Chemistry Reference
In-Depth Information
Table 3.2
Addition of different electrophiles to the TES (triethylsilyl) glycolate
39j.
O
O
R
O
1) LiHMDS
O
O
O
O
O
OSiEt
3
2) RX
THF/ HMPA
-78 °C
OSiEt
3
O
O
O
O
O
O
40l-q
39j
Entry
Electrophile (RX)
Product
Yield (%)
de (%)
1
CH
2
=CHCH
2
I
40l
71
91
2
BnBr
40m
75
96
3
CH
3
CH
2
I
40n
83
88
4
PhCH
2
CH
2
I
40o
61
60
5
CH
3
(CH
2
)
4
I
40p
58
83
6
2-NphCH
2
Br
40q
71
91
3.3
Aldol Addition
Along with the work of Heathcock cited earlier, pioneering work on the use of
carbohydrates as chiral auxiliaries was carried out by the group of Brandänge
on the first, and at present sole, example of acetate-type aldolization. The aldol
condensation of the 3-
O-
acetate of diacetone-d-glucose with acetophenone was
performed under various conditions [21]. The best results were obtained by depro-
tonation of
42
with lithium
N
-isopropyl-(-)-menthyl amide (Li-IMA) leading to
tertiary carbinol
43
with 50% de in favor of the
(S)
-isomer (Scheme 3.15). The
influence of the complexation of Li
+
in these processes was pivotal, since addition
of MgCl
2
to the enolate of
42
favored the
(R)
-diastereoisomer of
43.
Köll has also used his d-xylofuranosyl oxazolidinone derivatives in propionate-
type aldol condensation [22]. Deprotonation of imides
21
with LiHMDS and then
quenching the enolates with simple aliphatic aldehydes gave
syn
(2
′
R
,3
′
S
)-aldols
D-
gluco
HO
CH
Ph
3
1) Li-IMA
THF, -78 °C
O
O
O
O
O
O
O
O
O
O
O
Ph
O
O
O
2)
43
(3
S
: 3
R
= 3 : 1)
42
O
Scheme 3.15
Acetate-type aldolization using diacetone-d-glucose as chiral auxiliary.
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