Page 884 - Advanced Organic Chemistry Part B - Reactions & Synthesis

860 9.18. In the original report of the reaction in Entry 8 of Scheme 9.6, it was found that
use of three equivalents of BF led to loss of stereoselectivity, but not yield.
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CHAPTER 9
Carbon-Carbon CH 3 CH 3 CH 3
Bond-Forming Reactions CH 3 CH 3 CH 3 CH CHCH 2 SnBu 3
of Compounds of Boron, CH 3 O 2 C
Silicon, and Tin CH 3 O 2 C CH O
HO
Product Composition
Total anti anti syn syn
Equiv BF 3
Yield anti-Felkin Felkin Felkin anti-Felkin
1 92 94–97 3–4 1 1
2 90 83–91 5–9 1–3 2–5
3 90 17 32
41 10
These results were attributed to a preference for an eight-membered chelated
transition structure that was lost in the presence of excess BF because of
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coordination of a second BF at the ester group. What objections would you
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raise to this explanation? What alternative would you propose?
CH 3 H OCH 3
O BF 3
CH 3 O

9.19. The aldehyde 19-A shows differential stereoselectivity toward the enantiomeric
stannanes S -19-B and R -19-B. The former aldehyde gives a single product in
high yield, whereas the latter gives a somewhat lower yield and a mixture of two
stereoisomers under the same conditions and is a mixture of two stereoisomers.
Propose TSs to account for each product and indicate the reasons for the enhanced
stereoselectivity of S -19-B.

OH
OTBDMS Bu 3 Sn OMOM OTBDMS
CH O
TBDMSO + BF 3 TBDMSO CH 3
CH 3
OTBDMS
OTBDMSOMOM
19-A (S)–19-B
only product (90%)
OTBDMS Bu 3 Sn OMOM BF 3 OTBDMS OH
CH O TBDMSO CH 3
TBDMSO +
CH 3 OMOM
OTBDMS 61% OTBDMS
(BR)–19-B +
19-B OH
OTBDMS
CH 3
TBDMSO
OMOM
OTBDMS
7%