The Intramolecular version of the Diels-Alder reaction' generates in addition to the characteristic six-membered ring another ring, usually of five or six members, either fused to or bridged across the cyclohezene unit. It appeared to us possihle that the intramolecular Mels-Alder reaction could be utilized to form medium/large ring structures as well. The realization of such a process is described herein.
The test substrate chosen was structure III which contains diene and dienophile units at a separation of 10 atome. The preparation of II?' 3 waa readily accomplished in two steps (34% overall yield):
(1) base-catalyzed (Nail) transesteriflcation of e-caprolactone (I) with 2 equiv. of 01~ (no solvent) at 25" to form the oily diene-alcohol II, 3 and (2) acylatiou with 2 equiv. of acryloyl chloride in the presence of 2.2 equiv. of pyridine in ether (3 hr. at 00).
Slow addition of III to benzonitrile at refbm resulted in the desired intramolecular reaction5 with formation (77% yield) of a mizture of the three Diels-Alder pro&&s IV, V, and VI in a ratio of ~a.
6.2 : 6.8 : 1. Although these substances could not be separated chromatographically, the mizture could be analyzed and characterized in a straightforward way. Eeductlon of the mizhlre using lithium aluminum hydride in ether at 0" afforded quantitatively the same three' isomeric unsaturated dtola (VII : VIII : IX =