The direction of rotation of the following thermal electrocyclic ring closures.;$\rightleftharpoons$;respectively is:
(a)Disrotory, disrotatory, disrotatory
(b)Conrotatory, conrotatory, conrotatory
(c)Disrotatory, disrotatory, conrotatory
(d)Disrotatory, conrotatory, disrotatory
Answer
Answer: A ✓ checked by 4AB · confidence high
The source book printed C; on checking, A is correct — see the explanation.
All three are thermal 6π (triene → cyclohexadiene) electrocyclisations, so all are disrotatory. The trans-fused product comes from disrotatory closure of the (Z,Z,E)-cyclononatriene, not from an 8π system. The book key is (a).
Explanation
Each reaction is the ring closure of a conjugated triene (6π electrons, 4n+2). Under thermal conditions a 4n+2 electrocyclisation is disrotatory.1) Cycloheptatriene ⇌ norcaradiene (bicyclo[4.1.0]heptadiene): 6π, disrotatory.2) (Z,Z,Z)-Cyclonona-1,3,5-triene → cis-bicyclo[4.3.0]nona-2,4-diene: 6π, disrotatory, giving the cis ring fusion.3) (Z,Z,E)-Cyclononatriene → trans-bicyclo[4.3.0]nonadiene: still 6π and disrotatory. The trans fusion comes from the E double bond in the starting triene, not from conrotation.Disrotatory, disrotatory, disrotatory. Answer: (a).