The
Complete Active Space Self-Consistent Field (CASSCF) computational
method, with the 6-31G* basis set, was used to examine six electrocyclic
rearrangements, each involving a 1,2,4,6-heptatetraene skeleton with
two variously located oxygen and/or nitrogen heteroatoms, as a way
to determine which, if any, are pseudopericyclic as opposed to pericyclic.
Primarily through the close examination of the active space orbitals,
but also considering transition structure geometries and activation
energies, it was concluded that rearrangements <b>3</b> → <b>4</b>, <b>5</b> → <b>6</b>, <b>7</b> → <b>8</b>, and <b>9</b> → <b>10</b> are pseudopericyclic
with two orbital disconnections each, whereas the <b>13</b> → <b>14</b> and <b>15</b> → <b>16</b> rearrangements
are pericyclic. Our conclusions agreed with those of others in two
of the four cases that had been studied previously by density functional
theory (<b>3</b> → <b>4</b> and <b>7</b> → <b>8</b>) but ran contrary to the previous conclusions that the <b>5</b> → <b>6</b> rearrangement is pericyclic and
that the <b>15</b> → <b>16</b> rearrangement is
pseudopericyclic. Our results are also compared and contrasted to
previous similar ones of ours involving the <b>3</b> → <b>4</b> electrocyclization (essentially pericyclic), the <b>11</b> → <b>12</b> [3,3] sigmatropic rearrangement (pseudopericyclic),
and similar [3,3] sigmatropic rearrangements (all pericyclic), and
detailed rationales for these latest results are provided
