Deca-2,4,6,8-tetraene (left) undergoes an electrocylic ring closing reaction at low temperature (-10 °C) in the
dark. Upon warming to 20 °C, a second electrocyclic ring closure occurs. The complete stereochemistry of the
final product is not shown in the scheme below.
The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the tetraene
fragment and triene fragments are provided in the diagram.
-10 °C
H
20 °C
H
1. Complete the table below to characterise the two reaction steps, assuming thermal conditions only. An
example is provided.
Number of
Antarafacial or
electrons
suprafacial?
Conrotatory or
disrotatory under
thermal
Woodward-
Hoffmann
involved
descriptor
conditions?
Example
antarafacial
Conrotatory
4 Πα
First step (-10 °C)
Second step (20 °C)
2. Which one of the structures below gives the best description of the final product? Rationalize your answer
fully: you should provide a mechanism, orbital sketches, and/or a brief written response.
H
H
H
H
H
H
