Radical cyclization by ipso substitution of the methoxy group: considerable effect of HMPA on samarium-mediated cyclization.
Alkyl radicals generated by treatment of thiocarbamates of conformationally favorable 3-alkyl-3-arylpropan-1-ols with tris(trimethylsilyl)silane and AIBN efficiently undergo intramolecular ipso substitution of the methoxy group, yielding the corresponding cyclized products. In contrast, either conformationally favorable or flexible 1-arylalkan-3- or 4-ones easily cyclize into five- or six-membered condensed rings by treatment with SmI(2) via ketyl radical intermediates. The addition of HMPA as cosolvent dramatically changes the cyclization mode of the SmI(2)-induced reaction, and the para-cyclization products are exclusively formed. This "HMPA effect" can be rationalized by the strong chelating ability of HMPA with the samarium atom.
Ohno
Hiroaki
H
Graduate School of Pharmaceutical Sciences, Osaka University, 1-6 Yamadaoka, Suita, Osaka 565-0871, Japan.
Wakayama
Ryutaro
R
Maeda
Shin-ichiro
S
Iwasaki
Hiroki
H
Okumura
Mitsuaki
M
Iwata
Chuzo
C
Mikamiyama
Hidenori
H
Tanaka
Tetsuaki
T