Stereochemical Control via 1,3-Asymmetric Induction

Reactions of the a-methyl- -benzyloxy aldehyde 97 with allyltri-n-butylstannane 98 are summarized in Table 11-7 [83]. While little stereocontrol is observed in the BF3 OEt2-promoted allylation reaction, the chelate-controlled reaction catalyzed by either TiC or SnCl4 is much more selective, favoring formation of the Cram-chelate adduct 100 with up to 98 2 selectivity. The chelate transition state 101, where C-C bond formation occurs anti to the aldehyde a-methyl group, rationalizes the observed stereoselective formation of 100. Although the BF3-OEt2-cata- 

Although silyloxy groups do not participate in chelates under most conditions, Evans and co-workers have recently reported that the terr-butyldimethylsilyl-pro-tected / -alkoxy aldehyde 102 undergoes highly diastereoselective chelate-con-trolled allylation reactions with allyl and /J-methyl allyltri- -butylstannanes 98 using Me2AlCl (2.5 equiv) as the Lewis acid promoter (Eq. (11.2)) [89]. 

The crotylation reactions of a-methyl chiral aldehydes (e.g. 97) with Type II crotylmetal reagents can give up to four products (e.g., 105-108). The syn,syn- A-duct 106 and the 3,4-.vyn-4,5-anh diastereomer 107 can be obtained with useful levels of diastereoselectivity via the reaction of 97 with the achiral ( )- and (Z)-crotyltri-u-butylstannanes 10 under appropriate conditions (Table 11-8) [53]. 

The BF OEta-catalyzed addition of crotyltri-n-butylstannane 10 (a 55 45 mixture of E- and Z-isomers was used in this experiment) to the ff-silyloxy aldehyde 97b furnishes the 5yn,.v u-dipropionate adduct 106 with 95 5 diastereoselectivity. 

The stereochemistry of adduct 106 is consistent with formation through either the Felkin, synclinal transition state 109 or the Felkin, antiperiplanar transition state 110 (Fig. 11-11). A discussion of the merits of these two transition states appears earlier in this chapter (Section 11.1.2). 

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