly functionalised azatricyclononanes.

A broadly applicable and practical oligomeric (salen) Co catalyst for enantioselective epoxide ring-opening reactions

The (salen) Co catalyst (4a) can be prepared as a mixture of cyclic oligomers in a short, chromatography-free synthesis from inexpensive, commercially available precursors. This catalyst displays remarkable enhancements in reactivity and enantioselectivity relative to monomeric and other multimeric (salen) Co catalysts in a wide variety of enantioselective epoxide ring-opening reactions. The application of catalyst 4a is illustrated in the kinetic resolution of terminal epoxides by nucleophilic ring-opening with water, phenols, and primary alcohols; the desymmetrization of meso epoxides by addition of water and carbamates; and the desymmetrization of oxetanes by intramolecular ring opening with alcohols and phenols. The favorable solubility properties of complex 4a under the catalytic conditions facilitated mechanistic studies, allowing elucidation of the basis for the beneficial effect of oligomerization. Finally, a catalyst selection guide is provided to delineate the specific advantages of oligomeric catalyst 4a relative to (salen) Co monomer 1 for each reaction class.

Development of tag-free photoprobes for studies aimed at identifying the target of novel Group A Streptococcus antivirulence agents

We previously reported the identification and development of novel inhibitors of streptokinase (SK) expression by Group A Streptococcus (GAS), originating from a high throughput cell-based phenotypic screen. Although phenotypic screening is well-suited to identifying compounds that exert desired biological effects in potentially novel ways, it requires follow-up experiments to determine the macromolecular target(s) of active compounds. We therefore designed and synthesized several classes of chemical probes for target identification studies, guided by previously established structure-activity relationships. The probes were designed to first irreversibly photolabel target proteins in the intact bacteria, followed by cell lysis and click ligation with fluorescent tags to allow for visualization on SDS-PAGE gels. This stepwise, 'tag-free' approach allows for a significant reduction in molecular weight and polar surface area compared to full-length fluorescent or biotinylated probes, potentially enhancing membrane permeability and the maintenance of activity. Of the seven probes produced, the three most biologically active were employed in preliminary target identification trials. Despite the potent activity of these probes, specific labeling events were not conclusively observed due to a considerable degree of nonspecific protein binding. Nevertheless, the successful synthesis of potent biologically active probe molecules will serve as a starting point for initiating more sensitive methods of probe-based target identification.

A practical method for the synthesis of highly enantioenriched trans-1,2-amino alcohols

A highly enantioselective addition of phenyl carbamate to meso-epoxides has been developed to efficiently generate protected trans-1,2-amino alcohols. This transformation is promoted by an oligomeric (salen)Co-OTf catalyst and has been used to prepare two useful 2-aminocycloalkanol hydrochlorides in enantiopure form on a multigram scale from commercially available starting materials.

Synthesis of eudistomin C and E: improved preparation of the indole unit

An improved synthesis of the indole unit, a key intermediate for eudistomin C, was established utilizing Makosza's indole synthesis. A concise total synthesis of eudistomin E was achieved on the basis of the improved synthesis.

Chemoselective C(sp3)H Bond Activation for the Preparation of Condensed N-Heterocycles

Condensed N-heterocycles were prepared by using C-H activation reactions catalyzed by Pd(OAc)2 (5 mol %) and (p-tolyl)3P (10 mol %). The key step of these ring closures is chemoselective intramolecular C-H activation of the methyl group at position 2 of the pyrrole ring. Functionalized 9H-pyrrolo[1,2-a]indoles and pyrrolo[1,2-f]phenanthridine derivatives were prepared in good yields. The preparation of some complex N-heterocycles by using successive reactions is also described.

Acid-Catalyzed Rearrangement of Diaryl Hydrazides for the Efficient Synthesis of Functionalized 2,2‘-Diamino-1,1‘-biaryls

Diaryl hydrazides prepared from the Pd(0)- or Cu(I)-catalyzed coupling reactions of aryl hydrazides and aryl halides underwent the acid-catalyzed rearrangements to a series of previously unknown 2,2'-diaminobiaryls in good overall yields. The mildness of the reaction conditions allows the presence of various functional groups.