Transannular Diels–Alder cyclization of substituted 13-membered trienes. An approach to the B.C.D.[6.6.5] rings of the 14β-hydroxysteroids

The transannular Diels–Alder strategy has been applied to three 13-membered macrocyclic trienes whose characteristics are: a cis-trans diene and a cis methoxymethyl enol ether as the diénophile. The successful trials led to three 6.6.5 tricyles having a trans-syn-cis geometry exactly as in the B.C.D. rings of the 14β-hydroxysteroids. Key words: transannular, Diels–Alder, 14β-hydroxysteroids, 13-membered rings.

Studies on the transannular Diels–Alder reaction of 15-membered macrocyclic trienes containing a trans-trans diene. Part I: Synthesis and isomerization of various model cyclopentadecatrienes

The transannular Diels–Alder reaction of 15-membered macrocyclic trienes with a trans-trans-cis (TTC) olefin geometry can lead to A.B.C.[6.6.7] tricyclic products of trans-syn-trans (TST) and cis-syn-cis (CSC) stereochemistry whereas the TTT isomers can produce the trans-anti-cis (TAC) and cis-anti-trans (CAT) tricycles. In order to study the influence of the position (pro-6.6 or pro-6.7) and the nature (alkyl, alkoxymethyl, or formyl) of the dienophile substituent, a set of 10 model cyclopentadecatrienes was prepared. The synthesis of appropriately functionalized trisubstituted dienophile and diene synthons, as well as their coupling reactions affording acyclic precursors, is described in this paper (first in a series of two). A mild and efficient macrocyclization protocol yielded the required methyl- or alkoxymethyl-substituted cyclic substrates. Further transformations led to the formyl-substituted ones. In particular, the TTT macrocycles containing an enal as a dienophile could be completely isomerized to the corresponding TTC thermodynamic isomers in acidic aqueous media. Keywords: transannular, Diels–Alder, 15-membered ring, A.B.C.[6.6.7] tricycles, diterpene.