Rearrangement asymmetric Ireland

Asymmetric Ireland-Claisen Rearrangements. Chiral enolates derived from the boron complex (5) and allyl esters rearrange with excellent selectivity upon warming to —20 °C for a period of 1-2 weeks (eqs 9 and 10). As discussed above, the geometry of the intermediate enolate can be controlled by appropriate choice of base and solvent, thus allowing access to either syn or anti configuration in the product The reaction can be completed in 2-4 days with little erosion in selectivity when run at 4 °C. 

As mentioned for the Ireland-Claisen rearrangement, asymmetric Claisen rearrangement reactions are well known.Chiral Lewis acids have been designed for... 

Chiral Auxiliary Mediated Asymmetric Ireland-Claisen Rearrangements 4.6.6.1 Chiral Clycolates... 

To date the most general chiral auxiliary mediated asymmetric Ireland-Claisen rearrangement is that of Corey et al. (Scheme 4.44) [47]. They found that treatment of crotyl propionates and related esters afforded good yields, diastereoselectivities and enantioselectivities of the pentenoic acid products. The rearrangements also occurred at significantly lower temperature than the silyl ketene acetals. A key advantage of the chemistry is that the chiral auxiliary attachment, Ireland-Claisen rearrangement, and auxiliary removal all occur in one pot. 

Corey et al. used the asymmetric Ireland-Claisen rearrangement in the synthesis of j8-elemene (Scheme 4.131) [124], Rearrangement of the Z-(0)-B-ketene acetal via the chair transition state afforded the trienic acid in good yield with complete diastereo- and enantioselectivity. 

Synthetic highlights Racemization and enantiomerization are two consequences of configurational instability. Enantiomerically enriched 3-amino-1,4-BZDs have been generated via crystallization-induced asymmetric transformation. Enantiomerically pure target products have been obtained by asymmetric Ireland-Claisen rearrangement, highly enantioselective rearrangement of prochiral allylic esters to 1,2-disubstituted carboxylic acids and hydroboration of the terminal C=C bond, an efficient anti-Markovnikov hydratation process. 

Ireland and Varneys have reported asymmetric induction in this rearrangement by use of the enantiomerically pure secondary allylic akohol (1), obtained by resolution of the... 

The final approach was elegantly presented by Panek [44]. Several optically active ( )-crotylsilanes are available via stereoselective Ireland-Claisen rearrangement of enantiomerically pure vinylsilanes. Addition of the chiral crotylsilanes to acetals or to mixtures of aldehyde and trimethylsilyl methyl ether is effected by la to afford homoallylic ethers in exceedingly high diastereo- and enantioselectivity (Sch. 13). Occasionally a stoichiometric amount of la is required for allylation of aliphatic acetals, preserving the excellent level of asymmetric induction. The synthesis of (-F)-macbecin I involving triple use of the strategy imderscores the utility of the la-catalyzed asymmetric allylation [44c]. 

The most important feature of the Ireland variant is the control of the enolate geometry which allows the internal asymmetric induction (simple diastereoselectivity) of the rearrangement to be controlled (see Section 1.6,3.1.1.4.1.2. p 3412). 

Konno, T. Kitazume, T. Synthesis of trifluoromethylated compounds with four consecutive asymmetric centers via sequential [3,3]-Ireland-Claisen rearrangement and iodolacto-nization. Tetrahedron Asymmetry 1997, 8, 223-230. 

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