Gold-Catalyzed Nucleophilic Ring-Opening Reaction of 2-Alkynylazetidines with Alcohols.

The reaction of 2-alkynylazetidines and alcohols with a gold catalyst is described. A variety of substituted δ-amino-α,ß-unsaturated ketones were synthesized via gold-promoted nucleophilic attack of alcohols followed by ring-opening of azetidine ring.

Indium(III) bromide-mediated ß-selective thioglycosylation of 1,2,4-O-orthoacetylglucose derivatives.

1,2,4-O-Orthoacetyl-α-d-glucose possesses a skew-boat glucopyranose ring whose steric strain is expected to show high reactivity. This study describes the ß-selective thioglycosylation of 1,2,4-O-orthoacetyl-α-d-glucose. Indium(III) bromide catalyzes the reaction in trifluoromethylbenzene at ambient temperature in the presence of molecular sieves 4A, resulting in the corresponding thioglycoside product with perfect ß-selectivity and high yields. The presented glycosylation might be useful for the synthesis of functional molecules and natural products possessing sugar moieties.

Catalytic and Aerobic Oxidative Biaryl Coupling of Anilines Using a Recyclable Heterogeneous Catalyst for Synthesis of Benzidines and Bicarbazoles.

In this study, a heterogeneous rhodium-catalyzed oxidative homocoupling reaction of anilines utilizing molecular oxygen as the sole oxidant is reported. Employing a commercially available and recyclable Rh/C catalyst enabled the oxidative dimerization of various anilines, including N,N-disubstituted and N-monosubstituted anilines, as well as diarylamines, triarylamines, and carbazoles. Additionally, the catalytic protocol was extended to the ortho-ortho coupling of anilines, affording 2,2'-diaminobiphenyls with high regioselectivity. Notably, the developed approach provides rapid access to diversely functionalized benzidines and diaminobiphenyls in an operationally simple, practical, and environmentally friendly manner.

Synthesis of an Ellagitannin Component, the Macaranoyl Group with a Tetra-ortho-Substituted Diaryl Ether Structure.

Herein, a practical synthesis of the macaranoyl group contained in ellagitannins, i.e., a C-O digallate structure with a tetra-ortho-substituted diaryl ether bond, is described. The methodology involved an oxa-Michael addition/elimination reaction between a brominated ortho-quinone monoketal and a phenol with a hexahydroxydiphenoyl moiety in the presence of 18-crown-6 under dark conditions, followed by reductive aromatization. The existence of rotamers originating from the constructed ether moiety is discussed as well.

Synthesis of diaryl ether components of ellagitannins using ortho-quinone with consonant mesomeric effects.

Methods for synthesizing C-O digallate structures, the basic unit of diaryl ether components of natural ellagitannins, are described. In the designed building block derived from gallic acid, consonantly overlapped mesomeric effects enhanced its electrophilicity. This building block demonstrated substantial reactivity to improve the synthesis of dehydrodigalloyl, tergalloyl, and valoneoyl groups.

Aerobic Intramolecular Oxidative Coupling Using Heterogeneous Catalyst for Synthesis of N-Substituted 2,3,6,7-Tetramethoxycarbazoles.

A heterogeneous metal catalyst enabled the intramolecular oxidative coupling of diarylamines to form carbazoles with molecular oxygen as the sole oxidant. Rh/C had efficient catalytic activity and allowed the catalyst loading to be reduced to 0.1 mol% while maintaining excellent yields of carbazoles. This reaction is operationally simple in an open-to-air setup, and provides a green and atom-economical process for an efficient synthetic approach to N-substituted carbazoles.

A Highly Selective Palladium-Catalyzed Aerobic Oxidative Aniline-Aniline Cross-Coupling Reaction.

The first catalytic oxidative aniline-aniline cross-coupling reaction using oxygen as the terminal oxidant is reported. Anilines possessing a pyrrolidino group can be preferentially oxidized under mild aerobic conditions and reacted with other anilines to afford a variety of nonsymmetrical 2-aminobiphenyls with high selectivities. A heterogeneous palladium catalyst is used for the dehydrogenative cross-coupling of anilines with structurally diverse arenes. This reaction does not require stoichiometric oxidants and is an economical and environmentally friendly method.

Structural Revisions in Natural Ellagitannins.

Ellagitannins are literally a class of tannins. Triggered by the oxidation of the phenolic parts on ß-pentagalloyl-d-glucose, ellagitannins are generated through various structural conversions, such as the coupling of the phenolic parts, oxidation to highly complex structures, and the formation of dimer and lager analogs, which expand the structural diversity. To date, more than 1000 natural ellagitannins have been identified. Since these phenolic compounds exhibit a variety of biological activities, ellagitannins have potential applications in medicine and health enhancement. Within the context of identifying suitable applications, considerations need to be based on correct structural features. This review describes the structural revisions of 32 natural ellagitannins, namely alnusiin; alnusnin A and B; castalagin; castalin; casuarinin; cercidinin A and B; chebulagic acid; chebulinic acid; corilagin; geraniin; isoterchebin; nobotanin B, C, E, G, H, I, J, and K; punicalagin; punicalin; punigluconin; roxbin B; sanguiin H-2, H-3, and H-6; stachyurin; terchebin; vescalagin; and vescalin. The major focus is on the outline of the initial structural determination, on the processes to find the errors in the structure, and on the methods for the revision of the structure.

A unified strategy for the synthesis of highly oxygenated diaryl ethers featured in ellagitannins.

Ellagitannins are a family of polyphenols containing more than 1,000 natural products. Nearly 40% of these compounds contain a highly oxygenated diaryl ether that is one of the most critical elements to their structural diversity. Here, we report a unified strategy for the synthesis of highly oxygenated diaryl ethers featured in ellagitannins. The strategy involves oxa-Michael addition of phenols to an orthoquinone building block, with subsequent elimination and reductive aromatization. The design of the building block--a halogenated orthoquinone monoketal of gallal--reduces the usual instability of orthoquinone and controls addition/elimination. Reductive aromatization is achieved with perfect chemoselectivity in the presence of other reducible functional groups. This strategy enables the synthesis of different diaryl ethers. The first total synthesis of a natural ellagitannin bearing a diaryl ethers is performed to demonstrate that the strategy increases the number of synthetically available ellagitannins.

Roxbin B is cuspinin: structural revision and total synthesis.

Prompted by the outcome that the synthesized roxbin B was not identical to the natural roxbin B, the structural determination process and spectral data were re-examined, with the finding that roxbin B was very likely to be 1-O-galloyl-2,3-(R);4,6-(S)-bis-O-hexahydroxydiphenoyl-ß-d-glucose (cuspinin). Because the (R)-axial chirality is rare in natural products when the hexahydroxydiphenoyl group bridges the 2- and 3-oxygens, the proposed structure of cuspinin was confirmed by the total synthesis, leading to the conclusion that roxbin B is the same as cuspinin.

High-yield total synthesis of (-)-strictinin through intramolecular coupling of gallates.

This paper describes a total synthesis of (-)-strictinin, an ellagitannin that is 1-O-galloyl-4,6-O-(S)-hexahydroxydiphenoyl (HHDP)-ß-D-glucose. In the study, total efficiency of the synthesis was improved to produce a 78% overall yield in 13 steps from D-glucose. In the synthesis, formation of the 4,6-(S)-HHDP bridge including the 11-membered bislactone ring was a key step, in which intramolecular aryl-aryl coupling was adopted. The coupling was oxidatively induced by CuCl2-n-BuNH2 with perfect control of the axial chirality, and the reaction conditions of this coupling were optimized thoroughly to achieve the quantitative formation of the bridge.