better than with KBr in terms of the higher yields and lesser reaction time for the oxidative aromatization of 1, 4-DHP. This is attributed to the more oxidation potential of iodonium ion compared to bromonium ion. The reaction time for oxidative aromatization of all 1, 4-DHPs with DMP / I2 is, therefore, typically less than one hour.
Conclusions
In conclusion, a transition metal free oxidative aromatization of Hantzsch 1,4-DHPs was achieved efficiently under almost neutral conditions, by using DMP in combination with molecular iodine or KBr. The mild reaction conditions, no dealkylation at the 4-position of 1,4- DHPs, short reaction time, and easy work-up procedure are some of the important features of the reaction. Extension of this method to the preparation of other heterocyclic compounds is under way in this laboratory.
Experimental Section
General Procedures. Melting points are uncorrected. IR spectra were recorded on a Perkin- Elmer FTIR-1710 spectrophotometer. 1HNMR spectra were recorded at 400 MHz in CDCl3 using TMS as internal standard. Dess-Martin periodinane was purchased from was purchased from Spectrochem.
General procedure for oxidative aromatization of 1,4-DHP using DMP and molecular iodine
To a magnetically stirred mixture of 1, 4-dihydropyridine (1 mmol) and iodine (1 mmol) in 15 mL of CH2Cl2, DMP (1 mmol) was added. The resulting reaction mixture was stirred for the period of time as shown in Table 1. After ascertaining the completion of the reaction by TLC, the reaction mixture was treated with sodium thiosulphate and sodium bicarbonate (10 % each). The reaction mixture was then extracted with CH2Cl2 (15 mL x 2).The combined organic layer was dried using anhydrous sodium sulphate and the solvent was distilled. The product was purified by short silica gel column using the mixture of petroleum ether and ethyl acetate (9:1) to afford the corresponding pyridine derivatives.
General Procedure for oxidative aromatization of 1,4-DHP using DMP and KBr
To a magnetically stirred solution of the 1, 4-dihydropyridine (1 mmol) and KBr (1 mmol) in CH3CN (10 mL), DMP (1 mmol) was added at room temperature. The reaction mixture was stirred for the time as indicated in Table 1. TLC monitored the progress of the reaction. After the completion of the reaction, water (10 mL) was added, extracted with CH2Cl2 (2 x 10 mL) and dried over anhydrous sodium sulphate. The solvent was removed under reduced pressure and the
