Tetraacylethenes as Dienophiles and Hetero Dienes in Two‐Step Diels–Alder Reactions

Oxidation with barium manganate of the enol of tetraacetylethane ( 10 ) affords tetraacetylethylene ( 7a ) in good yield. Treatment of the 1,3‐diketones 15a and b with iodosobenzene in the presence of boron trifluoride does not result in oxidative coupling yielding 10 and diacetyldibenzoylethane 17 , respectively, as has been reported by Moriarty et al. Instead the known difluoroborylenolates 16 are formed. Oxidative coupling of the sodium enolate of 15b with iodine affords the meso ‐tetraketone meso ‐ 17 and diacetyldibenzoylethylene ( E )‐ 7b besides small amounts of rac ‐ 17 and the bisenol 19 . In the presence of 1,4‐diazabicyclo[2.2.2]octane, meso ‐ 17 and 19 tautomerise yielding identical equilibrium mixtures of meso ‐ 17 , rac ‐ 17 , and 19 (26:29:45). Dehydrogenation with 5,6‐dichloro‐2,3‐dicyanobenzoquinone of meso ‐ 17 yields ( E )‐ 7b and an isomer (77:23) which was assigned structure ( Z )‐ 7b on the basis of spectroscopic evidence. – Tetraacetylethylene ( 7a ) reacts with 1,2‐dimethylenecyclohexane to afford two different [4+2]cycloadducts, viz. the spirocyclic dihydropyran 25 (30 %) and the tetraacetyloctalin 27 . Whereas the latter is stable, the former isomerises to the latter on heating. Formation of both [4+2]cycloadducts and the isomerisation 25 → 27 are interpreted by invoking the zwitterion 26 as common intermediate. Monoolefines that cannot generate stable carbenium ions don't give [4+2]cycloadducts with 7a . In contrast, α‐methylstyrene yields the oxa Diels–Alder product 30 . The structures of ( E )‐ 7b , 19 , 25 , and 27 are established by X‐ray diffraction analyses.