Two new effective methodologies have been adopted for the preparation of 4-(2′-bromobenzyloxy)benzopyran-7-ones
In present time, the chemistry of radical cyclisation has been developed as a ripe field in all the aspects starting from polymers to synthesis of organic and inorganic compounds and such reactions have been applied to the synthesis of a number of natural products, pharmaceuticals, and others which mainly include heterocyclic compounds as a core structure [
Aryl radical cyclisation has been at the forefront for constructing carbon-carbon bonds in organic synthesis [
Recently, microwave enhancement of organic reactions has attracted the chemists [
It is well known that microwave irradiation enhanced the reaction rates of Pd-catalysed C–C coupling such as the Stille, Suzuki, Heck, and Sonogashira reactions [
In spite of increasing interest of microwave irradiation technique, the use of this methodology in free-radical-based organic synthesis is less common [
Previously we prepared [
Preparation of radical precursors 3
Entry | Product | R1 | R2 | R3 | R4 | MWI time (A) | % yield (A) | % yield (B) | mp°C (lit. [67] mp°C) |
---|---|---|---|---|---|---|---|---|---|
(1) | H | H | H | H | 6 min | 74 | 80 | 143 (142) | |
(2) | H | H | H | OCH3 | 4 min | 76 | 82 | 122 (122) | |
(3) | H | CH3 | H | H | 8 min | 85 | 88 | 157 (158) | |
(4) | H | CH3 | H | OCH3 | 8 min | 75 | 81 | 167 (168) | |
(5) | CH3 | H | CH3 | H | 7 min | 70 | 78 | 164-165 (165) | |
(6) | CH3 | H | CH3 | OCH3 | 7 min | 82 | 84 | 170 (170) | |
(7) | H | H | CH3 | H | 10 min | 89 | 93 | 115 (116) | |
(8) | H | H | CH3 | OCH3 | 10 min | 76 | 86 | 145 (144) |
Method A: LiOH, TBAB, DMF, 40–50°C; Method B: MWI (4–10 min), TBAB, K2CO3.
Additionally, we have also exploited the microwave irradiation protocol in synthesising the radical precursor
The mixtures were adsorbed on potassium carbonate and irradiated in an open Erlenmeyer flask in a microwave oven for 4
We found that the above alkylation reaction using microwave irradiation (MWI) has advantages over conventional conditions [
Previously we reported [
Bu3SnH, AIBN, PhMe, MWI (2.45 GHz, 1 h) in open vessel.
In summary, this paper describes new and efficient methodologies involving microwave irradiation to synthesise 12
All the reagents were obtained from commercial sources and used as received. Except methanol (which was HPLC grade), the remaining solvents were dried and distilled before use. Elemental analyses and mass spectra (ESI+) were performed at the Indian Institute of Chemical Biology, Kolkata. UV absorption spectra were recorded in EtOH on a Shimadzu UV-2401 PC spectrophotometer (
LiOH (1.75 mmol) was added in three portions over 30 min to a solution of
4-Hydroxy[1]benzopyran-2-ones
A mixture of
All these products are known compounds and were easily identified by comparison of their spectroscopic data and mp’s with those reported [
White solid; mp 122°C; IR (KBr) (
White solid, mp 141–42°C; IR (KBr) (
The authors are grateful to the Department of Science and Technology (New Delhi), Government of India for providing financial support under Fasttrack Scheme (SR/FT/CS-049/2008), and P.K. Basu dedicates this paper in the commemoration of his beloved late father, Sri Bharati Bhusan Basu. A. Ghosh is grateful to DST (New Delhi) for providing research fellowship (JRF). P. K. Basu is thankful to his departmental colleague Professor L. Thander for his continuous help.