ORDER
REPRINTS
3604
REDDY,REDDY,AND GIRIDHAR
with b-keto esters) is an established fact in the literature.1 Recent investiga-
tions for the Pechmann reaction employed various reagents such as zeolites,
clays and sulfonic acid resins resulting in simplified product recovery and
reduction of undesirable wasteful products.2–4 However, the zeolite and clay
based catalysts are highly susceptible to deactivation due to pore blocking
and hydration. Application of cationic exchange resins as condensing agents
results in better yields,5 however, most of the cationic exchange resins are
not resistant enough to high temperatures.
In this communication we report a new efficient ecofriendly W/ZrO2
solid acid catalyst for the synthesis of substituted coumarins from resorcinol
and substituted resorcinols with ethyl acetoacetate and ethyl-a-methylace-
toacetate in good yields. The reaction of resorcinol with ethyl acetoacetate
gives methylumbelliferone (4-methyl-7-hydroxycoumarin) a perfumery
ingredient for the insecticide Hymecromone.6 The tungstate modified zirco-
nia solid acid catalyst possesses many advantages such as easy to handle,
non-corrosive, low-cost, high activity, and high recycling capacity.
The sulfated zirconia solid superacid catalyst has attracted much
attention recently because of its ability to catalyze many acid catalyzed
reactions more effectively.7 However, the SO42ꢀ/ZrO2 catalyst gets deacti-
vated easily by loosing sulfate ions and thereby recycling of the catalyst is
highly restricted. Very recent investigations reveal that molybdenum or
tungsten doped ZrO2 is an alternative catalyst8–12 in reactions requiring
stronger acid sites (H0 ꢁ ꢀ 14.52).13 The major advantage of Mo-(or W-)
modified ZrO2 over sulfated catalyst is that they do not suffer from dopent
loss during thermal treatments and also undergoes significantly less deacti-
vation during catalytic runs. Although, solid superacids have been
attempted in petrochemical industries, their application in synthetic organic
chemistry remains unexplored.
The synthesis of substituted coumarins from resorcinol and substi-
tuted resorcinols with ethyl acetoacetate and ethyl-a-methylacetoacetate
are presented in Table 1. From entries 1–4 the reaction is carried out with
ethyl acetoacetate; and from entries 5–8 the reaction is carried out with
ethyl-a-methylacetoacetate. As can be noted from Table 1, the catalytic
activity of W/ZrO2 was found to be more promising. Different mechanisms
have been put forward to account for this reaction in the literature.5
The resorcinol as a result of resonance has a center of high electron density
at the position ortho to the hydroxyl group. Addition then takes place across
the double bond in the enolic form of the b-keto ester and results in good
yields. In case of orcinol, the position para to a methyl group is deactivated
for an electrophilic reagent. The activity of orcinol will therefore be less than
that of resorcinol. In case of pyrogallol, the third hydroxyl group deactivates
the position meta to it for an electrophilic reagent resulting in less yield.