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24 VOL. 35, NO. 1 • 2002 Figure 1. Pinane-Based Versatile Reagents (PVR). eq 1 eq 2 Figure 2. Elaboration of Allylboration-Derived Homoallylic Alcohols. eq 3 eq 4 eq 5
B–O bond. 15 The mechanism involves a similar six-membered cyclic transition state, and much success has been achieved with pinane-based chiral auxiliaries. 16 3. Preparation of Allylboranes The simplest synthesis of allylboranes involves the treatment of allylmetals with borane derivatives, such as haloboranes or alkoxyboranes (eq 10, 11). 9 Bubnov and coworkers have reported the synthesis of bis(allylboranes) from dilithioisobutylene (eq 12). 17 Allylboronates can be prepared by a one-carbon homologation (Matteson homologation) of vinylboronates with halomethyllithium (eq 13). 18 Treatment of vinyllithiums eq 6 eq 8 with halomethylboronates can also be used to prepare “crotyl”boronates stereospecifically (eq 14). 19 Brown and coworkers have reported a three-carbon homologation procedure for the preparation of higher crotylboronates (Scheme 1). 20 Suzuki and coworkers have shown that catalytic hydroboration of 1,3-dienes with catecholborane in the presence of Pd(0) catalyst results in the formation of crotylborane derivatives (eq 15). 21 Another procedure for preparing stereospecific “crotyl”boranes involves the haloboration of terminal acetylenes, followed by Negishi coupling and Matteson homologation protocols (Scheme 2). 22 VOL. 35, NO. 1 • 2002 eq 7 eq 9 eq 10 eq 11 eq 12 eq 13 Yet another procedure for the preparation of “crotyl”boranes involves the hydroboration of substituted allenes. Although several hydroboration products are possible, the predominant product observed is the E “crotyl”borane (eq 16). 23 4. Asymmetric Allylboration eq 14 Asymmetric allylboranes that react with aldehydes to produce chiral homoallylic alcohols have attained considerable importance in the art of stereoselective synthesis of highly sophisticated, conformationally nonrigid systems. 5,8 Here, not only the enantioselectivity, but also the diastereoselectivity of the reaction is highly important. Accordingly, numerous searches 25
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Page 3 and 4: Aldrich Chemical Co., Inc. 1001 Wes
Page 5 and 6: Boron-Based Reducing Agents for the
Page 7 and 8: Figure 3. Stoichiometric Boron-Base
Page 9 and 10: orane reductions 29,30 of 1,4-diphe
Page 11 and 12: α-Hydroxy Acetals R Reduction Cond
Page 13 and 14: and 5 provided moderate ee’s in t
Page 15 and 16: Other reductions using 14, 64c,d 16
Page 17 and 18: Masuda, H.; Ito, K.; Hirao, A.; Nak
Page 19 and 20: The synthesis of biaryl compounds v
Page 21 and 22: 52,396-8 [87199-18-6] 3-Hydroxyphen
Page 23 and 24: Josiphos is a family of chiral phos
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Page 37 and 38: Scheme 4. Tartrate Ester Auxiliary
Page 39 and 40: eagent with aldehydes at low temper
Page 41 and 42: from 2- and 3-carene turned out to
Page 43 and 44: Scheme 22. Application of Crotylbor
Page 45: Abstr. 1968, 68, 29743j. (11) Refer
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