Novel conversion of 3-(α-hydroxybenzyl)flavones to 3-benzoylchromones and 3-cyanoflavones with NaN3/TFA

Article abstract of DOI:10.1016/S0040-4039(00)00730-9

On treatment with NaN₃/TFA, 3-(α-hydroxybenzyl)flavones are converted to 3-benzoylchromones and 3-cyanoflavones, plausible mechanisms for which have been suggested. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00730-9

Tetrahedron Letters 41 (2000) 4929±4931
Novel conversion of 3-(a-hydroxybenzyl)¯avones to
3-benzoylchromones and 3-cyano¯avones with NaN3/TFA
Asok K. Mallik,* Falguni Chattopadhyay and Sankar P. Dey
Department of Chemistry, Jadavpur University, Calcutta-700 032, India
Received 17 February 2000; accepted 4 May 2000
Abstract
On treatment with NaN3/TFA, 3-(a-hydroxybenzyl)¯avones are converted to 3-benzoylchromones and
3-cyano¯avones, plausible mechanisms for which have been suggested. # 2000 Elsevier Science Ltd. All
rights reserved.
Keywords: 3-(a-hydroxybenzyl)¯avones; sodium azide/TFA; 3-benzoylchromones; 3-cyano¯avones.
On extending our current studies on reactions of E-3-benzylidene¯avanones (1)1 to the Schmidt
reaction some novel observations were obtained. Thus, on treatment with NaN3/TFA in air several
compounds yielded 3-benzoylchromones (2) by loss of the 2-aryl group as the corresponding
aniline.1d An aerial oxidation of intermediate 3-(a-hydroxybenzyl)chromones or corresponding
tri¯uoroacetates has been suggested to explain the occurrence of such products. 3-(a-Hydroxy-
benzyl)¯avones (3) are readily obtainable from 1 by reaction with NBS,1a and as 3 could be
considered as oxidation products of 1 we became interested in studying the reaction of the former
with NaN3/TFA. The novel aspects of this reaction are presented in this communication.
When the unsubstituted 3-(a-hydroxybenzyl)¯avone 3a was treated with 1 equivalent of NaN3
in TFA (rt, four days) it underwent ca. 80% conversion yielding 3-benzoylchromone (4a) as the
only product in moderate yield. Complete conversion of the substrate under the same reaction
* Corresponding author. Fax: 91-33-4731484; e-mail: piyas@juphys.ernet.in
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00730-9

4930
conditions was observed by use of 1.5 equivalents of NaN3. However, in this case another
product, characterised as 3-cyano¯avone (5a), was obtained in poor yield. The second product
could be obtained as the major product by use of still greater proportions of NaN3. Similar
results were obtained from other 3-(a-hydroxybenzyl)¯avones 3b±f (Table 1). On making the
aqueous solution left after ether extraction of the diluted reaction mixture alkaline, an aniline
corresponding to the phenyl group of the a-hydroxybenzyl moiety was obtained in each case.
Table 1
The mechanisms delineated in Scheme 1 may be suggested for the formation of 4 and 5 from 3.
In order to account for the results, migration of the phenyl group of the a-hydroxylbenzyl moiety
to an electron de®cient nitrogen becomes necessary in all the examples. This prompted us to study
the reaction of 3g (3 with R1=R2=H, R3=NO2) and in this case no product of the series 4 and 5
was obtained.3
Thus, we report a novel result originating from a simple reaction of 3-(a-hydroxybenzyl)-
¯avones (3). Although the benzopyrone moiety present in 3 is responsible for the formation of 4,
the transformation 3!5 appears to be a reaction typical of secondary alcohols. To our knowl-
edge, among a number of existing reports of reactions of secondary alcohols with HN3,4 none
mentions splitting of such compounds into amines and cyanides. The present study therefore has
potential to be extended to many other structurally related substrates.

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Scheme 1.
Acknowledgements
The authors thank the authorities of IICB, Calcutta and RSIC, Bose Institute, Calcutta for
NMR and mass spectral measurements. Financial assistance from the UGC, New Delhi and the
authorities of Jadavpur University is gratefully acknowledged.
References
1. (a) Dhara, M. G.; De, S. K.; Mallik, A. K. Tetrahedron Lett. 1996, 37, 8001. (b) Mallik, A. K.; Dhara, M. G.;
Chattopadhyay, F. Indian J. Chem. 1998, 37B, 1164. (c) Dey, S. P.; Mallik, A. K. Indian J. Chem. 1999, 38B, 400;
(d) Mallik, A. K.; Chattopadhyay, F. Indian J. Chem. 1999, 38B, 889.
2. The 3-benzoylchromones 4a±d are known compounds.1d Selected data of 5: 5a mp 151±152ꢀC. Anal.: found C, 77.41;
H, 3.81; N, 5.40%; calcd for C16H9NO2 C, 77.72; H, 3.67; N, 5.66%. IR (ꢀ, KBr, cm^1): 2210 (CꢁN), 1645 (CˆO). 1H
NMR (300 MHz, CDCl3) ꢁ 7.54 (1H, dt, J=7.6 and 0.9 Hz, H-6), 7.59±7.71 (4H, m, Ar-H), 7.80 (1H, ddd, J=7.6,
7.2 and 1.5 Hz, H-7), 8.11±8.15 (2H, m, H-20,60) and 8.29 (1H, dd, J=8.0 and 1.5 Hz, H-5). 13C NMR (75 MHz,
CDCl3): ꢁ 98.36 (C-3), 113.96 (CꢁN), 118.28 (C-8), 122.03 (C-4a), 126.25 (C-6), 126.87 (C-5), 128.75 (C-30,50),
129.16 (C-20,60), 130.16 (C-10), 133.34 (C-40), 135.25 (C-7), 155.46 (C-8a), 170.84 (C-2) and 174.14 (C-4). EIMS: m/
z 247(M+). Compound 5b: mp 169±170ꢀC. Compound 5c: mp 206±207ꢀC. Compound 5d: mp 202±203ꢀC.
3. The product in this case was a viscous liquid which has been assigned the structure 6. IR (ꢀ, KBr, cm^1): 2105
(azide), 1640 (¯avone CˆO). H NMR (300 MHz, CDCl3): ꢁ 6.06 (1H, s, H-a), 7.41 (2H, d, J=8.4 Hz, H-200,600),
1
7.44±7.57 (7H, m, Ar-H), 7.74 (1H, ddd, J=8.7, 7.5 and 1.6 Hz, H-7), 8.06 (2H, d, J=8.7 Hz, H-300,500) and 8.22
(1H, dd, J=7.8 and 1.5 Hz, H-5).
4. March, J. Advanced Organic Chemistry; John Wiley and Sons: New York, 1992; p. 1092 and references cited therein.