Stereo- and regioselectivity in Diels–Alder reactions of 1,3-azaphospholo[5,1-a]isoquinoline and -[1,5-a]pyridine

Microwave‐assisted synthesis and diels–alder reactions of 1,3‐azaphospholo[5,1‐a]isoquinolines

“…The reaction of 1-methyl-3-ethoxycarbonyl-1,3-azaphospholo[1,5-a]pyridine [4] 4 with 2,3-dimethyl-1,3-butadiene and sulfur under similar conditions (Methods A and B) however does not take place even after irradiating the reactants for 30 min at the maximum power. These results are similar to those observed for this substrate under thermal conditions [20].…”

“…The products 2a,b and 3a,b were identified on the basis of physical and spectral data reported earlier [17,20]. The yields of the products and reaction times under conventional and microwave conditions are shown in Table 1.…”

“…All these methods being based on the conventional synthesis require the use of the appropriate solvents and in some cases they occur quite slowly. [20].…”

Cycloaddition reactions of heterophospholes

“…The reaction with isoprene in presence of sulfur proceeds regiospecifically to give 3 only, but in presence of methyl iodide the regioselectivity is lowered and two regioisomers 4 (62%) and 4 (38%) are formed (Scheme 2). The structure of 3 (R 4 = H) has been confirmed by X-ray crystal studies [38]. Although 3-ethoxycarbonyl-1,3-azaphospholo[1,5-a]pyridine was found to be unreactive towards 2,3-dimethylbutadiene [Bansal, R. K.; Gupta, N. (unpublished results)], 1,3-bis(ethoxycarbonyl) derivative 2 reacted with 2,3-dimethylbutadiene and methyl iodide smoothly at room temperature to give 4 [38].…”

“…In the reaction with a cyclic diene, like cyclopentadiene, the kinetically preferred endo-isomer is formed at low temperature. [37,38]. The reaction with isoprene in presence of sulfur proceeds regiospecifically to give 3 only, but in presence of methyl iodide the regioselectivity is lowered and two regioisomers 4 (62%) and 4 (38%) are formed (Scheme 2).…”