Cycloadditions, 53. New spirocyclic chiral dienes — synthesis and diastereoselective Diels‐Alder reactions with N ‐phenylmaleimide

Synthesis and diastereoselective Diels‐Alder reactions of new spirocyclic chiral dienes are reported. Starting with diketene and (−)‐menthone ( 1 ), we prepared the diastereomeric diketene‐menthone adducts 2 and 3 . Further derivatization and olefination afforded four chiral dienes 12 ‐ 15 . Diastereoselective Diels‐Alder reactions of these dienes with N ‐phenylmaleimide (NPM) under high‐pressure conditions yielded the corresponding cycloadducts 16 ‐ 19 . Olefination of 6‐diethylphosphonomethyl‐2,2‐dimethyl‐1,3‐dioxin‐4‐one 8 with formaldehyde led to unexpected compounds 9 ‐ 11 resulting from the originally formed olefination product.