Base-catalyzed Degradations of Carbohydrates. 111. Formation of a Novel Perester from 1-0-Acetyl-3-deoxy-2,4,6-tri-0-methyl-

G. 0. ASP IN ALL,^ R. R. KING, AND ZOFIA PAWLAK~ Department of Chemistry, Trent University, Peterborough, Ontario

Received July 21, 1972

l-O-Acetyl-3-deoxy-2,4,6-tri-O-methyl-~-hex-2-enopyranose (1) reacts with m-chloroper- benzoic acid to give a novel perester, which has been assigned the structure 1,2-0-(1 '-m-chloroperbenzoyl- oxyethylidene)-2-methoxy-4,6-di-O-methyl-a-~-gIucopyranose (2), on the basis of spectral data and chemical degradation. The perester 2 and the derived acetate 3 undergo oxidative degradation on treatment with trifluoroacetic acid to give 3,5-di-0-methyl- (4) and 2-0-acetyl-3,5-di-0-methyl-D- arabinonolactone (5), respectively. Reductive decomposition of the acetylated perester 3 yields 1,3-di-0- acetyl-4,6-di-O-methyl-a-o-arabino-hexopyranosulose (6), which, on treatment with acetic anhydride in pyridine, gives successively 1,2,3-tri-0-acetyl-4,6-di-O-methyl-~(-~-eryhro-hex-2-enopyranose (7), 2-0- acetyl-l-deoxy-4,6-di-O-methyl-cc-~-e~hro-hex-l-enopyranose-3-uose (8), and 5-acetoxy-2-(methoxy- methyl)-4H-pyran-4-one (9).

Le 0-acetyl-1 dkoxy-3 tri-0-methyl-2,4,6 a-D-erythuo-hexeno-2 pyranose (1) riagit avec I'acide m-chloroperbenzofque pour conduire a un perester nouveau auquel a Cte attribuee la structure 0-(m- chloroperbenzoyloxy-1 6thylidene)-1,2 methoxy-2 di-0-methyl-4,6 a-D-glucopyranose (2), sur la base des donnees spectrales et degradation chimique. Le perester 2 et le derive acetate 3 subissent une degradation oxydante par traitement a I'acide trifluoroacetique qui conduit respectivement au di-0-methyl-3,5 (4) et 0-acetyl-2 di-0-methyl-3,s D-arabinonolactone (5). La decomposition du perester acttyle 3 par reduction mene au di-0-acetyl-1,3 di-0-methyl-4,6 a-D-arabino-hexopyranosulose (6) qui, apres traitemenr a l'anhydride acetique dans la pyridine, conduit successivement aux tri-0-acetyl-1,2,3 di-0-methyl-4,6 a-D-erythro-hexeno-2 pyranose (7), 0-acCty1-2 dkoxy-1 di-0-methyl-4,6 a-D-erythro-hexeno-1 pyranose ulose-3 (8), et acetoxy-5 (methoxymethy1)-2-4H-pyranone-4 (9).

[Traduit par le journal] Can. J . Chem., 51. 388 (1973)

3-Deoxyhex-2-enopyranoses, which are formed on alkaline degradation of 2,3-di- and higher 0-substituted hexopyranoses, and their deriva- tives (1-4) are very susceptible to degradation by acid. The possible formation, by oxidation of 3- deoxyhex-2-enopyranoses, ofy-pyrones as chemi- cally stable derivatives has been explored. 2,3,4,6- Tetra-0-methyl-D-glucose was chosen as an ap- propriate model compound for a reducing sugar so that the fate of all substituents during the transformations might be studied. 3-Deoxy-2,4,6-tri-0-methyl-D-erythro-hex-2-

enopyranose (1, 2), formed on alkaline degrada- tion of 2,3,4,6-tetra-0-methyl-D-glucose, was

'Presented in part at the Canadian Chemical Con- ference of the Chemical Institute of Canada, Halifax, Nova Scotia, May 31-June 2, 1971.

'Present address to which correspondence should be sent: Department of Chemistry, York University, Downsview, Ontario.

3 0 n leave (1969-1970) from the Department of Organic Chemistry, University of Warsaw, Warsaw, Poland.

treated with acetic anhydride to give the 1- acetate 1 having the a-D-configuration. When the unsaturated sugar acetate was treated with rn- chloroperbenzoic acid in ether a crystalline com- pound separated. Although this substance was relatively unstable and decomposed on prolonged standing at room temperature, a satisfactory elemental analysis was obtained for a perester of molecular formula C,,H,,O,Cl and for which structures 2, 2a, and 2b were considered (see Scheme 1). The i.r. spectrum of the compound showed an hydroxyl band and a single carbonyl band at 3600 and 1760 cin-', and the n.m.r. spectrum showed the presence of an aromatic ring, three methoxyl groups, and a methyl group (z 8.15) of an acetate or modified acetate. The perester furnished a syrupy monoacetate (3) whose n.m.r. spectrum showed an incompletely resolved doublet (z 4.72, J,., = 1.1 Hz), attrib- utable to H-3. In the light of subsequent chemi- cal transformations which established the D- arabino configuration at C-3 to -5, the n.m.r. spectrum was characteristic of a 1,2-0-alkylidene

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ASPINALL ET AL.: DEGRADATION OF CARBOHYDRATES. 111

Me0 OH OOAc

OR OMe 2a 26

M e 0 0 - OAc MeOQAc

AcO OAc 0

derivative having a skew-boat conformation (5-8). The mass spectra of the perester and its monoacetate 3 showed no molecular ions, but prominent fragment ions corresponded to loss of m-chlorobenzoylperoxy groups (nzle 263 and 305, respectively) and to the formation of m- chlorobenzoic acid (mle 156 and 158) and species derived therefrom.

The presence of a peracid group in compound 2 was first suggested when treatment with tri-

fluoroacetic acid resulted in oxidative degrada- tion and the isolation of 3,5-di-0-methyl-D- arabinonolactone (4). The acetylated perester 3 likewise on treatment with trifluoroacetic acid underwent oxidative degradation to give 2-0- acetyl-3,5-di-0-methyl-D-arabinonolactone (5), whose structure was confirmed by its formation on acetylation of the lactone 4. These parallel reactions indicate that the 3-hydroxyl group in the perester does not appear to participate in

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390 CAN. J. CHEM. VOL. 51, 1973

the oxidative degradation (9, 10); furthermore, no products from C,-C, cleavage were detected.

Direct evidence for the peracid function was obtained by estimation of active oxygen by treating the perester 2 with potassium iodide - acetic acid in acetone ( l l ) , the decomposition also leading to a nearly quantitative recovery of m-chlorobenzoic acid. Treatment of the acetyl- ated perester 3 with zinc dust in acetic acid furnished 1,3-di-0-acetyl-4,6-di-0-methyl-a-D- ar.c/bino-hexpyranosulose (6), whose n.m.r. spec- trum showed infer alia a one-proton doublet (7 4.56, J,,, = 16.3 Hz) which may be assigned to H-3 in confirmation of the D-arabino con- figuration. Reaction of the glycosulose 6 with acetic anhydride in pyridine at room temperature gave a syrupy enediol acetate, 1,2,3-tri-0-acetyl- 4,6-di-O-metl1yl-a-~-erytI1ro-hex-2-enopyranose (7), whose structure was supported by the detec- tion of a nlolecular ion peak (mle 332) in the mass spectrum, the presence of an en01 ester band at 1780 cm-' in the i.r. spectrum, and the appear- ance in the n.m.r. spectrum of a new acetate signal, the disappearance of the H-3 signal, and the appearance of the H-4 signal as a doublet (T 5.57, J,,, = 15 Hz). The enediol acetate 7 when heated with acetic anhydride, yielded the ene- diolone acetate derivative, 2-0-acetyl-I-deoxy- 4.6-di-0-methyl-2-D-erj.fhr.o-hex- 1 -enopyranose ($1, and, on prolonged heating, the kojic acid derivative, 5-acetoxy-2-(niethoxymethy1)-4H- pyran-4-one (9). The structure of the latter coni- pound was confirmed by its formation on acetyl- a t ion of 5-hydroxy-2-(methoxymethy1)-4H- pyran-4-one (monomethylkojic acid (10) (12)).

The observations in this and the following paper (1 3) indicate that oxidation of derivatives of 3-deoxy-hex-2-enopyranoses does not provide a simple route to the formation of y-pyrones. Special comment, however, may be made on the structure of the novel perester 2, and on the transformation of glycosuloses into y-pyrones. Since hydroxy peresters, formed via epoxides, have been postulated as intermediates in the reaction of n~-chloroperbenzoic acid with cyclic en01 ethers (9, lo), structure 2a was first con- sidered for the perester. However, the 3-hydroxyl group does not participate in the oxidative degra- dation of the perester as shown by the conversion of bhe hydroxy perester and its acetylated deriva- tive, when treated with trifluoroacetic acid, into the lactones 4 and 5 respectively. Since reaction of glycosulose 6 with m-chloroperbenzoic acid in trifluoroacetic acid resulted in forination of

the l a ~ ~ o n e 5, it is probable that reaction of the peresters 2 and 3 with trifluoroacetic acid pro- ceeds by hydrolysis of the ketal function, fol- lowed by intermolecular oxidation of the derived glycosulose with the liberated peracid.

We may consider here three possible structures for the perester and Scheme 2 indicates possible pathways in which reaction of the hex-2-eno- pyranose 1 with m-chloroperbenzoic acid affords a 2,3-anhydro-2-methoxy-~-1?1a1?no-derivative as an intermediate. It is shown in the following paper (13) that the corresponding 2,3-anhydro-2- methoxy-D-allo-derivative does not react further with m-chloroperbenzoic acid. The formation of a perester of structure 2a by direct opening of the D-manno-epoxide with peracid as an external nucleophile appears unlikely. On the other hand epoxide opening by the neighboring 1-acetoxy group would give an intermediate cyclic acetox- onium ion which could react with further peracid to give a perester of structure 2 if attack took place at C-I' or of structure 26 by attack at C-2. The bicyclic structure 2 for the perester is assigned since only a single carbonyl band at 1760cm-I is observed in thei.r. snectrum whereas the acetylated derivative 3 shows two bands at 1770 and 1740 cm-I assignable to perester and acetate groups. More importantly the signal in the p.m.r. spectrum assigned to H-3 in the acetyl- ated derivative is characteristic of those of H-3 in other 1 ,2-0-alkylidene-a-D-glucopyranose de- rivatives where ring fusion results in distortion of the pyranose ring from a chair to a skew-boat conformation (5-8). In contrast, the n.m.r. pa- rameters for two 2,2'-di-0-substituted hexopy- ranose derivatives, which are reported in the following paper (13), indicate that both com- pounds exist in undistorted ,C, conformations. There is at present no expegimental evidence concerning the configuration at C-1' of the oxyethylidene group.

The overall conversion of glycosuloses into derivatives of kojic acid (14) involves elimina- tions of the 1-0- and 4-0-substituents, and migration of the keto function froin C-2 to -3. For any particular transformation the sequence in which these steps occur will be governed in part by the nature of the leaving groups in the p-eliminations. In a recent study Lichtenthaler and Heidel (15) have isolated an intermediate

\ ,

enolone acetate in the formation of kojic acid in which the first step involves the elimination of the 4-0-substituent. Enediol derivatives have been postulated as intermediates in the migration

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ASPINALL ET A L DEGR4DATION O F CARBOHYDRATES I11

OMe

I OMe

2b

- MeOb + OAC@ T AcOH

Me0 Me0 - OAc

AcOgH O ~ C G A C AcO OAc

Me0 b-, -k I:& T OAC' -- k20

Me0 - OAc f+

~ ~ 0 ' ~ ~ c - o OAC o OAC

of the keto function (16), but have not previously been detected. In the present investigation an enediol acetate, 7, has been characterized as the first product in the conversion of the glycosulose 6 into the y-pyrone 9. Generation of the 3-keto function and elimination of the I-0-substituent take place with the formation of the enediolone 8. The second elimination involving loss of the 4-0-substituent, in this case methoxy, a poor leaving group, does not take place readily. A possible mechanism for the conversion of glycos- ulose 6 via enediol acetate 4 into enolone acetate 8 is shown in Scheme 3.

Experimental Melting points are uncorrected and were determined

on a Kofler hot-stage microscope. Optical rotations were measured in chloroform solution with a Perkin-Elmer model 141 polarimeter at room temperature. The i.r.

spectra as films or in Nujol mulls and U.V. spectra in ethanol solution were determined on Unicam SP 200 and 800A spectrophotometers. The n.m.r. spectra were re- corded on a JEOL C-60HL spectrophotometer in deuterochloroform with tetramethylsilane as internal standard. Mass spectra were recorded on an A.E.I. MS 12 mass spectrometer. Plates for t.1.c. were prepared with Kieselgel GF 254 (Merck) as adsorbent, and the dried plates were sprayed with ethanolic 5% sulfuric acid. Light petroleum refers to the fraction of b.p. 40-60". Solutions were concentrated below 50' under reduced pressure. Microanalyses were determined by A. B. Gygli, Toronto, Ontario, and by F. and E. Pascher, Bonn, West Germany.

l-O-Acetyl-3-deoxy-2,4,6-tri-O-methyl-~-~-mythro- hex-2-mopyranose ( I )

3-Deoxy-2,4,6-tri-0-methyl- a - D - erythro - hex-2- eno- pyranose (1, 2) (0.8 g) was treated with acetic anhydride (4 ml) in pyridine (10 ml) for 18 h at 0" and 6 h at room temperature. The reaction mixture was poured into ice- water (150ml), the solution was extracted with chloro- form, and the extract was washed with sodium hydrogen

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392 CAN. J. CHEI vf. VOL. 51, 1973

carbonate solution, dried, and concentrated. The residue was crystallized from ether-pentane to give l-O-acetyl-3- deoxy-2,4,6-tri-O-methyl-a-~-erythuo-hex-2-enopyranose (I) (0.5 g, 52%), m.p. 71-72", [a], +45" (c, 1.0); v,,, 1735 (ester C=O), 1670 cm-' (C-C); n.m.r. T 3.91 (one- proton singlet, H-1), 5.04 (one-proton doublet, J3,4 = 2 Hz, H-3), 6.44 (three-proton singlet, OCH,), 6.62 (six- proton singlet, 2 x OCH,), 7.90 (three-proton singlet, OCOCH,).

Anal. Calcd. for C11H1,06: C, 53.65; H, 7.28. Found: C, 53.76; H, 7.29.

1,2-0- (1'-m-Chloroperbenzoyloxy-ethylfdene) -2- methoxy-4,6-di-0-methyl-a-D-glucopyranoe (2)

The acetate 1 (0.38 g) was added to a cooled solution of m-chloroperbenzoic acid (0.34 g) in ether (3 ml) and the solution was kept at room temperature for 3 h and at 0" for 18 h. The crystals which separated were removed by filtration, washed with ether to remove peracid and m-chlorobenzoic acid, and recrystallized twice from ether to give the perester 2 (0.32 g, 48%), m.p. 123-124"; v,,,3600 (OH), 1760cm-' (perester C=O); n.m.r. T 2.12-2.73, (four aromatic protons), 4.43 (one-proton singlet, H-1), 6.60 (nine-proton singlet, 3 x OCH,), 8.13 (three-proton singlet, C-CH,). The supernatant liquid was washed with sodium sulfite (until the washings gave a negative reaction with starch-iodide paper), sodium hydrogen carbonate, and water, dried, and concentrated. Crystallization of the residue from ether afforded further perester 2 (0.08 g, 1273, m.p. 122-123", [a], - 14" (c 1.0). The same product was obtained when the oxidation was performed in dioxan or dichloromethane. T.1.c. showed that complete disappearance of starting material occurred only when 2 molar proportions of oxidant were used.

Anal. Calcd. for ClSHZ3O9C1: C, 49.76; H, 5.29; C1, 8.06.Found:C,49.49;H,5.16;C1,8.11.

The mass spectrum showed no molecular ion but prominent peaks were detected at mle 263 (M - C1C6- H4C03), and 156 and 158, and 139 and 141 (isotope pairs). Active oxygen in the perester 2 was estimated by degradation with potassium iodide - acetic acid in acetone solution followed by titration of liberated iodine with sodium thiosulfate (11). The value was 5% in excess of that calculated in the basis of the proposed structure. In addition, a virtually quantitative yield of m-chloro- benzoic acid (identified by m.p., mixed m.p., and i.r. suectrum) was obtained.

3-0-Acetyl-I,2-0- (1'-m-chloroperbenzoyloxy-ethylidene) - 2-methoxy-4,6-di-0-methyl-a-D-glucopyrane ( 3 )

The perester 2 (0.3 g) was kept in acetic anhydride (2 ml) and pyridine (5 ml) overnight at room temperature. The reaction product was poured into ice-water (50 ml), the solution was extracted with chloroform, and the extract was washed, dried, and concentrated to give syrupy acetylated perester 3 (0.26 g), which was homog- enous on t.l.c., but was insufficiently stable for satis- factory elemental analysis; v,,, 1770 (perester C=O), 1740 cm-' (acetate C=O); n.m.r. T 2.14-2.73 (four aromatic protons), 4.45 (one-proton singlet, H-l), 4.62 (incompletely resolved one-proton doublet, splitting 1.1 Hz), 6.43, 6.63, 6.76 (three-proton singlets, 3 x OGH,), 7.88 (three-proton singlet, OCOCH,), 8.10 (three-proton singlet, C-CH,). The mass spectrum

showed no molecular ion but prominent peaks were detected at m/e 305 (M - CIC6H4C0,), and 156 and 158, and 139 and 141 (isotope pairs).

Degradation of Perester 2 in TriJuoroacetic Acid The perester 2 (0.44 g) was kept in trifluoroacetic acid -

water (9:1, 5 ml) at room temperature for 30 min. The solution was concentrated, the residue was dissolved in ethanol, and the solution was neutralized by shaking with Amberlite resin IR-45 (OH) and concentrated. The resulting syrup crystallized from toluene to give 3,5-di-0- methyl-D-arabinonolactone (4) (0.12 g, 67%), m.p. 71°, [a], +48" (c, 1.0), whose i.r., n.m.r., and mass spectra were identical to those of the L-enantiomer.

Degradation of Acetylated Perester 3 in TriJuoroacetic Acid

The acetylated perester 3 (0.3 g) was kept in trifluoro- acetic acid - water (9:1, 5 ml) at room temperature for 30 min. The reaction mixture was worked-up as described above and the product crystallized from ether-pentane to give 2-0-acetyl-3,5-di-0-methyl-D-arabinonolactone (5) (0.11 g, 58%), m.p. 53", [a], +27" (c, 1.0); v,,, 1800 (y-lactone), 1750 cm-' (acetate C=O); n.m.r. T 4.56 (one-proton doublet, J2., = 7 Hz, H-2), 6.59 (six-proton singlet, 2 x OCH,) 7.81 (three-proton singlet, OCOCH,); M + , 218.

Anal. Calcd. for CgH1406: C, 49.53; H, 6.42. Found: C, 49.49; H, 6.47.

The lactone 4 (0.12 g) was treated with acetic an- hydride (2 ml) and pyridine (5 ml) at 0" for 1 h and then at room temperature for 24 h. The reaction product was worked-up in the usual way and crystallization from ether-pentane furnished 2-0-acetyl-3,5-di-0-methyl-D- arabinonolactone (5) (0.08 g), m.p. and mixed m.p. 53" [aJD +27" (c, 1.0 in chloroform), whose i.r., n.m.r,, and mass spectra were indistinguishable from those of the previously isolated sample.

1,3-Di-0-acetyl-4,6-di-0-methyl-a-D-&o- hexopyranosulose ( 6 )

The acetylated perester (3) (1.12 g) in acetic acid (35 ml) was stirred with zinc dust at room temperature until (en. 45 min) t.1.c. (ethyl acetate) indicated that decom- position was complete. Chloroform was added to the reaction mixture, and the solution was filtered, neutral- ized by washing with saturated aqueous sodium hydrogen carbonate, dried, and concentrated to a residue which crystallized from light petroleum to give the glycosulose 6 (0.394 g, 58%), m.p. 62-63", [a], f47.7" (c, 1.3); v,,, 1755 cm-' (C-0); n.m.r. T 3.92 (one-proton singlet, H-1), 4.56 (one-proton doublet, J3,4 = 16.3 HZ, H-3), 6.47, 6.57 (three-proton singlets, 2 x OCH,), 7.76, 7.82 (three-proton singlets, 2 x OCOCH,); M + , 290.

Anal. Calcd. for C,zHISOn: C, 49.65; H, 6.25. Found: C, 49.11; H, 6.36.

Oxidative Degradation of Glycosulose 6 to Lacrone 5 Glycosulose 6 (60 mg) in trifluoroacetic acid (10 ml)

was treated with m-chloroperbenzoic acid (80 mg) for 24 h at room temperature. The reaction mixture was worked-up in the usual way and crystallization from ether-pentane furnished 2-0-acetyl-3,5-di-0-methyl-D- arabinonolactone (5) which was similar (m.p., and i.r. and n.m.r. spectra) to the previously prepared sample.

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ASPINALL ET AL.: DEGRADATION O F CARBOHYDRATES. 111 393

1,2,3-Tri-O-acetyl-4,6-di-O-methyl-cc-~-erythro-hex- 2-enopyranose (7)

Glycosulose 6 (310 mg) in acetic anhydride (8 ml) and pyridine (12 ml) was kept at room temperature for 4 h. The mixture was poured into ice-water, neutralized with sodium hydrogen carbonate, and extracted with chloro- form. The chloroform solution was concentrated, with pyridine being removed by co-distillation with toluene to give a virtually quantitative yield of crude enediol acetate 7. A sample of the syrupy product was purified by preparative layer chromatography (ethyl acetate - light petroleum, 1 :5) and had [a], +34" (c, 1.0); M+, 332; v,,, 1780 (enol acetate), 1760cm-' (acetate); n.m.r. T 3.55 (one-proton singlet, H-1), 5.57 (one-proton doublet, J4,5 = 15 HZ, H-4), 6.54, 6.58 (three-proton singlets, 2 x OCH,), 7.79, 7.85, 7.89 (three-proton singlets, 3 x OCOCH,).

Anal. Calcd. for CI4HZ0O9: C, 50.60; H, 6.07. Found: C, 50.45; H, 5.91.

2-O-Acetyl-l-deoxy-4,6-di-O-methyl-cc-~-erythro-hex-l- enopyvanose-3-ulose ( 8 )

Enediol acetate 7 (200 mg) was heated in acetic an- hydride (8 ml) and pyridine (12 ml) on a boiling-water bath for 6 h. The cooled solution was poured into ice- water, neutralized with sodium hydrogen carbonate, and extracted with chloroform, and the dried extract was concentrated to a syrup, in which t.1.c. indicated the presence of two new compounds. Preparative layer chromatography (ethyl acetate - light petroleum, 2:5) afforded the more labile major component 8 (83 mg, 52%) as a syrup which crystallized from ether - light petroleum and had m.p. 68-70", and [a] , +132" (c, 1.0); Mi , 230; h,,, 266 nm (log c 4.16); v,,, 1760 (acetate), 1690 (C=O), 1630 cm-' (C=C); n.m.r. T 2.61 (one-proton singlet, H-1), 5.94 (one-proton doublet, J4,5 = 17.5 Hz, H-4), 6.32, 6.52 (three-proton singlets, 2 x OCH,), 7.72 (three-proton singlet, OCOCH,).

Anal. Calcd. for Cl0Hl4O,: C, 52.17; H, 6.13. Found: C, 52.69; H, 6.35.

5-Acetoxy-2-(methoxymethyl)-4H-pyvniz-4-one ( 9 ) The second component from the above reaction mix-

ture was separated chromatographically as a syrup (14 mg, 10%) which crystallized from light petroleum to give 9, m.p. 50-51"; M + , 198; I,,, 251 nm (log s 4.00);

v,,, 1780 (acetate), 1670 (dienone), 1642 cm-I (C-C); n.m.r. T 2.12 (one-proton singlet, H-6), 3.53 (one-proton singlet, H-3), 5.73 (two-proton singlet, CH2), 6.53 (three- proton singlet, OCH,) 7.17 (three-proton singlet, OCOCH).

Anal. Calcd. for CgH1005: C, 54.54; H, 5.09. Found: C, 54.77; H, 5.21.

The hex-1-enopyranos-3-ulose 8 on further treatment with hot acetic anhydride and pyridine furnished the kojic acid derivative 9. Reaction of 5-hydroxy-2-(me- thoxymethy1)-4H-pyran-4-one (10) (12) with acetic an- hydride and pyridine afforded the acetate 9, which was identical (m.p. and mixed m.p., t.l.c., and i.r. and n.m.r. spectra) to the previously isolated sample.

The authors wish to acknowledge financial support from the National Research Council of Canada.

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