Auxiliary stein

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). 

The efficient transmetalation of allylic stannanes to allylboron reagents has generated an attractive methodology for asymmetric allylation. Corey and coworkers first described the use of enantiomers of bromoborane 228 (Scheme 5.2.51) for mild and quantitative transmetalation of allylstannane to yield the allylboron reagent 229. i The asymmetry in the bis-toluenesulfonamide of 228 is derived from l,2-diamino-l,2-diphenylethane, and both antipodes are readily available in high optical purity, by resolution of the starting diamines producing (R,R)- and (5, 5 )- Stein chiral auxiliaries in transmetalation product 229. 

Scheme 5.2.51 Use of the Stein chiral auxiliary in transmetalations of allylic stannanes...

Transmetalations of allenylstannanes represent an important conceptual development, which substantially increases the potential utility of these reagents. i 4 Corey established an early example via the transmetalation of 3-triphenylstannyl-l-propyne with (/ ,f )-B-bromo-4,5-diphenyl-l,3-diaza-2-borolidine (R,R-22S) yielding the intermediate formation of allenylborane 334. Low temperature condensations of 334 with aldehydes occurred via closed transition states in which the chirality of the Stein auxiliary determined the stereogenicity of the enantioenriched -alcohol 335 (Scheme 5.2.70). ... 

Many other allylborating agents (12-21) utilizing chiral auxiliaries derived from carenes, camphor, tartaric acid, stein, etc. are also known (Figure 2) (2i-33). Recently Villieras described an ester-containing chiral allylborating agent which could be used directly for the synthesis of a-methylene-y-butyrolactones (32). In this review, we have restricted our discussions to the synthesis of lactones via homoallylic alcohols derived from B-allyldiisopinocampheyl-borane, 1. 

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