[4 + 2]‐Cycloadditionen mit 5‐Ethenyl‐3,4‐dihydro‐2 H ‐pyran und 5‐Ethenyl‐2,3‐dihydro‐1,4‐dioxin

[4 + 2]‐Cycloadditions with 5‐Ethenyl‐3,4‐dihydro‐2 H ‐pyran and 5‐Ethenyl‐2,3‐dihydro‐1,4‐dioxine 5‐Ethenyl‐3,4‐dihydro‐2 H ‐pyran ( 2a ) and 5‐ethenyl‐2,3‐dihydro‐1,4‐dioxine ( 2b ) as hetero‐cyclic donor‐activated 1,3‐dienes are prepared by Wittig alkenylation of the aldehydes 1a and 1b . [4 + 2]‐Cycloadditions with dienophiles such as tetracyanoethene, β‐nitrostyrene, diethyl azodicarboxylate, phenyltriazolinedione, and diethyl mesoxalate yield the new heterobi‐ and ‐tricycles 3 – 7 .