Remarkable rate acceleration of intramolecular Diels-Alder reaction in ionic liquids

摘要

The intramolecular Diels-Alder reaction of an ester-tethered 1,3,9-decatriene system was significantly accelerated in ionic liquids such as [emim]BF_4, [bmim]BF_4 and [bdmim]BF_4. Under the present conditions, the IMDA reaction proceeded smoothly without the use of Lewis acid catalysts to give cis-fused bicyclic lactones in good yield with high diastereoselec-tivity.rnThe intramolecular Diels-Alder (IMDA) reaction is one of the most powerful strategies for the stereoselective construction of highly functionalized polycyclic systems. In the IMDA reactions, the reactivity of triene substrates and the stereochemical outcome are highly dependent upon the structure of the tether moiety between the diene and dienophile parts. For example, although the IMDA reaction of an ester-tethered 1.3,9-decatriene system requires a particularly high reaction temperature and long reaction time to obtain the desired cycloadduct in a reasonable yield, a cis-fused oxabicyclo[4.4.0]decenone framework can be constructed with excellent diastereoselectivity due to the stereoelectronic effect of ester functionality in the transition state (Scheme 1). It is known that the IMDA reactivity of triene substrates tethered by ester functionality is considerably lower than those of amide-and ketone-tethered trienes. The low reactivity of ester-tethered trienes is attributed to the conformational behavior of the ester functionality. That is, a repulsive dipole interaction between the two oxygen atoms in the ester functionality and the steric repulsion between the two alkyl substituents existing in the carboxylic acid part and in the alcohol part make the transoid form preferable relative to the cisoid form of triene substrates.