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Indian Journal of ChemistryVol. 44B, September 2005, pp. 1947-1949

Note

A simple conversion of E -3-benzylidene-flavanones to 3-benzoylflavones by chromic

acid oxidation

Asok K Mallik & Falguni ChattopadhyayDepartment of Chemistry, Jadavpur University,

Kolkata 700 032, India

E-mail: mallikak52@yahoo.co.in

Received 2 September 2003; accepted (revised) 4 May 2005

Oxidation of E -3-benzylideneflavanones with CrO3/AcOH

yields 3-benzoylflavones in moderate yield.

Keywords: Benzylideneflavanones, benzoylflavones, chromicacid, oxidation

IPC : Int .Cl .7 C 07 D 311/30

In connection with some other problems 1 we requiredseveral trans -3-benzylflavanones 1. For preparationof 1, we chose chromic acid oxidation of 3( S *)-

benzyl-4( S *)-hydroxy-2( S *)flavans 2 obtainable bysodium borohydride reduction 2 of E -3-benzylidene-flavanones 3. Thus, when three 3( S *)-benzyl-4( S *)-

hydroxy-2-( S *)flavans 2a-c were oxidised in this way,the first two compounds gave the desired productswhile the last one gave trans -3-( p-methoxybenzoyl)-flavanone 4 (Scheme I) .This observation led us tosuggest that under the reaction condition the initial

product 1c is readily converted to the radical 5 (having a scope of enjoying extra stability byresonance) which is then transformed to 4 (ref. 3).

It was envisaged that E -3-benzylideneflavanones 3 on treatment with chromic acid might generate theradical 6 which would have the scope of enjoyingextra stability like 5 and of transforming to 3-benzoyl-flavones 7. So, chromic acid oxidation of 3 might beone method for direct conversion of 3 to 7.

Thus, when the E -3-benzylideneflavanone 3a wastreated with CrO 3-HOAc at room temperature, thereaction was found to be slow and about four dayswere required for disappearance of the startingmaterial. But on refluxing, the reaction was completewithin 6 hr and under both the reaction conditionsonly one product was obtained. Analytical andspectroscopic data confirmed it to be 3-benzoyl-flavone 7a (Scheme II ). The study was then extendedto five other E -3-benzylideneflavanones and theresults are presented in Table I .

NaBH 4

CrO 3 CrO 3

O

O H

H R

O H

H

R

H OH

O

O

H

H

OMe

O

2

3 4

O

O

H

H

OMe

5

1a, 1b 1c

a : R = H, b : R = Cl , c : R = OMe

Scheme I

O

O

H

H

R

1 a : R = H, b : R = Cl, c: R = OMe

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An interesting feature observed in the 1H NMRspectra of 3-benzoylflavones 7 is that the ortho -

protons of their B-ring (H-2',6') are seen to appear atsomewhat higher field as compared to the same

protons of corresponding 3-unsubstituted flavones ( 7.80-7.90).

So, this would be a very simple method for directconversion of E -3-benzylideneflavanones 3 to 3-

benzoylflavones 7 and is a good addition to themethods reported earlier 4-6.

Experimental Section

All melting points are uncorrected. IR spectra wererecorded in KBr disc on a Perkin-Elmer 297spectrophotometer; 1H NMR spectra in CDCl 3 on aBruker AM-300L (300 MHz) spectrometer.

General procedure for the reaction of CrO 3 with E -3-benzylideneflavanones 3 : To a solution of E -3- benzylideneflavanone 3 (1 mmole) in glacial aceticacid (10 mL), a solution of CrO 3 (5 mmole) in thesame solvent (5 mL) was added and the resultingmixture was either kept at room temperature for 4days or at reflux for 6 hr. It was then poured intowater (100 mL) and aqueous solution of sodiummetabisulphite was added to reduce Cr(VI). Theresulting material was then extracted with chloroform(3 25 mL), washed with water, dried and con-

centrated. Column chromatography of the material soobtained furnished the pure products 7.

Analytical and spectral data of the products are asfollows:

7a : m. p. 114-15 , IR: 1672 (C=O), 1632(C=O) cm -1; 1H NMR : 7.33-7.56 (7H, m, Ar-H),7.60 (1H, br. d, J = 8.4 Hz, H-8), 7.63-7.67 (2H, m,H-2',6'), 7.77 (1H, ddd, J = 8.4, 7.8 and 1.5 Hz, H-7),7.91-7.94 (2H, m, H-2",6"), 8.25 (1H, dd, J = 8.1 and1.5 Hz, H-5); Found: C, 80.78; H, 4.18. C 22H14O3requires C, 80.97; H, 4.32%.

7b : m. p. 168-69 , IR: 1655 (C=O), 1615 (C=O)cm -1; 1H NMR : 3.82 (3H, s, OCH 3), 6.87 (2H,

pattern resembles a pair of triplets 7, J = 7.2, 2.7 or 2.1Hz, H-3",5"), 7.33-7.49 (4H, m, Ar-H), 7.59 (1H, br.d, J = 8.6 Hz, H-8), 7.66-7.69 (2H, m, H-2',6'), 7.76(1H, ddd, J = 8.1, 7.8 and 1.5 Hz, H-7), 7.89 (2H,

pattern resembles a pair of triplets 7, J = 6.9 and 2.1 or2.7 Hz, H-2",6"), 8.25 (1H, dd, J = 7.8 and 1.5 Hz, H-5); EIMS m/z (rel. intensity): 356 (100, M +), 328(84.6, M + CO), 314 (10.0), 279 (11.1), 249 (22.3),208 (8.1), 165 (11.5), 135 (70.7, MeO C6H4 C O

+),129(68.2), 107 (13.0), 92 (41.1), 77 (37.4); Found: C,77.31; H, 4.49. C 23H16O4 requires C, 77.52; H, 4.53%.

7c : m. p. 161-62 , IR: 1665 (C=O), 1620 (C=O)cm -1; 1H NMR : 3.95 (3H, s, OCH 3) 6.98 (1H, d, J =2.4 Hz, H-8), 7.03 (1H, dd, J = 8.8 and 2.4 Hz, H-6),7.35-7.43 (5H,m, Ar-H), 7.52 (1H, t, J = 8.1 Hz, H-

4"), 7.62-7.65 (2H, m, H-2',6'), 7.90-7.93 (2H, m, H-2",6"),8.15 (1H, d, J = 9.0 Hz, H-5); Found: C, 77.39;H, 4.66. C 23H16O4 requires C, 77.52; H, 4.53%.

7d : m. p. 171-72 , IR: 1665 (C=O), 1615 (C=O)cm -1; 1H NMR : 7.32-7.58 (8H, m, Ar-H), 7.63-7.65(2H, m, H-2',6'), 7.90-7.92 (2H, m, H-2",6"), 8.03(1H, dd, J = 1.5 and 0.9 Hz, H-5); Found: C, 73.01;H, 3.77. C 22H13O3Cl requires C, 73.23; H, 3.63%.

7e : m. p. 169-70 , IR: 1670 (C=O), 1630 (C=O)cm -1; 1H NMR: 2.49 (3H, s, CH 3), 7.34 (2H, patternresembles a pair of triplets 7, J = 8.7, 2.4 or 1.8 Hz,

CrO 3 , AcOH

rt / Heat

6

O

O

R 2

R 1

R 3

R 4

O

O

HO

R 2

R 1

R 3

R 4 O

O

R 2

R 1

R 3

R 4

H

3 7

a : R 1 = R 2 = R 3 = R 4 = H b : R 1 = R 2 = R 3 = H, R 4 = OMec: R 1 = R 3 = R 4 = H, R 2 = OMe d : R 1 = Cl, R 2 = R 3 = R 4 = He: R 1 = Me, R 2 = H, R 3 = R 4 = Cl f : R 1 = R 3 = R 4 = Cl, R 2 = H

Scheme II

Table I Results of CrO 3 oxidation of E -3- benzylideneflavanones

Yield (%)Substrate ProductAt Room Temp.

(4 days)Under Reflux

(6 hr)

3a 7a 40 443b 7b 38 423c 7c 37 323d 7d 43 413e 7e 35 383f 7f 46 48

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NOTES 1949

H-3',5'), 7.39 (2H, pattern resembles a pair of triplets 7, J = 8.4 and 1.5 or 2.4 Hz, H-3",5"), 7.48 (1H, d, J = 8.4 Hz, H-8), 7.57 (2H, pattern resembles a pair oftriplets 7, J = 8.7 and 1.5 or 2.4 Hz, H-2',6'), 7.58 (1H,

dd, J = 8.1 and 2.1 Hz, H-7), 7.85 (2H, patternresembles a pair of triplets 7, J = 8.7 and 2.4 or 1.5 Hz,H-2",6"), 8.00 (1H, br. s, H-5); Found: C, 67.55; H,3.64. C 23H14O3Cl2 requires C, 67.49; H, 3.45%.

7f : m.p. 166-67 , IR: 1665 (C=O), 1620 (C=O)cm -1; 1H NMR: 7.32-7.71 (8H, m, Ar-H), 7.90 (2H,d, J = 7.8 Hz, H-2",6"), 8.19 (1H, d, J = 2.7 Hz, H-5);Found: C, 61.15; H, 2.54. C 22H11O3Cl3 requires C,61.49; H, 2.58%.

Acknowledgements

The authors are thankful to the authorities of IICB,Kolkata and RSIC, CDRI, Lucknow for 1H NMR and

mass spectral measurements and to the UGC, NewDelhi for financial assistance.

References

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3 House H O, Modern Synthetic Reactions , (The Benjamin /Cummings Publishing Company, Menlo Park, California),1972 , 228.

4 Dhande V P, Thankwani P & Marathe K G, Tetrahedron , 44,1988 , 3015.

5 Chawla H M & Sharma S K, Bull Soc Chim Fr , 1990 , 656.6 Chawla H M & Sharma S K, Synth Commun , 20, 1990 , 301.7 Dyer J R, Application of Absorptions Spectroscopy of Organic

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