Stein-Based Allylboron Reagents

Corey and co-workers developed the highly enantioselective allylboron reagent 198 [127], whose chiral 1,2-diamino-1,2-diphenylethane (stein) auxiliary [254] serves as the source of asymmetry. In an extension of this methodology, Williams et al. have demonstrated the utility of the bromoborane 332 for the preparation of synthetically complex allylborane reagents [255] and have applied this methodology in two natural product syntheses [256, 257] (see below). 

Allyiboron reagent R,R)-19S reacts with a variety of achiral aldehydes, affording homoallylic alcohols 240 in excellent yields and enantiomeric excess (Table 11-19). Corey rationalized that adducts 240 should arise through transition state 333. The alternative transition state, 334, is disfavored due to unfavorable steric interactions between the a-methylene group of the reagent and the adjacent sulfonamide ligand. Note that the toluene substituents of the sulfonamides in transition states 333 and 334 are spatially oriented so as to avoid steric interactions with the phenyl substituents of the chiral auxiliary. 

The synthesis of bis-pyran 340 was initiated by the allylation reaction of aldehyde 336b with the allylboron reagent derived from allylstannane (S)-335 and 

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