Radical 4-penten-1 -oxyl

Figure 1.17 Ranges of rate constants for 5-exo-trig cyclization of 5-Hexen-l-yl, 4-Pentenyl-l-aminyl, 4-Pentenyl-l-aminium, and 4-Penten-l-oxyl radicals.

The 4-penten-l-oxyl radical (5) has been generated from the corresponding nitrite [39, 40], hydroperoxide [41], peroxycobaloxime [42], benzene (9-sulfenate... 

Scheme 3. Reactions top) and transition structures bottom) of 4-penten-l-oxyl radieal cyclizations. Quoted yields refer to c o(Bu3SnH) = 0.44 M. Shaded balls denote atoms and positions which are important in regiocontrol (6-endo-trig-reaction of 5) or in 5-exo-trig stereocontrol in substituted 4-penten-l-oxy 1-radicals (for example 12, Scheme 4, or 19 and 22, Scheme 5) [16, 45, 46]...

Cyclization of 2-substituted 4-penten-l-oxyl radicals leads to 2,4-cis-disubstituted tetrahydrofurans. The m-stereoselectivity improves in going from CH3 to C6H5 to C(CH3)3. 

Scheme 5. Stereoselective cyclization of substituted 4-penten-l-oxyl radicals 19 and 22 [16, 46[...

Exceptions to these general guidelines are observed in ring closure reactions of the l-phenyl-4-penten-l-oxyl radical and its /xrra-substituted derivatives, which provide both diastereomeric 5-exo-trig cyclization products in equal amounts [36, 48]. 

Cyclizations of 5-alkyl-substituted 4-penten-l-oxyl radicals are faster and frequently more selective than those of terminal unsubstituted derivatives [48, 53]. This finding is in accord with the electrophilic nature of alkoxyl radicals in addition reaction to C-C double bonds. 5-Alkyl- or 5-phenyl-substituted 4-pentenoxyl radicals, such as intermediates 25 or 26, were generated from a number of different sources. For example, alkyl nitrites [39], A-alkoxypyridine-2(l//)-thiones [46], and A-alkoxy-(/ -chlorophenyl)thiazole-2(3/f)-thiones [54] in photochemically induced reactions, and A-alkoxyphthalimides [55] or type IV radical precursors [53] in thermally initiated reactions have been applied for this purpose (Scheme 6). 

Hartung, J. and Kneuer, R., Ring closure reactions of disubstituted 4-penten-l-oxyl radicals towards a stereoselective synthesis of aflo-muscarine, Eur. J. Org. Chem., 1677, 2000. 

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