Page 113 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Bu Sn O SCF . 68 The Lewis acids promote ionization of the acetal to an oxonium 85
2
3 2
3
ion that acts as the electrophile. The products are -alkoxy ketones.
SECTION 2.1
Aldol Addition and
–
+
RCH(OR′ ) + MX n RCH O R′ + [R′OMX ] Condensation Reactions
n
2
O
2
+
RCH O R′ +R CH CR 3 RCHCHCR 3
OTMS R′O R 2
In some cases, the enolate can be formed directly in the presence of the acetal with
the Lewis acid also activating the acetal. 69
O O TiCl 4 O O
CH 3 CH 3 + CH O CH 3 O
3
O Et N 83%
3
CH 3
Dibutylboron triflate promotes both enol borinate formation and addition. 70

O O Ph
O Bu 2 BOTf
+ Ph O OH
O (i-Pr) 2 NEt
78%

Reactions with acetals can serve to introduce -alkoxy groups into complex molecules,
as in the following reaction. 71

TBDMSO O
TBDMSO CH 3 TBDMSO O OCH 3
TiCl 4 OPMB
CH 3 TBDMSO
(i -Pr) NEt
2
(CH O) CH OPMB CH 3 CH 3 CH 3
3
2
+ 52%
CH 3
It has been proposed that there may be a single electron transfer mechanism
for the Mukaiyama reaction under certain conditions. 72 For example, photolysis of
benzaldehyde dimethylacetal and 1-trimethylsilyloxycyclohexene in the presence of a

68 T. Sato, J. Otera, and H. Nozaki, J. Am. Chem. Soc., 112, 901 (1990).
69 D. A. Evans, F. Urpi, T. C. Somers, J. S. Clark, and M. T. Bilodeau, J. Am. Chem. Soc., 112, 8215
(1990).
70
L.-S. Li, S. Das, and S. C. Sinha, Org. Lett., 6, 127 (2004).
71 G. E. Keck, C. A. Wager, T. T. Wager, K. A. Savin, J. A. Covel, M. D. McLaws, D. Krishnamurthy,
and V. J. Cee, Angew. Chem. Int. Ed. Engl., 40, 231 (2001).
72
T. Miura and Y. Masaki, J. Chem. Soc., Perkin Trans. 1, 1659 (1994); T. Miura and Y. Masaki, J.
Chem. Soc., Perkin Trans. 1, 2155 (1995); J. Otera, Y. Fujita, N. Sakuta, M. Fujita, and S. Fukuzumi,
J. Org. Chem., 61, 2951 (1996).