Q1(e). Two transformations by electrocyclic relays
Complete the above transformations: (i) a (2Z,4Z,6E)-octa-2,4,6-triene converted into its (2E,4Z,6E) isomer; (ii) cis-bicyclo[4.3.0]nona-2,4-diene converted into the trans-fused diene. [schemes given in the paper]
Answer in brief
(i) Thermal disrotatory closure to trans-5,6-dimethylcyclohexa-1,3-diene, then photochemical conrotatory opening to the (2E,4Z,6E)-triene; (ii) photochemical conrotatory opening to (1Z,3Z,5E)-cyclonona-1,3,5-triene, then thermal disrotatory closure to the trans-fused diene.
Key points
- Each change inverts one stereocentre or C=C: impossible in one allowed step.
- Use a ring closure–ring opening relay, one step thermal, the other photochemical.
- (i) Heat: Z,Z,E triene closes disrotatory: the two CH₃ end trans.
- Light reopens the trans diene conrotatory: both CH₃ turn outward: E,Z,E.
- (ii) Light opens the cis-fused diene conrotatory; the strained E,Z,Z triene recloses on warming, disrotatory: trans ring fusion.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Topic notes: pericyclic reactions, with every Civil Services, IFoS and Geo-Scientist PYQ →
Coverselectrocyclic reactionWoodward–Hoffmann rulesconrotatorydisrotatoryhexatrienephotochemical ring opening
All parts of Question 1
- 1(a) Origin of the CN group in RCOO⁻ + BrCN10 marks
- 1(b) Benzyltrimethylammonium salt with sodium amide at low and at high temperature10 marks
- 1(c) 2-Cyclohexylpropan-2-ol with BF₃·OEt₂: products A and B10 marks
- 1(d) Two aldehydes towards a nucleophile in the presence of BF₃·OEt₂10 marks
- 1(e) Two transformations by electrocyclic relays10 marks