Electrocyclic cascade: one chain, two rings
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Advanced Synthesis · Free interactive demo
With no reagent at all, an (E,Z,Z,E)-tetraene closes twice. First an 8π electrocyclization joins its two ends into a cycloocta-1,3,5-triene; eight π electrons close conrotatory under heat, so the two methyl groups end up trans. Then the hexatriene inside that ring closes in a 6π electrocyclization; six π electrons close disrotatory, so a four-membered ring is fused cis to a cyclohexadiene.
This demo opens in Guided mode: the electrons that move are picked for you, and you draw where they go. Seven arrows in two pericyclic steps, with one question between them. Hints appear by themselves if you pause.
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The 8π–6π electrocyclization cascade, step by step
Electrocyclic reactions follow the Woodward–Hoffmann rules: thermal closures of 4n π-electron systems are conrotatory and those of 4n + 2 systems are disrotatory. This 8π–6π sequence is how nature and chemists make the endiandric acids: Black's biosynthetic proposal, confirmed in Nicolaou's synthesis, adds an intramolecular Diels–Alder reaction that drains the reversible 6π step.
The 4 steps of the demo, in the exercise's own words. Try drawing each one above before you read it.
8π electrocyclization
The two terminal carbons of the tetraene (C2 and C9) bond while the four π bonds shift to three. With eight π electrons (4n), the thermal closure is conrotatory, so the two outward-pointing methyl groups of the E,E termini end up trans on the new sp³ carbons of the eight-membered ring.
Which mode for the second closure?
The cyclooctatriene contains a hexatriene, which closes thermally in the disrotatory sense.
6π electrocyclization
Inside the eight-membered ring, the hexatriene closes: C3 and C8 bond and the three π bonds shift to two. The disrotatory motion fuses a four-membered ring cis to a cyclohexadiene, and the trans methyl groups end up on the cyclobutane.
7,8-Dimethylbicyclo[4.2.0]octa-2,4-diene
Two σ bonds and two rings have formed without any reagent, purely by orbital-symmetry-controlled electrocyclizations. The 6π step is reversible, so the bicyclic diene equilibrates with the cyclooctatriene. In the endiandric acid cascade, an intramolecular Diels–Alder reaction of the bicyclic diene follows on heating and drains this equilibrium.
How the mechanism is drawn
(2E,4Z,6Z,8E)-Deca-2,4,6,8-tetraene, toluene, room temperature to mild warming, dark