1,3-stereoinduction in radical reactions: Radical additions to dialkyl 2-alkyl-4-methyleneglutarates

Tin hydride-mediated radical additions to a series of alpha -methylene-glutarates 1, furnishing 2,4-dialkyl-substituted glutarates 3 are reported. The diastereoselectivity of hydrogen transfer to the intermediate adduct radicals 2, possessing a stereogenic center in gamma -position, was disappointing in the temperature range of -78 to 80 degreesC. However, the reactions proved to be able to proceed with excellent 1,3-diastereoselectivities under chelation-controlled conditions, depending on the steric impacts of 2- and 4-alkyl substituents as well as on the ester-alkyl moiety and choice of Lewis acid. Using MgBr2. OEt2 as additive, syn-selectivities of 98:2 were achieved upon initial tert-butyl radical addition at -78 degreesC. High anti-diastereoselectivities were observed in the MgBr2. OEt2-controlled pathway at 70 degreesC when smaller alkyl radicals such as cyclohexyl, ethyl, and methyl were applied. Interesting and uncommon temperature dependences were observed in the temperature range of -78 to 100 degreesC, revealing strong entropic effects in the transition states. A model that accounts for the opposed stereochemical outcomes under chelation-controlled conditions is presented.