Synthesis of Flavone Skeleton by Different Methods

Flavones (flavus = yellow), are a class of flavonoids based on the backbone of 2-phenylchromen-4-one. Flavones are mainly found in cereals and herbs. Flavones are biologically active compounds. Therefore number of synthetic methods were developed. In this mini revive we have tried to cover various synthetic strategies for the synthesis of flavones. Some of the well known methods used for synthesis of flavones are Baker & Venkatraman synthesis and Claisen-Schmidt condensation.


INTRODUCTION
Scheme18: One pot synthesis of flavones using ferric chloride is efficient method carrid out by Rajiv Karmarkar & co-worker 26

OH
Br    38 suggested the following methodologies respectively Heat,PhNO 2 Scheme 30: LiHDMS,THF H 2 SO 4 ,AcOH Scheme 31: Hydrogen peroxide is used as catalyst for this one pot method 39 O R OH

Flavones
(flavus = yellow), are a class of flavonoids based on the backbone of 2-phenylchromen-4-one Apar t from flavones other flavonoids are isoflavonoids, derived from 3-phenylchromen-4-one structure neoflavonoids, Flavones are well known for their various biological activities such as anticancer1 Anti -osteoporotic3, anti-diabetic4, etc. some of the examples as shown as under.Synthetic strategies of flaovones Tra d i t i o n a l l y, f l avo n e s h ave b e e n prepared by BakerVenkatramanrearrangement and Claisen-Schmidt condensation.whichinvolves the conversion of 2hydroxyacetophenones into benzoyl esters, followed by rearrangement in base to1,3diphenylpropane1,3diones which upon cyclization under acidic conditions furnishes flavones.On the other hand hydroxychalcone synthesized from 2hydroxyacetophenone anbenzaldehyde under ClaisenSchmidt conditions can undergo oxidative cyclization to furnish flavones ring.

is used for the synthesis of flavones 15 Scheme 8 : 16 Scheme 9 : Koneni & his group first time invented flavones in which oxygen of flavone come from watr molecule 17 Scheme10: A two step synthesis of flavones via Wacker oxidation is carried out in this process 18
Scheme 7: A Novel Synthesis of 4H-Chromen-4-ones via IntramolecularWittig Reaction is used for the synthesis of flavones15

Scheme 17 .
Scheme 15: Palladiumacetate is used catalyst for the synthesis of flavones23

3 OScheme19: Silica supported lewis acids indium chloride & indium bromide undergoes oxidative coupling to give flavones 27 Scheme 20 : Wet acetone is efficient catalyst for the one pot synthesis of flavones from 2 -hydroxy acetophenone & acetyl chloride 28 Scheme 21 :
Scheme19: Silica supported lewis acids indium chloride & indiumbromide undergoes oxidative coupling to give flavones27

Scheme 22 : 30 Scheme 23 :Scheme 24 : Iodo & bromo derivatives of flavones were synthesized by this method 32 Scheme 25 : Oxidative cyclisation followed by bromination is carried out by this process 33 Scheme 26 :
Scheme 22: Carbonylative couplig using Pd catalyst is invention of this method30

Scheme 27 : 3 O 29 :
Scheme 27: Wittig reaction is applied for the synthesis of flavones35

tellurim oxide is used for the oxidativ coupling method by the author KumarS& Sharma D 48 Scheme 41 :
Scheme 39: Jae In Lee et al47 suggested the following methodology