Page 880 - Advanced Organic Chemistry Part B - Reactions & Synthesis

856 d. It is observed that the stereoselectivity of cyclizative condensation of
aminoalkyl silane 6-F depends on the steric bulk of the amino substituent.
CHAPTER 9 Offer an explanation for this observation in terms of the transition structure
Carbon-Carbon for the addition reaction.
Bond-Forming Reactions
of Compounds of Boron,
Silicon, and Tin
Si(CH 3 ) 3
CH 2 CH 2 NHR N
ZnI 2 , 5 mol %
+ PhCH 2 CH O R H
CH 3 OH
CH 2 Ph
6-F
R Yield (%) trans:cis ratio
a 68 20:80
CH 3
88 58:42
PhCH 2
Ph 2 CH 73 >99:1
Dibenzocycloheptyl 67 >99:1
a
Ph(CH 3 ) 2 Si instead of (CH 3 ) 3 Si.
9.7. A number of procedures for stereoselective synthesis of alkenes involving
alkenylboranes have been developed. For each of the reactions given below,
show the structure of the intermediates and outline the mechanism in sufficient
detail to account for the observed stereoselectivity.
3
1) R Li 2
2
1) BHBr 2 ·SMe 2 R Li R 1 R
1
(a) R C CBr 7-A 7-B 2) I , MeOH
2
2) HO(CH ) OH 3) NaOH H R 3
2 3
1) Br 2 1) s-BuLi
2
1) BHBr ·SMe 2 R Li 7-E
2
1
(b) R C CBr 7-C 7-D 2) (i-PrO) B
) OH 3
2) HO(CH 2 3 2) NaOMe, MeOH
3) HCl
3
O 1) R Li 1 3
H 2 R R
7-F 7-G
) OH 2) I , MeOH
HO(CH 2 3 2 H R 3
3) NaOH
(c) 1 2
2
R 2 BCl NaOCH3 R R
1
R C CH 7-H
I H H
LiAlH 4 2
(d) 1) R Li R 1 R 2
2
1) BHBr ·SMe 2 7-I
2
1
R C CH 2) I , MeOH
2
2) HO(CH ) OH 3) NaOH H H
2 2
9.8. Suggest reagents and reaction conditions that would be effective for the following
cyclization reactions:
(a) OH
CH O
OCH OCH 3 CH CHOCH 2 OCH 3
2
SnBu 3
(b) SnBu 3 CH 2
O O
CH 2 C H
2 5
O O
CH 3 SCH 3
CH S SCH 3
3