Diastereofacial differentiation asymmetric synthesis

The vast majority of stereoselective reactions in organic chemistry, especially in asymmetric synthesis, involve diastereofacial differentiation of a trigonal, sp -atom. The stereoelectronic considerations outlined in the introduction to this chapter afford a rare opportunity in stereoselective synthesis the preparation of a chiral organometallic in which the carbon bearing the metal is stereogenic. As a result of the... 

Curran identified Oppolzer s camphor sultam (cf. 81) [77] as a superb chiral auxiliary for the asymmetric [l,3]-dipolar cycloadditions of nitrile oxides (Scheme 18.17) [78-80]. The resulting cycloadducts were generally obtained with high diastereofacial selectivity (dr 90 10). It was proposed that the observed facial differentiation was the result of attack by the nitrile oxide from the top face of conformer 84. This was suggested to be favored on the basis of electronic and steric effects, a proposition that finds support in an X-ray structural analysis of 81 [78, 80]. Cycloaddition of 81 with the nitrile oxide generated in situ from 80 led to isoxazoline 82 in 89 % yield (dr= 92 8). This product constituted a key intermediate in Curran s total synthesis of 83, a bicyclic ketal isolated from ambrosia beetles [80]. 

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