Stereocontrolled construction of 1,7-dimethyl A.B.C.[6.6.6] tricycles. Part II. Transannular Diels-Alder reaction of 14-membered macrocycles containing cis-dienophiles

The synthesis and transannular Diels-Alder reactions of 14-membered macrocyclic trienes containing a methyl-substituted diene and a methyl-substituted cis-dienophile moiety are described. As a result of the dienophile cis geometry the 1,7-dimethyl A.B.C.[6.6.6] tricycles obtained from the Diels-Alder reaction have a different stereochemistry at the four chiral centers than similar tricycles that were the subject of the preceding paper in this series. Thus trans-syn-cis (TSC) tricycle 30 was the sole product obtained from the trans-cis-cis (TCC) macrocycle 1 b. In a similar fashion TCC macrocycle 1 d afforded only trans-syn-cis (TSC) tricycle 31 . On the contrary, the transannular Diels-Alder reaction in cis-trans-cis (CTC) macrocycle 1 a and trans-trans-cis (TTC) macrocycle 1 c led to a mixture of the same four tricyclics ( 30 , 32 - 34) but in different ratio. The above experimental results are rationalized by taking into consideration the interconversion of macrocyclic trienes 1 a, 1 b, and 1 c. Pathways for these interconversions are also proposed.