Page 696 - Advanced Organic Chemistry Part B - Reactions & Synthesis

H CH CH CHCH 671
(e) 2 3
THPOCH CH C CH C C PROBLEMS
2
2
THPOCH CH 2 H
2
4 9
(f) O C H H
C H COCH 3 C C
4 9
CH O C H
3
5 11
CH
(g) CH CH 3 3
3 CH 3
O
O
O
(h) CH O O
OCH 3 CH O
3
O
CH O H
3
CH 3 CH O OCH 3
3
CH 3 O
CH 3

7.14. Catalytic amounts of chiral amino alcohols both catalyze the reactions of
alkylzinc reagents with aldehydes and induce a high degree of enantioselec-
tivity. Two examples are given below. Formulate a mechanism for this catalysis.
Suggest transition structures consistent with the observed enantioselectivity.

N(CH 3 ) 2
PhCH O + (C H ) Zn + CH 3 CH 3 (S )-PhCHC H
2 5 2
2 5
OH OH
Ph
H ) Zn
PhCH O + (C 2 5 2 + H (R )-PhCHC H
2 5
N OH
OH
(CH ) CCH 2
3 3
7.15. When 4-substituted 2,2-dimethyl-1,3-dioxolanes react with Grignard reagents,
the bond that is broken is the one at the oxygen attached to the less-substituted
-carbon. What factor(s) are likely the cause for this regioselectivity?

R
R
CH MgBr
3
O O (CH ) COCHCH OH
2
3 3
CH 3 CH 3 R = Ph, c-C H
6 11