Synthesis of some dihydropyrans by utilizing crotonoyl cyanides as heterodienes in a very mild condition

The reaction of crotonoyl cyanide 1 and its bromo derivative 2 with trimeth ylene 3 affords the dihydropyrans 6 and 7. respectively. Similarly the comp ound 1 on reaction with acrolein dimethyl acetal 4 and acrolein diethyl ace tal 5 gave the isomeric products 8a/8b and 9a/9b, respectively; the cis-iso mer being the predominant one. On the other hand the bromo derivative 2 on treatment with the compound 5 yields only the cis-isomer 10. BF3. Et2O cata lysed epimerisation of 11a gives the other epimer 11b in a mild condition w ith reasonable yield.