ORGN 364 |
| Synthesis of phosphonylpyrazoles reported in the literature involve multi-step reaction sequences. Since many pyrazole derivatives show biological activities, facile and convenient methods for the synthesis of phosphonylpyrazoles appeared desirable. To this end, we developed a strategy which involves a base mediated 1,3-dipolar cycloaddition (1,3-DC) of diethyl-1-diazo-2-oxopropylphosphonate 2 (Bestman-Ohira reagent, BOR) with nitroalkenes 1. In the presence of a suitable base (e.g. K2CO3, NaOEt etc), BOR 2 underwent acyl cleavage followed by regioselective 1,3-DC with a variety of nitroalkenes 1 affording exclusively a single regioisomer of pyrazole 3. The formation of 3 was presumably taking place via spontaneous elimination of HNO2 (HBr when X = Br) from the initial cycloadduct. The structure of 3 was unambiguously established by single crystal X-ray analysis. Under the optimized conditions, a variety of alkyl, aryl and heteroaryl nitroalkenes reacted with BOR 2 in the presence of NaOEt in EtOH and afforded the pyrazoles 3 in 45-77% yield.
|
|
New Reactions and Methodology
8:00 AM-12:00 PM, Tuesday, 12 September 2006 Moscone Center -- Room 133, Oral
Division of Organic Chemistry |
