Synthesis of the B-ring of FR182877. Investigation of the reactions of 6-fumaryl 1,3,8-nonatrienes

Investigation of the intramolecular Diels–Alder reactions of 6-fumaryl 1,3,8-nonatrienes, substituted at the C5 by a vinyl group, to afford the B-ring of FR182877 are reported. The synthesis of the required 1,3,8-nonatriene was achieved quickly and in high yield. 6-Fumaryl 1,3,8-nonatrienes substituted at the C5 by a vinyl group were found to undergo competing tandem sigmatropic rearrangement/Diels–Alder cyclisation when heated under standard Diels–Alder cyclisation conditions. This rearrangement became the exclusive pathway when the reaction was performed in the presence of a Lewis acid. As expected from modeling studies, the major intramolecular Diels–Alder cyclisation product was the desired exo-trans adduct, which was required for the synthesis of FR182877. Intrigued by the rearrangements, a number of alterations were made to the 1,3,8-nonatriene. Replacement of the fumaryl group by an acetyl group resulted in the diminished reactivity of the 1,3,8-nonatriene with neither rearrangements nor cycloadditions observed. Variation of the C5 substituent was found to be very important in determining the π-diastereoselectivity of the Diels–Alder cyclisation.