2. The symbolism [i + j + ...] for a cycloaddition identifies the numbers i, j, ... of electrons in the interacting units that participate in the @T06446@ of reactants to products. In this notation the reaction (a) and (b) of the preceding paragraph would both be described as [2+4] cycloadditions, and (c) as a [2+2+2] cycloaddition. The symbol a or s (a = @A00377-1@, s = @A00377-2@) is often added (usually as a subscript after the number to designate the stereochemistry of addition to each fragment. A subscript specifying the orbitals, viz. σ, π (sigma, pi) with their usual significance) or n (for an orbital associated with a single atom only), may be added as a subscript before the number. Thus the normal Diels–Alder reaction is a $$\left [4_{\text{a}}\: +\: 2_{\text{s}} \right ]$$ or $$\left [_{\pi }4_{\text{a}}\: +\: _{\pi }2_{\text{s}} \right ]$$ cycloaddition, whilst the reaction:
would be a $$\left [14_{\text{a}}\: +\: 2_{\text{s}} \right ]$$ or $$\left [_{\pi }14_{\text{a}}\: +\: _{\pi }2_{\text{s}} \right ]$$ cycloaddition. (N.B. Square brackets [...] are used in the descriptions based on numbers of electrons.) Cycloadditions may be @P04491@ or (non-@C01234@ stepwise @S05970@ . The term 'dipolar cycloaddition' is used for cycloadditions of 1,3-@D01753@.