Aldehydes enantioselective indium-mediated allylation

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. 

Anomalous yy/z-diastereoselectivity of indium-mediated coupling of aldehydes with bromides (Z)- and (E)A2 has been reported (Scheme 45). The reaction affords high syn-selectivity (43/44) regardless of the allylic bromide geometry. Preliminary studies on the enantioselective indium-mediated allylation have been attempted and it has been found that the desired products can be obtained in moderate yield with high. //-selectivity and enantioselectivity (Equation (23)).99... 

Scheme 4.5 Enantioselective indium-mediated allylation reaction of aldehydes.

A chemoenzymatic methodology has been developed using indium-mediated allylation (and propargylation) of heterocyclic aldehydes under aqueous conditions followed by Pseudomonas cepacia lipase-catalyzed enantioselective acylation of racemic homoallylic and homo-propargylic alcohols in organic media.192... 

Variable enantioselectivities are observed during allylation of aldehydes in the presence of ligand 35. P-Hydroxyaldehydes can be allylated in the unprotected form with a chiral allyltitanium species 36, whereas a-hydroxyaldehydes are readily prepared from 37 by various organometallic reactions. Cinchona alkaloids, being commercially available and inexpensive, are suitable chiral promoters for the indium-mediated allylation, even though they induce only moderate enantioselectivities. Allylation such as that effected by 38 affords products useful for substrate-based alkylation. ... 

Indium mediates a highly enantioselective Barbier-type allylation of both aromatic and aliphatic aldehydes, using a chiral ethanolamine auxiliary, readily recoverable by acid extraction.193 Barbier coupling of aldehydes can be carried out in water using tin(II) chloride, with cobalt(II) acetylacetonate as catalyst.194... 

An indium-mediated enantioselective allylation of aldehydes has been realized in dichloromethane with (—)-cin-... 

The indium-mediated enantioselective allylation of aldehydes is realized in the presence of external chiral ligands (-I-)-cinchonine and (-)-cinchonidine (Tab. 8.6) [38]. By... 

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