The synchronous 6π-electron Diels-Alder reaction is a [4 + 2]-cycloaddition (i.e. suprafacial with respect to both components), as exemplified by the reaction to the right.
Thus as the total number of antarafacial components is 0, which is even, the reaction is symmetry-allowed.  This prediction agrees with experiment as the Diels-Alder reaction is a rather facile pericyclic reaction.