Unusual conversion of 5-hydroxy-2(5H)-furanone in aqueous solution

Article abstract of DOI:10.1023/A:1025137914137

The conversion of 5-hydroxy-2(5H)-furanone into succinic acid in aqueous solution has been detected experimentally for the first time, indicating the possibility of forming and hydrolyzing its previously unknown tautomeric forms. The accelerating effect of increased pH values and temperature on the reaction has been established. A scheme is proposed to form succinic acid from 5-hydroxy-2(5H)-furanone.

Full text of DOI:10.1023/A:1025137914137

Chemistry of Heterocyclic Compounds, Vol. 39, No. 5, 2003
UNUSUAL CONVERSION OF 5-HYDROXY-
2(5H)-FURANONE IN AQUEOUS SOLUTION
V. V. Poskonin and L. A. Badovskaya
The conversion of 5-hydroxy-2(5H)-furanone into succinic acid in aqueous solution has been detected
experimentally for the first time, indicating the possibility of forming and hydrolyzing its previously
unknown tautomeric forms. The accelerating effect of increased pH values and temperature on the
reaction has been established. A scheme is proposed to form succinic acid from 5-hydroxy-2(5H)-
furanone.
Keywords: 5-hydroxy-2(5H)-furanone, succinic acid, solution equilibrium.
5-Hydroxy-2(5H)-furanone (1), used in the synthesis of heterocyclic compounds [1-5], is the cyclic form
of cis-β-formylacrylic acid (2) (Scheme 1). At the basis of the mobile equilibrium existing between structures 1
and 2 lies a mechanism of ring-chain tautomerism [6-8]. Previously we developed conditions assisting a shift in
this equilibrium in the direction of the cyclic form 1. This permitted its separation from the mixture with
tautomer 2 formed on oxidation of furfural with hydrogen peroxide in the presence of vanadium compounds
[1,9,10].
Scheme 1
COOH
OHC
HO
O
O
1
2
However recent results obtained on investigating the behavior of hydroxy-furanone 1 in aqueous media
have indicated the possibility of carrying out tautomeric–hydrolytic conversions of it under definite conditions
more complex than Scheme 1. The formation of succinic acid (3) in aqueous solutions of lactol 1 at pH 1-10 has
been established by us for the first time.
On heating an aqueous solution of furanone 1 (concentration 1.2 M) at 60°C for 3 h the yield of acid 3
was 3%. On increasing the heating time to 24 h the yield grew to 13%. Increasing the acidity of the medium (by
introducing the appropriate quantity of perchloric acid) was the same as reducing the temperature and led to a
slowing of the process of forming product 3. On the other hand, in media of increased basicity (at pH > 8), the
reaction suddenly accelerated. Thus at 70-80°C in NaOH solution of pH about 9-10 practically complete
conversion of compound 1 into acid 3 was observed in a few minutes.
__________________________________________________________________________________________
Kuban State Technological University, Krasnodar 350072, Russia; e-mail: k-obh@kubstu.ru,
badovskay@kubstu.ru. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 688-691, May,
2003. Original article submitted September 20, 2000.
594
0009-3122/03/3905-0594$25.00©2003 Plenum Publishing Corporation

A polarographic investigation was carried out on dilute solutions of furanone 1 (concentration 1·10-4 M,
pH 0-4) blown through with nitrogen and maintained for 1 year at room temperature in the absence of oxygen of
the air. The results obtained confirmed the acceleration of the conversion of compound 1 into acid 3 on reducing
the proton concentration.
Product 3 was isolated from the reaction mixture as the diethyl ester 4. The spectral and other
characteristics of this ester, and of the acid 3 obtained from it by acid hydrolysis, corresponded with the data of
standard samples of the compounds indicated.
The results presented above enabled acid 3 to be considered as the final product of previously unknown
tautomeric conversions of furanone 1.
Among the main products of oxidation of furfural with aqueous hydrogen peroxide 2(3H)- and
2(5H)-furanones 5 and 6 were detected by one of us in [11]. Their formation under these conditions is explained
by isomerization of the unstable intermediate reaction product, 2-hydroxyfuran (7), by a type of keto-enol
equilibrium (Scheme 2).
Scheme 2
O
O
5
O
O
7
CHO
OH
O
O
6
We propose that a similar mechanism, linked with migration of the proton found at position 5 of the
lactone ring and occurring without ring opening, is also possible for hydroxyhydrofuranone 1. In accordance
with this mechanism compound 1 is in equilibrium with extremely unstable tautomeric forms, viz.
2,5-dihydroxyfuran (8), 5-hydroxy-2(3H)-furanone (9), and succinic anhydride (10). The last compound is
hydrolyzed in aqueous medium with the formation of succinic acid 3 (Scheme 3). The effect described above of
acids and bases on the reaction is in agreement with this Scheme.
Scheme 3
HO
HO
OH
O
O
8
H2O
1
O
O
O
10
O
9
+
EtOH, H
–H2O
HOOC
COOH
EtOOC
COOEt
4
3
The addition of analytical reagents, reacting specifically with anhydrides of dicarboxylic acids
(NH2OH·HCl + FeCl3 [12]) and with enols (FeCl3, ethanol [13]), to furanone 1 or to solutions of it in anhydrous
solvents under various conditions (at different temperatures, in the presence of acids or bases, or without them)
595

did not lead to detection of the desired grouping. This result may be explained by the fact that in the absence of
water the equilibrium between furanone 1 and its tautomers 8-10 is shifted practically completely in the
direction of the first compound. In aqueous medium this equilibrium may be displaced in the direction of the
indicated intermediates due to their rapid irreversible conversion into the final product 3.
The use of the analytical reagents indicated above as traps for tautomers 8-10 proved to be impossible
due their rapid competing reaction with the initial substance 1, leading respectively to the oxime and Fe(III) salt
of β-formylacrylic acid.
The succinic acid 3 formed in the peroxide oxidation of furfural is therefore probably the conversion
product not only of its known precursors 5 and 7 but also of hydroxyhydrofuranone 1 (through its postulated
unstable forms 8 and 9).
The established possibility of the fairly ready conversion of 5-hydroxy-2(5H)-furanone (1) in aqueous
media into succinic acid 3 has been taken into consideration by us when planning syntheses based on it.
EXPERIMENTAL
Procedure for Obtaining Succinic Acid (3) from 5-Hydroxy-2(5H)-furanone (1). A solution of
furanone 1 (1.0 g, 10 mmol) in water (8.6 ml) was stirred at 60°C for 3-24 h. The reaction mixture was
evaporated at reduced pressure. Ethanol (5 ml), chloroform (10 ml), and a small quantity of TsOH were added
to the residue, and the mixture was boiled for 10 h with azeotropic distillation of water. The esterification
products obtained were analyzed by GLC in a Chrom 4 (Czechoslovakia) instrument with a flame-ionization
detector and a glass column (300 × 0.3 cm). Carrier gas was nitrogen at 30 ml/min. The carboxylic acid ethyl
esters were determined quantitatively on polymethylphenylsiloxane oil (PPMS-4), (15% on Chromaton N-AW
DMCS) at a column temperature of 150°C, evaporator and detector temperatures were 220°C. The time for
diethyl ester 4 to emerge was 345-355 sec.
Procedure for Detecting Dicarboxylic Acid Anhydrides in Nonaqueous Solutions of 5-Hydroxy-
2(5H)-furanone. A test sample (1-2 drops) of the reaction mixture, obtained by heating aqueous solutions of
furanone 1 at various pH values at 60-80°C, was mixed with freshly prepared reagent solution (3-4 drops)
containing a 5% alcoholic solution of FeCl3, acidified with HCl, and saturated with NH2OH·HCl. The mixture
was evaporated and water (several drops) was added to open the anhydride. No change was observed in the
color of the mixture in each case.
Procedure for Detecting Enols in Nonaqueous Solutions of 5-Hydroxy-2(5H)-furanone. A 1%
alcoholic solution (1-2 drops) of FeCl3 was added to an alcohol solution of the reaction mixture test sample. No
change was observed in the color of the solution.
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