Alcohols chiral appendage

Predicting the diastereoselection of the 12 + 21 cycloaddition can be done according to the principles developed previously. Even if facial selectivity is not always satisfactory, the method is still of synthetic interest if the diastereoisomers produced in the photoreaction can be separated easily. After removal of the chiral appendage and recovery of the chiral auxiliary, the cyclobutane derivatives can be obtained with a high percentage of ee. Esters of chiral alcohols, chiral dioxenones, lactams, or ketals have been used as temporary chiral auxiliaries. 

The presence of a chiral appendage on an imactivated ethylenic substrate may also induce a diastereoselective carbolithiation Thus, cinnamaldehyde has been transformed into a gem-amino alcoholate [53], with the lithium monoamide of an enantiomerically enriched 1,2-diamine [54] (Scheme 32). This chiral sub-... 

In addition, catalysts 2b and 6 have also been successfully applied to the KR of other aUyhc alcohols, such as that depicted in Scheme 3.2 [16h, 21]. On the other hand, Miller et al. [30] have demonstrated that a peptide could be employed as acylation catalyst for the KR of a functionaHzed cyclohexenol, which was recovered in enantioselectivity of >98% ee (Scheme 3.2). In addition, moderate selectivity factors (s < 4.5) have been obtained by using a novel chiral 4-dimethylaininopyridine (DMAP) bearing a sulfoxide as a chiral appendage [31], whereas the use of a novel C2-symmetric diferrocenyl-pyrrolidinopyridine nucleophihc catalyst allowed promising selectivity factors of up to 6 to be obtained [32]. Moreover, the first example of chiral N-heterocycHc carbenes as acylation catalysts for the KR of alcohols was reported, albeit providing low enantioselectivities (<37% ee) [33]. 

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