Enantioselective reactions allylzincation

Diastereoselective allylzincations of achiral cyclopropenone acetals, addressing the control of the relative configuration at the newly formed C-C bond (centers C3 and C4) and of chiral derivatives such as 20 (R = H), addressing chirality transmission through a spiro carbon center, have been investigated. In the presence of bis(oxazoline) ligands, enantioselective reactions of this kind can also be performed. ... 

Nakamura et al.141 reported a closely related reaction, that is, the enantioselective addition of allylzinc reagent to alkynyl ketones catalyzed by a bisox-azoline catalyst 137. High ee values were obtained in most cases (Scheme 2-55). 

Bis(oxazoline) ligands have been shown to be useful in the many carbon-carbon bond-forming reactions previously listed. They have also been used in a myriad of other carbon-carbon bond-forming reactions. For example, Nakamura and coworkers used bis(oxazoline) ligands ent-2, ent-22, and ent-39 in ligand-induced enantioselective allylzincation. This reaction consisted of the transformation of the cyclopropenone acetal 198 into allylic cyclopropanone acetal 199 in yields ranging from 73 to 90% with selectivities from >98 2 for the isomer shown to 1 99 (Fig. 9.57). 

Several methods promoted by a stoichiometric amount of chiral Lewis acid 38 [51] or chiral Lewis bases 39 [52, 53] and 40 [53] have been developed for enantioselective indium-mediated allylation of aldehydes and ketones by the Loh group. A combination of a chiral trimethylsilyl ether derived from norpseu-doephedrine and allyltrimethylsilane is also convenient for synthesis of enan-tiopure homoallylic alcohols from ketones [54,55]. Asymmetric carbonyl addition by chirally modified allylic metal reagents, to which chiral auxiliaries are covalently bonded, is also an efficient method to obtain enantiomerically enriched homoallylic alcohols and various excellent chiral allylating agents have been developed for example, (lS,2S)-pseudoephedrine- and (lF,2F)-cyclohex-ane-1,2-diamine-derived allylsilanes [56], polymer-supported chiral allylboron reagents [57], and a bisoxazoline-modified chiral allylzinc reagent [58]. An al-lyl transfer reaction from a chiral crotyl donor opened a way to highly enantioselective and a-selective crotylation of aldehydes [59-62]. Enzymatic routes to enantioselective allylation of carbonyl compounds have still not appeared. 

Reaction of the homochiral organozinc reagent 24 or its antipode 25 to cyclopropenone acetals leads to enantioselective allylzincation (see Table 5). ... 

Following their earlier findings on the enantioselective allylzincation of cy-clopropenone acetals [43], Nakamura and co-workers investigated the same reaction with imines of aromatic aldehydes [44]. In an initial experiment, ( )-ben-zaldehyde N-phenyl imine (59) was treated with preformed chiral allylic zinc reagent 57a (R=z-Pr) obtained by mixing a lithiated bis(oxazoline) (55, R=z-Pr) with an allylic zinc bromide (56a) (Scheme 22). The allylation product was obtained in 95% yield, but the enantioselectivity was only 6% ee. Fortunately, the enantioselective allylzincation of Z- (i.e., cyclic) aldimines was more successful. 

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