Auxiliary to the Site Undergoing Reaction

Control of the configuration of the alkene center remote from an auxiliary group presents a challenge. For auxiliary groups such as the dimethylpyrrolidine amide the remote carbon of the alkene is beyond the reach of the auxiliary. Thus, addition of cyclohexyl radical to 10 proceeds without regioselectivity, and stereoselectivity is only observed for the products formed by addition to the end of the alkene bearing the auxiliary group [8]. 

Different strategies have been developed to solve this problem. Curran and his collaborators have developed auxiliaries based upon Rebek s imide, shown in the unsymmetrical alkene 11 [18, 19]. Addition of /ert-butyl radical to 11 proceeds with good regio- and stereoselectivity. Thus, addition occurs at 0°C with 97 3 regioselectivity, preferential addition being on the carbomethoxy-bearing carbon. The major regioisomers are formed as an 88 9 mixture of diastereomers. The major di-astereomer formed results from addition to the top face of the alkene, as shown in structure 11. 

Oxazolidinones have proven to be extremely useful auxiliary groups in a variety of synthetic reaction types. Thus, the Evans auxiliaries are useful in the control of configuration in enolate alkylations and concerted cycloadditions, to name a few of the more important applications. Sibi and his collaborators at North Dakota State University have pioneered the use of these auxiliary groups in radical transformations mediated by Lewis acids [21-24]. Consider the general conformational questions that arise in a carboximide, such as 12, derived from an oxazolidinone (Eq. 18). The conformer 12 is disfavored by steric factors while 13 and 14 have similar steric demands. In the absence of any chelating Lewis acid, one expects that 13 would be of lower energy than 14 because of the opposed dipoles of the anti carbonyls in this conformation. 

Conformation 14, in fact, has parallel carbonyl dipoles that destabilize this arrangement. In the presence of Lewis acids, however, this conformation can serve as 

Steric factors slow the rate of intermolecular addition of radicals to non-terminal alkenes such as 1, 2 and 10-12. In the absence of a Lewis acid additive, radical additions to the cinnamate 16 or the crotonate 17 are inefficient at —78 °C because radical reduction is faster than radical addition [21, 22, 25]. The reaction is also nonselective in the absence of Lewis acid. When stoichiometric amounts of Lewis acids are added to the reaction of isopropyl radical with 16, however, both the yield and the diastereoselectivity increase significantly. Thus, Yb(OTf)3 gives a product yield of 90% in a ratio of 18a/19a of 45 1. This record P diastereoselectivity in radical addition is comparable to or better than that obtained under ionic conditions. Use of catalytic Yb(OTf)3 (10 mol%) gives only a slight reduction in selectivity. Addition to the crotonate 17 promoted by Yb(OTf)3 gives a product ratio of 18b/ 19b of 25 1. 

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