Abstract
Cycloadditions of the α,β‐unsaturated‐acyl cyanides 1–3 with (Z)‐or (E)‐1‐bromo‐2‐ethoxyethene (4) may be performed at moderate temperatures and provide in good yields the 3‐bromo‐2‐ethoxy‐3,4‐dihydro‐2__H__‐pyran‐6‐carbonitriles 5–7, respectively (Scheme 1). Diastereoisomeric pairs of products result at room temperature merely from the ‘endo’‐ and ‘exo’‐transition states; more complex mixtures appear above 60° as a consequence of (Z)/(E)‐isomerization of 4. The relative stability of the anomers of 5 and 6 is explored by treatment with BF~3~·Et~2~O. Acid alcoholysis (MeOH or EtOH) of 5 leads to acetals 9a, b of 4‐bromo‐5‐oxopentanoate. Alkyl (2__Z__,4__E__)‐5‐ethoxypenta‐2,4‐dienoates 12, 17, and 20, are formed in alcoholic alkoxide solutions from 5, 6, and 7, respectively, which is compatible with the intermediacy of 2‐alkoxy‐2__H__‐pyrans and their valence tautomers, α,β‐unsaturatedacyl cyanides. Methoxide addition to the CN group competes with dehydrobromination in case of 5; it leads to 3‐bromo‐3,4‐dihydro‐2__H__‐pyran‐6‐carboximidate 13 (ca. 50% at −20°) which can be hydrolyzed to the methyl carboxylate 14. DBU (1,8‐diazabicyclo[5,4,0]undec‐7‐ene) in benzene converts 5 to 6‐ethoxy‐2‐oxohexa‐3,5‐dienenitrile (11), the ring‐opening product of an obviously unstable 2‐ethoxy‐2__H__‐pyran; the same reagent dehydrobrominates 6 to 2‐ethoxy‐4‐methyl‐2__H__‐pyran‐6‐carbonitrile (15). HBr Elimination from 7 takes place with great ease in presence of pyridine, or even during chromatography on alumina, and leads to the stable ethyl 6‐cyano‐2‐ethoxy‐2__H__‐pyran‐4‐carboxylate (18); this dimerizes at room temperature to give a 1:3 mixture of tricyclic adducts ‘endo’‐21 and ‘exo’‐21. The structure of the latter is established by an X‐ray crystallographic analysis.