When an allyl anion undergoes a cycloaddition under thermal conditions with ethylene (bonds forming at the ends of the π-systems),the cycloaddition is described as

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The cycloaddition between an allyl anion (which has 3 π electrons) and ethylene (which has 2 π electrons) under thermal conditions is described as a [2+4] cycloaddition because it involves the addition of a 4 π-electron system (allyl anion) with a 2 π-electron system (ethylene). The process is suprafacial because both reactants approach each other from the same face to form the cycloadduct.

When an Allyl Anion Undergoes a Cycloaddition Under Thermal Conditions