Synlett

SynlettDOI: 10.1055/s-0039-1691657© Georg Thieme Verlag Stuttgart · New YorkArticle in Thieme eJournals:Table of contents  |  Full text

A simple and efficient cationic Fe(III)/TEMPO-catalyzed oxidative cyclization of aroyl hydrazones has been developed for the synthesis of 2,5-disubstituted 1,3,4-oxadiazole derivatives. The reaction offers a broad scope, good functional-group tolerance, and high yields under mild conditions in the presence of O2.

3,3-Dialkylated oxindoles (1,3-dihydro-2H-indol-2-ones), particularly those containing C3 quaternary stereogenic centers, occupy an important place in organic synthesis and drug discovery. The radical 1,2-alkylarylation of activated olefins with alkyl radicals has emerged as the most atom- and step-economical approach to 3,3-dialkylated oxindoles. This article covers important developments in the area of oxidative radical alkylation/cyclization cascade reactions of α,β-unsaturated amides toward the synthesis of alkyl-substituted oxindoles by employing a range of alkyl-radical precursors and various reaction conditions.

As a part of the ‘bottom-up’ campaign for the precise preparation of carbon nanotubes, the chemical synthesis of carbon nanohoops is observing rapid progress, with a number of milestone achievements, over the past decade. With simple carbon nanohoops (e.g. cycloparaphenylenes) now no longer elusive targets, this Synpacts article highlights latest synthetic advances to further build up nanohoops’ π-systems. Works reviewed herein include the study explaining the unsuccessful Scholl reaction method, the preparation of a carbon nanohoop consisting solely of hexabenzocoronene units, syntheses of π-extended carbon nanohoops employing the ring-closing metathesis method, and the anthracene photodimerization/cycloreversion method for anthracene-incorporated carbon nanohoop synthesis.1 Introduction2 Some Latest Syntheses of π-Extended Carbon Nanohoops3 Conclusion

Acyclic conjugated ynenediones tethered to an alcohol or carboxylic acid are converted into furanyl-substituted cyclic ethers or lactones in a single step by treatment with the tetrahydrothiophene. Modest levels of diastereocontrol can be achieved in some cases where the presence of a substituent on the tether results in the creation of a second stereogenic centre upon formation of the cyclic ether or lactone.

A palladium-catalyzed regioselective ortho-halogenation of 2-pyridyl sulfoxides via a C–H activation pathway has been reported. Under the conditions established, this reaction proceeded smoothly and could tolerate a variety of functional groups under mild conditions.

Asymmetric catalytic activities of various organocatalysts bearing double hydrogen-bonding donor units showing different pK
a values were examined for direct aldol reactions of cyclohexanone with aromatic aldehydes. Organocatalyst with motif exhibiting the highest acidity resulted in the corresponding aldol products with the highest enantioselectivity. A good correlation has been observed between the asymmetric catalytic activity for direct aldol reactions and acidities of double hydrogen-bonding donating units.

A route to the oxabicyclic cores of the HIV cytoprotective quinolone alkaloids, waltheriones C and D, is described. The approach relies on a stereospecific transannular bromoetherification followed by reductive debromination. The route can also be rendered enantioselective via enzymatic reduction of a key intermediate (>99:1 er).

A domino reaction sequence has been evaluated that begins with union of novel dihydrooxazinone precursors with 2-alkynyl-substituted benzaldehyde components through aldol condensation. Ensuing operations, including alkene isomerization, Diels–Alder, and retrograde Diels–Alder with loss of CO2 occurs in the same reaction vessel to provide polysubstituted tricyclic pyridine products.

Iminophosphoranes reacted with CS2 at –5 °C to produce the isothiocyanates, which were treated with primary amine to give thioureas in 73–91% yields. The subsequent reaction of thioureas with alkyl bromides in the presence of solid K2CO3 produced 2-alkylthiopyrimidin-4(3H)-ones in 68–88% yield via tandem intramolecular cyclization–isomerization–S-alkylation.