Fast synthesis of some flavones under microwave irradiation lJI

: Reactions under microwave irradiation occur under solvent free conditions. Such reactions occur in much shorter time than the usual ones. Fast synthesis of flavone from o-hydroxyacetophenone has been achieved in three steps under microwave irradiation.

Some flavones have been synthesised by Baker-Venkataraman method under microwave irradiation. MW irradiation is an efficient and environmentally-benign method to activate various organic transformations to afford products in higher yields in shorter reaction periods and involving a very small amount of solvent 1 • 2 . Fast synthesis of flavones (4a-d) from o-hydroxyacetophenones has been achieved by us in three steps under microwave irradiation according to Scheme 1.

Results and discussion
The literature method 3 of benzoylation of ohydroxyacetophenone (Ia) uses pyridine as base and the reaction mixture requires pouring into large volume of hydrochloric acid to get the benzoyl derivative precipitated. However, benzoylation has been achieved in step 1 of the Scheme l by !>imply shaking (1) with sodium hydroxide solution (and benzoyl chloride) without the need to pour the reaction mixture into hydrochloric acid. Hence, the use of both pyridine and hydrochloric acid has been avoided. In step 2, methanol in very small volume was used instead of pyridine in large volume according to the literature method 3 . Use of methanol instead of pyridine obviates the disadvantages of the use of the latter. In the step 3, microwave irradiation for only 3 min was required instead of 1 h of heating under normal heating.
The yield in each step is also increased considerably.
The method reported here leads to a notabe improvement in reaction conditions for flavone synthesis : the reaction time is reduced to only a few minutes in the third step by using microwave dielectric heating, yield is improved, and the procedure is simplified by doing away with the large volume of pyridine.

Experimental
Reactions were carried out under atmospheric pressure in an open vessel adapted to a microwave oven. Compounds were identified by melting points which were found to be in good agreement with literature data. Flavones (4a-d) were also identified by IR, 1 H NMR and mass spectra.
M.ps. were taken in open capillaries and are uncorrected. Purity of the compound was checked by TLC. IR spectra were recorded on Perkin-Elmer 157 spectrometer in KBr. 1 H NMR spectra were recorded in CDCI 3 on a Broker WM 400 MHz spectrometer. using TMS as an internal reference. Mass spectra were measured on Jeol JMS-300 spectrometer at 70 eV.
o-Benzoyloxyacetophenones (2a-d). General procedure: 0.025 mol of o-hydroxyacetophenone (la-d) was suspended in 40 ml of 2 N NaOH solution in a wellcorked conical flask followed by the addition of 4.9 g (4 ml, 0.035 mol) of benzoyl chloride with constant shaking for about 20 min. The solid benzoyl derivative (2a-d) was filtered off, washed with 5 ml of ice-cold methanol and then with 5 ml of water and recrystallised from metha- Step 3 preheated in an oven at 100°. Stirring was continued for 15 min resulting in the separation of the yellow potassium salt of the product. The reaction mixture was cooled to room temperature and acidified by adding with stirring 25 ml of 10 percent aqueous acetic acid. The pale yellow precipitate so obtained was dried and recrystallised from 836 methanol.