Zhang H, Hay EB, Geib SJ, Curran DP * University of Pittsburgh, USA
Radical Cyclizations of Cyclic Ene Sulfonamides Occur with β-Elimination of Sulfonyl Radicals to Form Polycyclic Imines.
J. Am. Chem. Soc. 2013;
135: 16610-16617
Key words
polycyclic imines - radical cyclization - β-sulfonyl radical elimination
Significance
Reported is a radical cyclization of cyclic ene sulfonamides to give stable bicyclic and tricyclic imines in good yields. Mechanistically, an α-sulfonamidoyl radical, which is generated after the initial radical cyclization, undergoes β-elimination to form the imine product and a phenylsulfonyl radical. In a related method, 3,4-dihydroquinolines can also be synthesized by radical translo-cation (1,5-hydrogen transfer) reactions of N-(2-halophenylsulfonyl)tetrahydroisoquinolines. In both cases, the very stable sulfonamides are cleaved under mild reductive conditions.
Comment
Imines are highly versatile synthetic intermediates, which are commonly made by condensing the corresponding aldehydes and ketones with amines. The radical cyclization methodology described in this paper provides alternative bond construction and reaction conditions. It is also synthetically useful due to the facile cleavage of the strong N–SO2Ar bond; these are hard to remove otherwise, for example, by hydrolysis or reduction.
