Zur Stereoselektivität der intramolekularen Diels‐Alder‐Reaktion von 1,7,9‐Decatrien‐3‐onen zu Octalonderivaten

Stereoselectivity of Intramolecular Diels‐Alder Reactions of 1,7,Q‐Decatrien‐3‐ones to Octalone Derivatives 2‐Alkenyl‐substituted 2‐siloxycyclopropanes 1 and 2 serve as protected enone equivalents and are converted into key building blocks 5–8 in excellent yields. After desilylation and ring cleavage these compounds provide 1,7,9‐decatrien‐3‐ones 9–12, which undergo smooth intramolecular Diels‐Alder reaction at room temperature or approximately 100°C. The resulting octalone derivatives 13–16 are formed with exclusive or preferred cis connection of the rings. The relative configurations of the cycloadducts are secured by equilibration experiments and NMR spectroscopy (selective proton decouplings). For 11 → 15 and 12 → 16 the cis selectivitiy can remarkably be enhanced by trifluoroacetic acid catalysis. This high noninduced diastereoselectivity is caused by the endo approach of the reactive molecular sites, whereas the configuration of the third stereogenic centre (C‐2) is determined by the folding of the linking carbon chain. Preference of a boat conformation with equatorial ester group endo‐Beq may be assumed and appears to be general for intramolecular cycloadditions of such trienones. This effect as well as the influence of further substituents at the diene or dienophile portion on stereoselectivity and the reactions of other model compounds are discussed.