Page 146 - Advanced Organic Chemistry Part B - Reactions & Synthesis

118 Another promising boron enolate is derived from (−)-menthone. 133 It yields E-boron
enolates that give good enantioselectivity in the formation of anti products. 134
CHAPTER 2
)
Reactions of Carbon CH(CH 3 2
Nucleophiles with CH 3 CH 3
Carbonyl Compounds CH 3 (CH 3 CH ) BCl R R′CH O R R′
2 2
R H
O OB(CH 2 menth) 2 O OH
Et N
3
H , i -C H , R′ = C H , i-C H , c-C H , Ph
R = C 2 5 3 7 2 5 3 7 6 11
The boron enolates of -substituted thiol esters also give excellent facial selectivity. 135
CH(CH )
3 2
H CH menth
2
(CH 3 CH ) BCl X SR
2 2
R′CH O R′ R′ O B
COSR COSR X
XCH 2 menth
Et N OH O CH 2
3
X = Cl, Br, OCH Ph
2
The facial selectivity in these chiral boron enolates has its origin in the steric effects
of the boron substituents.
Several chiral heterocyclic borylating agents have been found useful for enantio-
selective aldol additions. The diazaborolidine 14 is an example. 136
Ph Ph

ArSO N NSO 2 Ar
2
B
Br 14
Ar = 3,5-di(trifluoromethyl)phenyl
Ph Ph

TsN NTs O OH
B CH 3
O CH(CH )
Br (CH ) CHCH O 3 2
CH 3 2
CH 3 3 CH 3
i -Pr NEt 85% yield, 98:2 syn:anti, 95% e.e.
2
Derivatives with various substituted sulfonamides have been developed and used
to form enolates from esters and thioesters. 137 An additional feature of this chiral
auxiliary is the ability to select for syn or anti products, depending upon choice of
reagents and reaction conditions. The reactions proceed through an acyclic TS, and
diastereoselectivity is determined by whether the E-or Z-enolate is formed. 138 t-Butyl
esters give E-enolates and anti adducts, whereas phenylthiol esters give syn adducts. 136

133
C. Gennari, Pure Appl. Chem., 69, 507 (1997).
134 G. Gennari, C. T. Hewkin, F. Molinari, A. Bernardi, A. Comotti, J. M. Goodman, and I. Paterson, J.
Org. Chem., 57, 5173 (1992).
135
C. Gennari, A. Vulpetti, and G. Pain, Tetrahedron, 53, 5909 (1997).
136
E. J. Corey, R. Imwinkelried, S. Pikul, and Y. B. Xiang, J. Am. Chem. Soc., 111, 5493 (1989).
137 E. J. Corey and S. S. Kim, J. Am. Chem. Soc., 112, 4976 (1990).
138
E. J. Corey and D. H. Lee, Tetrahedron Lett., 34, 1737 (1993).