Q1(g). Two thermal electrocyclic steps

Answer in brief

A is cis,cis,trans-cyclodeca-1,3,5-triene; B is trans-bicyclo[4.4.0]deca-2,4-diene.

Key points

• At 140 °C the strained cyclobutene opens by a thermal 4π conrotatory process, breaking the shared fusion bond to form a conjugated diene.
• The original cyclooctene C=C lies next to one end of that diene, so the ten-membered ring contains three conjugated double bonds.
• At 220 °C the six-π triene segment undergoes thermal disrotatory electrocyclisation, giving a fused six/six skeleton.
• Conrotatory opening generates one trans double bond; the cis,cis,trans triene then closes disrotatorily to the trans-fused bicyclic diene.