The cell labelled A is adjacent to all cells except one.
Therefore this cell must contain either 1 or 8 as these are the
only numbers having only one adjacent number.
So we know the central column is either as shown or
flipped upside down
There are two cells that the 3 can occupy (those in the same
row as 1) and, thinking ahead, each leaves just one cell that
the 6 can occupy allowing the 4 and 5 to be kept apart.
So 4 arrangements in total: