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 1b. In a similar fashion TCC macrocycle 1d afforded only trans-syn-cis (TSC) tricycle 31. On the contrary, the transannular Diels–Alder reaction in cis-trans-cis (CTC) macrocycle 1a and trans-trans-cis (TTC) macrocycle 1c 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 1a, 1b, and 1c. Pathways for these interconversions are also proposed.
The racemic and chiral synthesis of trans-cis-cis macrocyclic triene 5 is described. Heating this compound at 262 °C leads via a transannular Diels–Alder reaction to the tricyclic structure 6, which can be further transformed into tricyclic compound 7. This work constitutes a preliminary study for the synthesis of corticoids.