Copper‐Catalyzed Sulfonyl Radical‐Enabled Regioselective Cyclization of 1,6‐Enynes
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Abstract A copper‐catalyzed cascade cyclization of 1,6‐enynes with sulfonyl hydrazides for producing various sulfonyl‐substituted γ ‐lactams is described. This sulfonyl radical‐enabled cyclization reaction features excellent regioselectivity, mild conditions and broad substrate scope, enabling the formation of new C−S and C−C bonds through radical sulfonylation, intramolecular cyclization and hydrogen abstraction. Moreover, relevant experiments have been carried out to probe a possible mechanism.Keywords:
Sulfonyl
Radical cyclization
Hydrogen atom abstraction
An unprecedented tandem radical sulfenylation/cyclization reaction of N-arylacrylamides with sulfonyl hydrazides has been developed in the presence of iodine for the selective synthesis of 3-(sulfenylmethyl)oxindoles and 3-sulfenyl-3,4-dihydroquinolin-2(1H)-ones. Preliminary mechanistic studies showed that sulfonyl hydrazides decomposed completely at an early stage to thiosulfonates and disulfides, both of which underwent tandem radical sulfenylation/cyclization with N-arylacrylamides at a late stage.
Sulfonyl
Tandem
Radical cyclization
Radical initiator
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β-Aminoenones react with monoalkyl hydrazines to give regioselectively 1,3,5-trisubstituted pyrazoles. The mechanism and level of regioselectivity depend on both the substitution pattern of the substrates and the reaction conditions. When the least bulky substituent is attached at the β-position of the enone, a high regioselectivity is obtained, usually higher than that from equivalent β-diketones. If the β-substituent is the bulkiest group, the regioselectivity decreases. Nevertheless, in the conditions reported in this paper, it is possible to prepare pyrazoles not obtainable from β-diketones.
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Pauson–Khand reaction
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Pyrazole is the ubiquitous sub-structure of many natural products and biologically active compounds. In this respect, its regioselective synthesis drew a lot of attention from synthetic organic chemists, leading to the development of various regioselective modifications of the classical Knorr cyclocondensation reaction and alternative methods. This review covers the period of 2003 to the middle of 2009 on the regioselective synthesis of pyrazoles, which are categorized into four reaction types: (i) modified Knorr condensation reaction using 1,3-dicarbonyl surrogates; (ii) 1,3-dipolar cycloadditions approach; (iii) regioselective direct substitution reaction of the pyrazole ring system; and (iv) other methods. Keywords: Pyrazole, regioselective dipolar cycloaddition, regioselective condensation, regioselective cross-coupling
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Two Se-phenyl 7-cyclopropyl(selenohept-6-enoate) esters were prepared and their reactions with tributylstannane and a free-radical initiator were studied. In both cases the initial acyl radical cyclization proceeded smoothly to give cyclopropylmethyl radicals which then suffered clean ring opening to homoallylic radicals. In the case of the initial substrate carrying a phenyl group at the 7-position a further radical rearrangement involving δ-hydrogen abstraction occurred to give, after chain transfer with the stannane, an α-alkylidenecyclohexanone.
Stannane
Hydrogen atom abstraction
Radical cyclization
Tandem
Fragmentation
Free-radical reaction
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Abstract A copper‐catalyzed cascade cyclization of 1,6‐enynes with sulfonyl hydrazides for producing various sulfonyl‐substituted γ ‐lactams is described. This sulfonyl radical‐enabled cyclization reaction features excellent regioselectivity, mild conditions and broad substrate scope, enabling the formation of new C−S and C−C bonds through radical sulfonylation, intramolecular cyclization and hydrogen abstraction. Moreover, relevant experiments have been carried out to probe a possible mechanism.
Sulfonyl
Radical cyclization
Hydrogen atom abstraction
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We highlighted the applications of sulfonyl hydrazides in radical cyclization of alkenes, which generally has two radical conversion modes, including sulfonyl radicals and sulfoxide radicals, with an emphasis on their reaction mechanisms.
Sulfonyl
Radical cyclization
Radical initiator
Free-radical reaction
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Abstract A perfectly regioselective and sequential method for the preparation of orthogonally protected glucopyranosides has been developed. An acyl group was introduced at C(4)‐OH by organocatalysis with >99 % regioselectivity. TBDPS, Boc, and BOM groups were sequentially introduced into the 4‐ O ‐acyl‐glucopyranoside at C(6)‐OH, C(2)‐OH, and C(3)‐OH, respectively, with ca. 100 % regioselectivity in each step.
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Abstract The 1‐halo‐5‐hexenyl phenyl sulfones (I) undergo radical induced ring closure to yield the cyclopentyl sulfone (IIa), together with the fused thiopyran dioxides (III).
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Tandem
Radical cyclization
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ABSTRACT N-Chloro-2,3,4,4,5,6-hexachlorocyclohexa-2,5-dienylideneamine was used as a new, mild and highly regioselective chlorinating reagent in the chlorination of phenol and o-cresol in CCl4, DMF and CH3CN. The effects of C2H5OH, C5H5N, DMF and Et3N on the regioselectivity in CCl4 have also been examined.
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