Q1(e). Thermal 6π electrocyclisation of a dienyl cyclohexene

Introduction. The substrate is a 1,3,5-hexatriene: the ring C=C (C-1=C-2 of the cyclohexene, carrying the methyl) is conjugated with both C=C bonds of the side chain. On heating, a hexatriene closes to a cyclohexa-1,3-diene by a 6π electrocyclic reaction. With 6 π electrons the thermal reaction is disrotatory (HOMO ψ₃ has like-phase termini on the same face).

Reading of the question: ring closure needs the C=C next to the ring to be cis (Z); the dienoate is therefore taken as the (2E,4Z) isomer of the published reaction. The all-E triene drawn in the paper would first have to isomerise.

Scheme: The new σ bond links C-2 of the ring (bearing CH₃) and the ester-bearing carbon.

1. Bond formed. The termini of the triene are the methyl-bearing ring carbon and the carbon carrying CO₂Et. They bond to each other to form a second six-membered ring; the three π bonds become one σ bond and a new 1,3-diene. The product is an ethyl 4a-methyl hexahydronaphthalene-carboxylate: a decalin skeleton with an angular methyl and a cyclohexadiene ring.
2. Concerted, stereospecific. Thermal 6π closure is disrotatory. The groups at each terminus are fixed as “inside” or “outside” by the alkene geometry: at the ring terminus the CH₃ is inside (it is cis to the chain) and the ring carbon bearing OCH₂OCH₂Ph is outside; at the ester terminus the (2E) ester is outside and H inside.
3. Relative stereochemistry. In a disrotatory closure the two inside groups end up cis. So the angular CH₃ and the H next to it are cis, and the angular CH₃ and CO₂Et are trans on the new ring.
4. Facial selectivity. The two disrotatory modes (both termini rotating “up” or “down”) give two diastereomers with respect to the existing OCH₂OCH₂Ph centre. The termini rotate so that the new C–C bond forms on the face away from the neighbouring bulky alkoxy group; the angular methyl is then cis to it (shown).

Conclusion. At 240°C the triene undergoes a thermal, disrotatory 6π electrocyclisation to a bicyclic cyclohexadiene (hexahydronaphthalene) with a new quaternary carbon. The angular CH₃ and the ring-junction H are cis, so CH₃ and CO₂Et are trans; the existing alkoxy centre controls which face closes, so one main diastereomer is formed.

Application. Thermal and photochemical hexatriene closures are the key steps of vitamin D chemistry (photochemical ring opening of 7-dehydrocholesterol to previtamin D, thermal ring closure of previtamin D to the pyrocalciferols) and of many decalin-terpenoid syntheses.