Page 149 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Scheme 2.6. (Continued) 121

9 i SECTION 2.1
S O OTBDPS OH OTBDPS
S O Aldol Addition and
O CH Condensation Reactions
S N O Sn(OTf) 2
+ i Pr NEt S N O
O 2 O
CH 2 OCH 2 Ph CH 3
CH(CH 3 ) 2 CH 2 OCH 2 Ph
–78°C
CH(CH 3 ) 2
96%
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anti selective using C H AlCl. 139 The anti selectivity is proposed to arise as a
2
5 2
result of the greater size requirement for the complexed aldehyde with C H AlCl.
2
5 2
These reactions both give a different stereoisomer than the reaction done without the
additional Lewis acid. The chiral auxiliary is the source of facial selectivity.
R R R R
B OH B OH
O O H O O
O O R O O
CH CH 3
3 R R
O N TiCL O N
O N O N 4
O R O
H CH 3 CH 3
H CH(CH ) H CH(CH )
CH(CH ) Al(C H ) Cl 3 2 CH(CH ) 3 2
3 2
3 2
5 2
2
2,3-anti 2,3-syn
R = C H , (CH ) CH, (CH ) CHCH , (CH ) C, Ph
2 5 3 3 3 2 2 3 3
With titanium enolates it was found that use of excess (3 equiv.) of the titanium
reagent reversed facial selectivity of oxazolidinone enolates. 140 This was attributed to
generation of a chelated TS in the presence of the excess Lewis acid. The chelation
rotates the oxazolidinone ring and reverses the facial preference, while retaining the
Z-configuration syn diastereoselectivity.
O
O CH(CH )
O O OH (CH 3 2 O N 3 2 O O OH
) CH
N O
3
O N R Cl Ti O R Cl Ti O O N R
4
O O
CH 3
) CH 3 R CH 3 CH 3
CH(CH 3 2 )
CH(CH 3 2
normal transition structure chelated transition structure
139 M. A. Walker and C. H. Heathcock, J. Org. Chem., 56, 5747 (1991).
140
M. Nerz-Stormes and E. R. Thornton, Tetrahedron Lett., 27, 897 (1986); M. Nerz-Stormes and
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