Synlett

para-Quinones feature extensively as targets and/or intermediates throughout a number of chemical and biological subdisciplines, highlighting the importance of efficient preparative methods. This Synpacts article provides an overview of ring forming approaches to para-hydroquinones and para-benzoquinones, concluding with our recent contribution concerning the development of 2,5-bis(tert-butyldimethylsilyloxy)furans as vicinal bisketene equivalents in the Diels–Alder reaction.1 Introduction2 Ring Forming Approaches to para-Quinones2.1 Hauser–Kraus Annulation2.2 Moore–Liebeskind Rearrangement2.3 Wulff–Dötz Reaction2.4 Oxidative Bergman Cyclization2.5 Diels–Alder Strategies2.5.1 Ketene–Enol Equivalents2.5.2 Bisketene Equivalents3 Toward an Improved Bisketene Equivalent4 Conclusion

An amine-free sulfonylation of various alcohols was developed that uses 4-methylpyridine N-oxide in the presence of 4Å molecular sieves at room temperature. This mild method gives various sulfonylated products in high yields, and can be applied to base-sensitive substrates.

A synthesis of 3-alkyl-2-arylindoles was performed by sequential oxidation and reduction of 2-(2-nitrophenyl)ethanols that were prepared by base-catalyzed three-component reactions of vinylarenes, aldehydes, and various pronucleophiles, including nitroalkanes, thiols, and malonates. In addition to indoles, a selective synthesis of an N-hydroxyindole was accomplished. The highly nucleophilic character of transient benzylic anions in DMSO was also clarified for the three-component reactions.

Amidation of amino acids using dichloro(methyl)(3,3,3-trifluoropropyl)silane (MTFPSCl2) and imidazole is described. MTFPSCl2 activates the carboxy group and protects the α-amino group of amino acids. The amidation proceeded with 19 amino acids and 19 amines, including α-branched amines and anilines; the corresponding amino acid amides were synthesized in good-to-high yields (up to 96%) with low-to-no racemization.

A series of polyoxometalate-ionic liquid catalysts that combine the features of a polyoxometalate and an ionic liquid, with the introduction of acidity and miscibility, have been developed to promote the Ritter reaction. Among them, [BSmim]CuPW12O40 [BSmim = 1-methyl-3-(4-sulfobutyl)-1H-imidazol-3-ium] displayed the highest activity for the amidation of a variety of alcohols with nitriles, delivering the corresponding amide products in good to excellent yields. Furthermore, the reaction can be easily scaled up to a gram scale without losing efficiency. This process therefore provides an appealing way to prepare amides by a Ritter reaction using a polyoxometalate–ionic liquid-based catalyst.

In the absence of transition-metal catalysts and additional oxidants, arylhydroxylamines can undergo an unconventional O-sulfinylation/[2,3]-sigmatropic rearrangement/rearomatization cascade with trifluoromethylsulfinyl chloride to rapidly and efficiently synthesize versatile ortho-sulfonylated aromatic amines. This Synpacts article describes the discovery, further study, and practical application of the rearrangement reactions in arylhydroxylamine compounds and highlights our recent advances in this area.1 Introduction2 Discovery of a New Reaction3 O-Sulfinylation of Arylhydroxylamines Followed by [2,3]-σ Rearrangement4 Conclusion

The design and synthesis of new macrocycles with well-defined cavities represent a promising avenue for the development of new supramolecular hosts. Moreover, the ability to diversify a macrocycle through chemical manipulations enables the fine-tuning and tailoring of properties. In this report, the synthesis and functionalization of thiapillar[6]arene, a pillar[6]arene analogue in which the bridging methylene groups are replaced by sulfurs, are described. First, we demonstrate the scalable synthesis of the parent thiapillar[6]arene. Next, the diversification of thiapillar[6]arene is demonstrated via functionalization of the phenols and oxidation of the sulfur atoms. The solid-state structures of two thiapillar[6]arene derivatives are reported, and the effect of sulfur oxidation state on the macrocyclic conformation is discussed. All sulfone derivatives described were found to demonstrate high luminescence quantum yields (ΦF = 0.43–0.66) in CH2Cl2 with emission maxima between λ = 404 and 462 nm. Lastly, assessment of the electrochemical properties of the sulfone derivatives by square-wave voltammetry revealed electron-accepting ability owing to the oxidation of the sulfur atoms, with four reduction events observed for the analogues surveyed. Overall, this work implicates thiapillar[6]arene as a modular scaffold amenable for further applications in host–guest chemistry and sensing.

o-Iodoxybenzoic acid (IBX) is an oxidation reagent that has surged into prominence in the last two decades. It is cost-effective, environmentally benign, and readily prepared from o-iodobenzoic acid. However, its insolubility in common organic solvents and explosive attributes upon impact and heating are debilitating disadvantages. The development of modified IBXs (mIBXs) that exhibit improved solubility and enhanced reactivity, and obviate explosive attributes by judicious manipulation of the structure of IBX has been an incessant endeavor. In this account, common organic solvent-soluble mIBXs developed in our research group are collated with a discussion of the rationale underlying the design principles. Steric build-up around the iodoxolone moiety that is responsible for strong intermolecular interactions within the crystal lattice of IBX constitutes the key consideration in the design and development of modified λ5-iodanes that are reactive and sparingly soluble in common organic solvents. In situ generation of mIBXs from precursor iodo-acids in the presence of Oxone® permits their employment as organocatalysts for facile oxidative transformations. Reactive mIBXs generated in situ from precursor modified iodo-acids (mIAs, II) in the presence of Oxone® may offer unrivaled prospects for cost-effective oxidations. Applications of mIBXs, generated in situ or otherwise, for efficient oxidations are consolidated.1 Introduction2 Design and Synthesis of Modified IBXs and their Precursors Iodo-Acids3 Catalytic Oxidations with Modified IBXs3.1 Oxidation of Alcohols3.2 Oxidation of 1,2-Diols3.3 Conversion of Diols into Lactones3.4 One-Pot Oxidative Cleavage of Olefins3.5 One-Pot Transformation of Olefins into α-Bromo- and α-Azidoketones4 Conclusions

A phosphine-promoted intermolecular annulation reaction of functionalized 3-benzoyl coumarin with alkynone has been disclosed. This reaction was found to be highly dependent on the nucleophilicity of the phosphine. Two classes of coumarin-fused cyclopentanones were selectively afforded in moderate to good yields with excellent diastereoselectivities under the mild reaction conditions.

The conventional synthesis of glycosyl chlorides from thioglycosides relies on sequential oxidation and chlorination. A one-pot synthesis of glycosyl chlorides is warranted as an alternative method. Here, we report a one-pot synthesis of glycosyl chlorides from thioglycoside precursors. The transformation was mediated at low temperatures by bromodiethylsulfonium bromopentachloroantimonate (BDSB) as a mild oxidant with Bu4NCl as an additive. Armed thioglycosides afforded the corresponding α-glycosyl chlorides in moderate to good yields under the optimized conditions. Low conversions and yields were obtained when the less-reactive disarmed thioglycosides were used. Unexpectedly, BDSB-mediated oxidation of thioglycosides without the addition of Bu4NCl also afforded the α-glycosyl chlorides in moderate yields. We suggest a mechanism involving the transfer of chloride ions from the nonnucleophilic bromopentachloroantimonate (SbCl5Br) anion to the oxocarbenium ion.