A Novel Route to 2‐Ethoxybutadienes; Diels‐Alder Reaction of 1,2‐λ 5 ‐Oxaphosphorines

2‐Ethoxybutadienes—enol ethers of α,β‐unsaturated carbonyl compounds universally applicable as synthons can be prepared from primary alkyl halides, carboxylic acid derivatives and aldehydes as alkyl‐ and aryl‐substituted derivatives (1) as well as acyl derivatives (2) . Intermediary acylphosphoranes cyclize in part to 1,2‐λ 5 ‐oxaphosphorines, which function as dienes in Diels‐Alder reactions.