- Photoreactions of Phenyl-Substituted N-(Pent-4-enyl-1-oxy)pyridine-1(1H)-thiones
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A series of hitherto unknown N-(pent-4-enyl-1-oxy)pyridine-2(1H)-thiones (6) were prepared from substituted pent-4-enyl tosylates or benzylic chlorides.On irradiation with incandescent light heterocycles 6 liberated alkoxy radicals 2, which were studied for rearrangement reactions.Surprisingly, all transformations involving the 1-phenylpent-4-enyl-1-oxy radical (2a), for example, to give the substituted thioether 8, 2-bromomethyl-5-phenyltetrahydrofuran (11), or the tetrahydrofuran 14a, were not stereoselective.On the other hand 2-, 3-mono-, and 1,5-disubstituted pent-4-enyl-1-oxy radicals 2d-e and 2g cyclized in good yields and with good to excellent stereoselectivities to give the corresponding 2,4-cis- and 2,3-trans-phenyltetrahydrofurfuryl radicals 3d-e, and the trans-2-benzyl-5-methyl substituted intermediate 3g.The major reaction mode of the 4-phenylpent-4-enyl-1-oxy radical (2f) was the 6-endo cyclization, which afforded 3-phenyltetrahydropyran (13f) as the major product (endo:exo=93:7) after trapping with hydrogen donors.According to the experimental data of the present study, the unusual reactivity of the 1-phenylpent-4-enyl-1-oxy radical (2a) in 5-exo-trig ring closures could be caused by a coplanar arrangement of the benzyloxy moiety in the transition state of the cyclization.This interaction would lock the radical center in 2a in a preferred conformation, which would result in similar steric effects for both cis- and trans-cyclizations. - Keywords: alkoxy radicals; cyclizations; pyridinethiones; radicals; tetrahydrofurans
- Hartung, Jens,Hiller, Margit,Schmidt, Philipp
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p. 1014 - 1023
(2007/10/03)
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- Some Studies on syn-Elimination Reactions
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On heating to 220 deg C the 2-alkylthiopyridine N-oxide (6) was found to undergo syn-elimination giving ethyl acrylate.Sulphoxide (15) was more prone to syn-elimination than sulphoxide (16) demonstrating the importance of β-hydrogen acidity in the rate of sulphoxide syn-eliminations.
- Crich, David,Lin, Linda B. L.
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p. 2928 - 2941
(2007/10/02)
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