Auxiliary on the Site Undergoing Reaction

The first examples of control of stereochemistry in free-radical addition to alkene radical traps utilized 2,5-dimethylpyrrolidine carboxamides as the chiral auxiliary [8j, In this approach, the symmetry of the pyrrolidine reduces the conformational options for the a, -unsaturated carboxamide. Thus, the preferred Z conformation for carboxamides of 2,5-dimethylpyrrolidine is shown in Fig. 1 (structure 1). Single-crystal X-ray analysis of several dimethyl pyrrolidine carboxamides, including 1 and 2, have been carried out, and these structures support the notion that the preferred ground state conformation for these carboxamides is as shown [9], The X-ray structures for I and 2 are shown in f ig. 1. 

In the Cl symmetry strategy, the conformation of the carbonyl carbon-amide 

Experiments have recently been reported that utilize Oppolzer s sultam as an auxiliary group attached to an unsaturated oxime undergoing radical addition [13- 

The radical addition to 4 has been used as a basis for the synthesis of amino acids. Thus, the isopropyl radical adduct 5 has been converted to the amino acid 8 by reductive removal of the benzyloxy group of the major diastereomer R-5 by treatment with Mo(CO)6. Subsequent removal of the sultam auxiliary by standard hydrolysis afforded the enantiomerically pure D-valine without any loss of stereochemical purity. A variety of alkyl radicals were employed in the addition reaction, which gave the alkylated products with excellent diastereoselectivity, allowing access to a wide range of enantiomerically pure natural and unnatural amino acids. Even in the absence of Bu3SnH, treatment of 4 with alkyl iodide and EtsB at 20 °C gave the C-alkylated products with moderate diastereoselectivities. The use of 

Et2Zn instead of 136 as a radical initiator was also effective for the radical reaction. In these tin-free reactions, the mechanism of propagation is proposed to be a process in which boron or zinc Lewis acids complex to the oxime, promoting addition. Once addition of a radical to the complexed oxime occurs, fragmentation of the EtsB or Et2Zn complex generates an ethyl radical that propagates the chain. 

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