TY - JOUR
T1 - Tandem cycloaddition chemistry of nitroalkenes
T2 - Probing the remarkable stereochemical influence of the Lewis acid
AU - Denmark, Scott E.
AU - Seierstad, Mark
PY - 1999/3/5
Y1 - 1999/3/5
N2 - The influence of several Lewis acids on the stereochemical course of the [4 + 2] cycloaddition of nitroalkene 1 and chiral, nonracemic propenyl ether 8 has been examined. All of the Lewis acids examined favored ul relative diastereoselection ('exo' approach); TiCl4, TiBr3(Oi-Pr), SnCl4, and ATPh were the most selective. Within the titanium-based Lewis acids, it was found that increasing the halide-to-alkoxide ratio increased the degree of ul (relative) selectivity, as did switching from chloride to bromide. The internal diastereoselectivity was also dependent on the Lewis acid; most titanium isopropoxide-halides (bromide and chloride) and SnCl4 were highly selective for (1,3-lk) approach, with the selectivity increasing with increasing halide content. Two aluminum-based Lewis acids (MAPh and ATPh) were selective for the opposite sense of internal diastereoselection. The high lk (relative) diastereoselectivity observed only with TiCl2(Oi-Pr)2 is proposed to arise either from Coulombic stabilization of an endo approach or precomplexation of the vinyl ether to the Lewis acid. The switch in internal diastereoselectivity seen in the exo manifold is thought to arise from subtle changes in the steric nature of the Lewis acid-nitroalkene complex.
AB - The influence of several Lewis acids on the stereochemical course of the [4 + 2] cycloaddition of nitroalkene 1 and chiral, nonracemic propenyl ether 8 has been examined. All of the Lewis acids examined favored ul relative diastereoselection ('exo' approach); TiCl4, TiBr3(Oi-Pr), SnCl4, and ATPh were the most selective. Within the titanium-based Lewis acids, it was found that increasing the halide-to-alkoxide ratio increased the degree of ul (relative) selectivity, as did switching from chloride to bromide. The internal diastereoselectivity was also dependent on the Lewis acid; most titanium isopropoxide-halides (bromide and chloride) and SnCl4 were highly selective for (1,3-lk) approach, with the selectivity increasing with increasing halide content. Two aluminum-based Lewis acids (MAPh and ATPh) were selective for the opposite sense of internal diastereoselection. The high lk (relative) diastereoselectivity observed only with TiCl2(Oi-Pr)2 is proposed to arise either from Coulombic stabilization of an endo approach or precomplexation of the vinyl ether to the Lewis acid. The switch in internal diastereoselectivity seen in the exo manifold is thought to arise from subtle changes in the steric nature of the Lewis acid-nitroalkene complex.
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U2 - 10.1021/jo9820869
DO - 10.1021/jo9820869
M3 - Article
AN - SCOPUS:0033525699
SN - 0022-3263
VL - 64
SP - 1610
EP - 1619
JO - Journal of Organic Chemistry
JF - Journal of Organic Chemistry
IS - 5
ER -