[3 + 3] Annulation Based on 6-Endo-Trig Radical Cyclization: Regioselectivity and Diastereoselectivity
作者:Dale E. Ward、Yuanzhu Gai、Brian F. Kaller
DOI:10.1021/jo00129a024
日期:1995.12
The development of a [3 + 3] annulation strategy based on sequential ''two-electron'' and ''one-electron'' allylation of beta-substituted aldehydes and derivatives with the bifunctional isobutene conjunctive reagent 1 is described. The key step involves an unusual 6-endo-trig radical cyclization. Yields of 6-endo products are improved if the PhS group is oxidized to a PhSO(2) group prior to cyclization. The structural factors affecting the regioselectivity and stereoselectivity of the cyclization are examined. In general, the stereoselectivity of 6-endo-trig cyclization of 5-hexenyl radicals can be rationalized by conformational analysis of chairlike transition states and can be calculated effectively with an MM2 force field model, High 6-endo regioselectivity requires a strong driving force. Fragmentable allylic groups (R(3)Sn, PhSO(2), and to a lesser extent PhS) are shown to be sufficiently activating to achieve 6-endo regioselectivity.
[3+3] Annulation by sequential two electron and one electron allylation
作者:Dale E. Ward、Brian F. Kaller
DOI:10.1016/s0040-4039(00)92100-2
日期:1991.2
3-phenylthio-2-(trimethylsilymethyl)propene is a convenient conjunctive reagent for the preparation of methylenecyclohexanes via a [3+3] annulation. The trimethylsilyl group facilitates the Lewisacid catalyzed allylation of an aldehyde or acetal while the phenylthio group directs a 6-endo-trig radical cyclization reaction.