Carbonyl and Imine Allylation

Carbonyl allylation represents an important class of reactions that allows the construction of polypropionate motifs found in many bioactive natural products. Boron Lewis acids have been employed primarily to activate aldehyde and ketone substrates toward allylic tin and allylic silicon reagents. 

More than a decade ago, Yamamoto and coworkers demonstrated that the chiral acyloxyborane (6) is an excellent catalyst for the Sakurai-Hosomi allylation of aldehydes to furnish the homoallylic alcohols in good yields and enantioselectivities (Equation 60) [59]. This system remains to date, the most effective catalytic enantioselective allylation of aldehydes based on boron Lewis acids. 

Marsden and coworkers have recently reported that the reaction of cyclic al-lylsiloxane (101) with aldehydes, in the presence of BF3 OEt2, gives access to a 2,3,4-substituted tetrahydrofuran motif (102) and (103) found in the furanolignan family of natural products (Equation 61) [60]. This reaction proceeds with complete trans selectivity at the 3 and 4 positions. 

Recently, Suh and coworkers have reported a new method of entry into functionalized cycloalkylamines via a diastereoselective intramolecular amidoalkylation of N, 0-acetal trimethylsilyl ether possessing allylsilane (104), under the effect of BF3 OEt2, to give the corresponding cyclic amine (105) in high diastereoselectivity (Equation 62) [61]. 

More recently, Lewis acids have found use in the allylboration of aldehydes [62-64]. In these new conditions, the Lewis acid activates the reaction by rendering the boron atom in the allylboron reagent more electrophilic [65], thus allowing reactions to take place at significantly lower temperatures. A recent example is Kabalka and coworkers use of BF3 OEt2 catalysis to form highly functionalized homoallylic alcohols (Equation 63) [66]. 

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