L Le ev ve el l 2 2 – – C Ca as se e 1 19 9
C Ch he el la at te e- -C Co on nt tr ro ol ll le ed d C Ca ar rb bo on ny yl l A Ad dd di it ti io on n R Re ea ac ct ti io on ns s
K
Ke ey y p po oi in nt t: : N Nu uc cl le le l le le le l leo op op op ph hi il i il il i il i i i
li li l li li l lic ic ic ic i i a ad dd dd dd d dd d ddi di di d di di d dit ti ti ti t ti ti t tio io io io i ion n. . S St te te t te te te te t ter re re r re re re re r reo oc ch he em mi is is i is is is i i t tr t tr tr tr tr t t y
ry ry y
OR
i-Pr
Me
O
H
t-Bu
OTMS
SnCl 4
i-Pr
Me
O
H
OR
Me 2 AlCl
t-Bu
OTMS
Me
i-Pr
OR
OH
t-Bu
O
OH
t-Bu
O
Me
i-Pr
OR
OR
i-Pr
Me
OH
t-Bu
O
t-Bu
O
OR
i-Pr
Me
OH
> 97:3
> 95:5
C Ch he el la at te e- -C Co on nt tr ro ol ll le ed d C Ca ar rb bo on ny yl l A Ad dd di it ti io on n R Re ea ac ct ti io on ns s
The nucleophilic addition of pinacolone enolsilane 1 to syn-D-methyl-E-alkoxy aldehydes 2, promoted by Lewis acids, is highly stereoselective. Thus, when SnCl 4
was employed, aldols 3 were obtained preferentially with selectivities higher than
95:5. Instead, the stereoisomers 4 were isolated with nearly total stereoselectivity
when Me 2 AlCl (2 eq) was used to promote the reaction (Scheme 19.1a and b).
1
i-Pr
Me
O
H
t-Bu
OTMS
SnCl 4
i-Pr
OR
OH
t-Bu
O
i-Pr
Me
OH
t-Bu
O
2
3
4
R = Bn, 3:4 95:5
R = TBS, 3:4 99:1
1
OR
Me
OR
Scheme 19.1a
1 Bn = Benzyl; TBS = t-BuMe 2 Si; TMS = Me 3 Si.