Selectivity in Garratt–Braverman Cyclization of Aryl-/Heteroaryl-Substituted Unsymmetrical Bis-Propargyl Systems: Formal Synthesis of 7′-Desmethylkealiiquinone

Tetradehydro-Diels-Alder Reactions of Flexible Arylalkynes via Folding Inside a Molecular Cage

The tetradehydro-Diels-Alder (TDDA) reaction is a useful transformation for the rapid assembly of polycyclic scaffolds from simple linear precursors in a single synthetic step. However, the reaction requires careful substrate design and harsh reaction conditions to overcome the inherent entropic cost for this transformation. Herein we report an efficient site-selective TDDA transformation within a self-assembled Pd6L4 cage. Despite the large size, the flexibility of the employed substrates allows for efficient encapsulation within the host cavity. The rate of thermal cyclization of the encapsulated guest was found to be greatly enhanced, and high product selectivity for an unsymmetrical substrate was observed. The efficiency of this system relies on the precise conformational control of the substrate within the confined space of the host cage.

Synthesis of fused 1,2-naphthoquinones with cytotoxic activity using a one-pot three-step reaction

A method for the synthesis of fused 1,2-naphthoquinones, as analogues of biologically active natural terpene quinones, is described. The intermediate polycyclic naphthalenes were prepared by a one-pot palladium-catalysed process from simple alkynes, one of which was made from an optically pure biomass-derived levoglucosenone. The prepared methoxy-substituted naphthalenes were subsequently transformed in one step to 1,2-naphthoquinones by a trivalent-iodine-mediated oxidation. The naphthoquinone products were found to have cytotoxic properties.

Protecting-Group-Free Total Synthesis and Biological Evaluation of 3-Methylkealiiquinone and Structural Analogues

The modular protecting-group-free total synthesis of 3-methylkealiiquinone, an analogue of the marine alkaloid kealiiquinone, was accomplished in seven steps. A regioselectively constructed functionalized arylbenzimidazolone moiety and dimethyl squarate were used as the only two building blocks. A thermal ring expansion via 6π-conrotatory ring closure to build the quinone fragment gave rise to the desired linear analogue of the natural compound along with a nondescribed structurally attractive angular naphtho[1,2- d]imidazole regioisomer. The IC₅₀ values for the compounds were determined on three cancer cell lines.

Total synthesis of kealiiquinone: the regio-controlled strategy for accessing its 1-methyl-4-arylbenzimidazolone core

A practical, concise and straightforward total synthesis of kealiiquinone 1, a naphtho[2,3-d]imidazole alkaloid obtained from the Micronesian marine sponge Leucetta sp. was accomplished. The squaric acid chemistry to construct the 1,4-quinoid ring and the regioselective N-methylation through a benzo[c][1,2,5]selenadiazolium heterocycle are the key features in this report. The full details of the representative approaches involving the different attempted synthetic strategies are also presented. Finally a successful total synthesis of this complex secondary metabolite is described.

A practical, concise and straightforward total synthesis of kealiiquinone 1, a naphtho[2,3-d]imidazole alkaloid obtained from the Micronesian marine sponge Leucetta sp. was accomplished.

Does the Mechanism of the Garratt-Braverman Cyclization Differ with Substrates? A Computational Study on Bispropargyl Sulfones, Sulfides, Ethers, Amines, and Methanes

We studied the variation in mechanism among different bispropargyl substrates-sulfone, sulfide, ether, amine, and methane-toward Garratt-Braverman (GB) cyclization using density functional theory calculations. Isomerization and cycloaddition are the key steps in the GB cyclization. To compare the reactivity among the various substrates, we computed the free energy of activation (ΔG(⧧)) for the cycloaddition and the cyclization steps, whereas we used the theoretically computed pKa values for the isomerization steps. Our results suggest that the sulfones undergo a relatively fast isomerization followed by slower cyclization, while the ethers undergo a slow isomerization followed by easy cyclization. The methanes and amines are similar to the ethers, and the sulfides showed intermediate behavior. We extended our study to unsymmetrical substrates and compare the results with experiments that suggest the isomerization to be the rate-limiting step for bispropargyl ethers, while cyclization through a diradical intermediate is crucial to the rate for the bispropargyl sulfones. On the basis of these findings, we made predictions on the selectivity of unsymmetrical bispropargyl sulfones, amines, methanes, and sulfides. This is the first detailed mechanistic study on the GB cyclization of bispropargyl substrates other than sulfones.

Synthesis of 4,5,6-trisubstituted-1,3-dihydroisobenzofurans by virtue of palladium-catalyzed domino carbopalladation of bromoenynes and internal alkynes

An efficient hetero-annulation protocol has been developed for the construction of 4,5,6-trisubstituted-1,3-dihydroisobenzofuransviapalladium-catalyzed domino carbopalladation of bromoenynes and internal alkynes.

Synthesis of benzochromenes and dihydrophenanthridines with helical motifs using Garratt–Braverman and Buchwald–Hartwig reactions

X-ray structures of dihydrophenanthiridine (a) and benzochromene (b) showing greater helicity in (a).