Chemistry Reference
In-Depth Information
by the cycloaddition reaction of propargylic alcohols with 2-naphthol in the
presence of a Lewis acid catalyst (InCl
3
.
4H
2
O) under ball milling within 1 h
(Scheme 1.41).
39
Other Lewis acids such as ZnCl
2
or SnCl
4
also catalyzed the
cycloaddition reaction but the yields of the corresponding naphthopyrans
were poor. Using this methodology several substituted naphthopyrans were
synthesized. When propargyl alcohols bearing either a pyrazinyl group or a
pyridyl group were used the expected naphthopyrans were not formed. This
may happen due to the unwanted coordination of the pyrazinyl or pyridyl
ligand to the indium metal.
Although there are several routes for the synthesis of flavone, containing a
six-membered oxygen-containing ring, mechanically activated solid-state
synthesis of flavones by high-speed ball milling (HSBM) received consider-
able attention as it avoids the use of solvent and provides waste-free prod-
ucts in almost quantitative yields. The synthesis of flavones was
accomplished by cyclodehydration of 1-(2-hydroxyphenyl)-3-aryl-1,3-propa-
nediones in the presence of KHSO
4
by HSBM within 15 min (Scheme 1.42).
40
Kaupp and co-workers have reported the quantitative formation of six-
membered oxygen containing heterocycles by Michael addition followed by
rearrangement and cyclization under ball milling (Scheme 1.43).
41
Quanti-
tative yields of products were obtained without using any catalyst in a waste
free manner.
The two-component straightforward synthesis of 2,3,6,7-tetrahydro-4(5H)-
benzofuranone derivatives under solvent-free ball-milling was initiated by
Mn(OAc)
3
2H
2
O, which acts both as an oxidant and as a Lewis acid
(Scheme 1.44).
42
In the standardized conditions oxidative radical reactions
of 1,3-dicarbonyl compounds (dimedone, cyclohexane-1,3-dione) with
(aza)chalcones catalyzed by Mn(OAc)
3
2H
2
O afforded the products in
moderate to high yields within 1 h without the need of any external base.
This protocol is found to be widely applicable for a variety of chalcones. With
the isolation of a key intermediate 30 (Scheme 1.45) after running the
R
2
R
1
R
3
OH
R
2
O
InCl
3
.
4H
2
O
MM200,
30 Hz, 1 h
R
1
R
3
+
OH
76-97%
16 examples
R
1
,
R
2
,
R
3
= aryl, alkyl, heteroaryl
Scheme 1.41 Cycloaddition reaction for formation of naphthopyran derivatives
under ball milling.
OH
O
KHSO
4,
Silica gel
HSBM, 5-15 min.
R
2
R
2
R
1
R
1
O
O
O
R
1
= H, Cl, Br, CH
3
, OCH
3
, NO
2
, OH
R
2
= H, Cl, CH
3
, OCH
3
, NO
2
88-97%
14 examples
Scheme 1.42
Synthesis of flavones catalyzed by KHSO
4
under HSBM.
Search WWH ::
Custom Search