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Photoreactions of Phenyl‐Substituted N ‐(pent‐4‐enyl‐1‐oxy)Pyridine‐2(1 H )‐Thiones
Author(s) -
Hartung Jens,
Hiller Margit,
Schmidt Philipp
Publication year - 1996
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.19960020818
Subject(s) - chemistry , radical , pyridine , steric effects , thioether , medicinal chemistry , moiety , reactivity (psychology) , tetrahydrofuran , ring (chemistry) , stereochemistry , organic chemistry , medicine , alternative medicine , pathology , solvent
A series of hitherto unknown N ‐(pent‐4‐enyl‐1‐oxy)pyridine‐2(1 H )‐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‐bro‐momethyl‐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 ‐phenyltetra‐hydrofurfuryl 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.