endo/exo Stereoselectivity in DielsAlder Reactions of α,β‐Dialkylated Conjugated Enals to Cyclic 1,3‐Dienes: Intermediates in the Synthesis of (−)‐β‐Santalol and Its Analogs

Highly exo‐selective 4+2] cycloadditions of cyclopenta‐1,3‐diene 2a to α,β‐dialkyl conjugated enals 5 are compared with the analogous endo‐favored Diels? Alder reaction of cyclohexa‐1,3‐diene 7 . The exo‐stereoselectivity is lower in the homologous case of methylcyclopenta‐1,3‐diene 9 . This diastereoselectivity is discussed either in terms of a retro‐homo‐Diels? Alder reaction, associated with thermodynamic control, or with respect to either a competing hetero‐Diels? Alder/Claisen or Cope domino pathway, or retro‐Claisen/retro‐hetero‐Diels? Alder of the endo‐homo‐cycloadducts. These hypothetical mechanisms have been examined by DFT calculations at the MPW1K(CH₂Cl₂)/6‐31+G** level of theory for the AlCl₃‐mediated cycloadditions of 5d to 2a and 7 . Application of Corey's methodology to the γ‐halogeno‐α‐methyl‐substituted dienophiles 5a and 5b allowed an enantioselective preparation of known and useful intermediates for the synthesis of either the naturally occurring (?)‐β‐santalol or its potentially olfactive structural analogs.