respect to the 0=C-C=C bond is important to achieve good diastereoselectivity in radical addition (Figure 1). In contrast to the amide auxiliary the conformation of the ester group exists as an equilibrium mixture of s-cis and s-trans isomers.3•12 Radical cyclization of (-)-8-phenylmenthyl (£)-2-octen-7-ynoate (l)13 and (-)-S-phenylmenthyl (£)-2-nonen-8-ynoate (2)13 under the conditions reported by Oshima and Utimoto (1 or 2, 90 mM; n-Bu3SnH, 1.5 equiv; Et3B, 1.1 equiv; toluene, 0 °C, under Ar)14 afforded (2-methylenecyclopentyl)acetate 5 and (2-methylenecyclohexyl)acetate 6, respectively, in good yield. However, only modest diastereoselectivity was observed (Table 1, runs 1 and 5)•15
Lewis Acid Promoted Diastereoselective Radical Cyclization Using Chiral α,β-Unsaturated Esters.-An efficient and convenient diastereoselective radical cyclization is presented, which shows a high degree of β-selectivity. The presence of a Lewis acid is essential to achieve a high level of selectivity, otherwise both products are obtained in a nearly 1:1 ratio. Formation of six-membered rings also proceeds with high diastereoselectivity even at 0 • C. -(NISHIDA, M.; UEYAMA, E.; HAYASHI, H.; OHTAKE, Y.; YAMAURA, Y.; YANAGINUMA, E.; YONEMITSU, O.; NISHIDA, A.; KAWAHARA, N.; J. Am. Chem. Soc. 116 (1994) 14, 6455-6456; Fac. Pharm.
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