A convenient and microwave-promoted novel protocol for the syntheses of diverse kinds of substituted benzopyrans from the corresponding variety of substituted hydroxy acetophenones and keto compounds using benzyltrimethylammonium hydroxide (Triton-B) under solvent-free conditions has been developed. This protocol is mild and efficient than the other reported methods.
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Initially, the syntheses of various substituted benzopyrans were achieved from the corresponding hydroxy acetophenones and keto compounds using various kinds of solid support like neutral and basic alumina. Better yields of the products were obtained in by using basic alumina as a solid support. Moreover, reaction was further carried out without using solid support where minute amount of substituted benzopyrans was observed. Keeping the basic nature of benzyltrimethylammonium hydroxide (Triton-B), we have tried a reaction of hydroxyl acetophenone with a keto compound, where it was realized that there is complete transformation of the starting materials into desired substituted benzopyran derivative. A comparative study of use of various mild bases in caring out this synthetic reaction was studied where it was further realized that best yields were obtained using Triton-B. Moreover, the advantages associated with Triton-B include the easy removal from the reaction mixture by simple filtration. Thus, various substituted hydroxyl compounds were reacted with variety of keto compounds using Triton-B under microwave conditions to afford the clean formation of the corresponding substituted benzopyrans in good to excellent yields. Hence, it was concluded that reaction works using Triton-B under solvent-free conditions and completed in very short time (5–8 min.) afforded very excellent yields (81–99%) of the desired substituted benzopyrans. The results were summarizes in Table
Conversion of substituted benzophenones into benzopyrans of general formula
Product | R1 | R2 | R3 | R4 | Time/min. | Yieldsb/% | Reference |
---|---|---|---|---|---|---|---|
|
H | H | CH3 | CH3 | 5 | 86 | [ |
|
H | OH | CH3 | CH3 | 5 | 89 | [ |
|
H | OH | CH3 | C2H5 | 8 | 85 | [ |
|
H | OH | R3 = R4 = Cyclopentyl | 7 | 88 | [ | |
|
H | OH | R3 = R4 = Cyclohexyl | 6 | 90 | [ | |
|
OH | H | CH3 | CH3 | 10 | 81 | [ |
|
OH | H | CH3 | C2H5 | 8 | 82 | [ |
|
OH | H | R3 = R4 = Cyclopentyl | 8 | 89 | [ | |
|
OH | H | R3 = R4 = Cyclohexyl | 8 | 86 | [ | |
|
NO2 | H | CH3 | CH3 | 8 | 85 | [ |
|
OCH3 | H | CH3 | CH3 | 5 | 91 | [ |
|
OCH3 | OH | CH3 | CH3 | 5 | 92 | [ |
|
OCH3 | OCH3 | CH3 | C2H5 | 4 | 94 | [ |
|
OCH3 | OCH3 | C2H5 | C2H5 | 4 | 99 | [ |
|
OCH3 | NO2 |
|
CH3 | 5 | 92 | [ |
|
OH | OCH3 |
|
|
5 | 83 | [ |
aAll the products were characterized by IR, NMR, and mass spectroscopic data.
bIsolated yields.
In conclusion, we developed a convenient and efficient protocol for the one-pot, two-component coupling of various substituted acetophenones with a variety of keto compounds using basic resin. This method generates the corresponding benzopyrans in good to excellent yields. Furthermore, this method exhibits substrate versatility, mild reaction conditions, and experimental convenience. This synthesis protocol developed is believed to offer a more general method for the formation of substituted benzopyrans essential to numerous organic syntheses.
Chemicals were procured from Merck, Aldrich, and Fluka chemical companies. Reactions were carried out under Argon. IR spectra 4000–200 cm−1 were recorded on Bomem MB-104-FTIR spectrophotometer using neat technique, whereas NMRs were scanned on an AC-300F, NMR (300 MHz) instrument using CDCl3 and
Substituted hydroxy acetophenone (0.01 mol) was added to a mixture of corresponding ketone (0.01 mol) and Triton-B (0.02 mol). The reaction mixture was irradiated under microwave at 800 W in a sealed tube. The reaction mixture was extracted with ethyl acetate thrice, and organic layer was concentrated to afford the desired substituted benzophenone derivative which was recrystallized with benzene/hexane.
Mp: 85–87°C; IR (KBr): 2978, 1690, 1604, 1573, 1456 cm−1; 1H NMR (300 MHZ, CDCl3)
Mp: 168°C; IR (KBr): 3469, 1652, 1611, 1570, 1492, 1459, 1428, 1415 cm−1; 1H NMR (300MHZ, CDCl3)
Mp: 98°C; IR(KBr):3128, 2965, 1648, 1574, 1489, 1376 cm−1; 1H NMR (300 MHz, CDCl3): 0.93(3H, t,
Author is thankful to Director General, Amity University Uttar Pradesh, Lucknow Campus, Lucknow, U. P., for his constant support and encouragement for research.
The authors confirm that there is no conflict of interests with the commercial identities used inside the paper.