Chemistry Reference
In-Depth Information
MBOA, 6-methoxy-2-oxo-2,3-dihydrobenzoxazole 12
MBOA 12, the resistance factor of wheat against fungi and insects, has been pre-
pared by various methods from 2-amino-5-methoxyphenol using either phosgene
(for a safe source, see Chapter 7) [13, 14], triphosgene [15], CDI [16], potassium
cyanate [17], or urea [18]. Good yields of 12 were obtained with triphosgene (75%)
and CDI (73%), whereas phosgene (15% or 12%), potassium cyanate (5%), and
urea (1%) were unsatisfactory.
reagent:
conditions:
COCl
2
AcOH, 30°C
yield: 15 %
or
COCl
2
pyridine
reflux, 2 h
12 %
or
75 %
(CCl
3
O)
2
CO
Et
3
N, THF
rt, 30 min
MeO
MeO
OH
O
O
H
or
NH
2
73 %
CDI
THF, reflux, 4 h
12
or
KOCN
5 %
or
urea
1. 160°C, 25 min, (
2 %
)
1 %
MeO
OH
O
N
H
NH
2
2. 165°C, 30 min, (
55 %
)
The two cyclocarbonylations of the aminophenol with phosgene (in either AcOH
[13] or pyridine [14]) afford only poor yields, probably due to the occurrence of side
reactions between the highly activated aminophenol and the very reactive phos-
gene. These results, coupled with the high toxicity of gaseous phosgene, make
these syntheses unsuitable for the production of 12. On the other hand, harmless
urea provides an even worse yield of 12 of 1.1%. With urea, 12 is formed by way of
a two-step synthesis; in the first step, 4-methoxy-2-hydroxyphenyl urea is formed in
just 2% yield, whereas the second step, its intramolecular cyclization by pyrolysis,
affords 12 in 55% yield. This is an instructive example, in that heat-powered (160-
165
C) reactions are largely unselective and are accompanied by a significant
amount of by-products, making them totally inapplicable for the production of 11.
Triphosgene, which has only 1/170th of the reactivity of phosgene (see Table 3.1 in
Chapter 3), and CDI, which has a similar reactivity as triphosgene, afford 12 in
good yields of 75% and 73%, respectively (see also Section 4.3.2.4).
o-(4-Nitrophenylene) carbonate 13
Cyclocarbonylation of 4-nitrocatechol forming o-(4-nitrophenylene) carbonate
(NPC) 13 has been accomplished with either phosgene (for a safe source, see
Chapter 7) or triphosgene [19].
