Deductions can be made in this order:  In column e, e1=D.  In column a, a1=B.  d3= either e4 or e2, so d3 can’t be E; d3=A or F.  But if d3=F, then either e2 or e4=F, and there would be no place for F in the upper right region.  So d3=A, e4=A, c1=A, b6=A, f5=A.  Now either d2 or f2=B, but if d2=B there would be no place for B in the d5-f3 region; so f2=B, d4=B, e5=B, c6=B.  Now f3=E, d6=F, f6=C, f4=D, a6=D, c3=D, b2=D.  In row 5, a5=C.  b1=C, d2=C, etc.