Enantiofacial selection asymmetric synthesis

Efforts have been made to apply r 3-allyltitanium chemistry to the asymmetric synthesis of homoallylic alcohols and carboxylic acids. The synthesis and reactions of chiral r 3 -allyl-titanocenes with planar chirality, or containing Cp ligands with chiral substituents, have been reported [6c,15,30—32]. The enantiofacial selectivity in the allyltitanation reactions has been examined for the complexes 12 [15], 13 [30], 14 [31], 15, 16, and 17 [32] depicted in Figure 13.2. 

In fact, the usefulness of chiral oxazoline enolates in asymmetric synthesis had been already demonstrated by Meyers [24]. Evans obtained enantiofacial selectivities (or enantiomeric excesses = e.e) equal to or greater than 99% (Table 9.4). 

Extensive use in synthesis has been made of the asymmetric epoxidation of (2,3 )-disubstituted allylic alcohols. With few exceptions enantiofacial selectivity is excellent (90-95% ee). The results for a number of epoxidations of allylic alcohols with smaller substituents are collected in Table 6A.6 [2,4,41,61b,79-84], while a variety of other compounds with larger groups are illustrated by structures 47-60. 

The asymmetric cyclisation of achiral olefinic organohthium reagents by a stereogenic alkah metal centre can be modulated by ( )-sparteine, which confers enantiofacial selectivity on the reaction such that the anionic cyclisation process discriminates between the enantiotopic faces of an unactivated C=C bond. Recently, modifications have been made to the well known hthium-ene cyclisation reaction whereby the subsequent expulsion of a thiophenoxide group yields a fused vinylcyclopropane. Moreover, allylic lithium oxyanion-induced reactivity and stereoselectivity in this intramolecular carbometallation has been demonstrated in the highly stereoselective synthesis of a natural bicyclo[3.1.0] hexane. ... 

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