Axial vs equatorial attack of Grignard reagent on dioxan-5-one and dithian-5-one
I've found the solution in Fleming: Molecular Orbitals-Reference edition:


  The dioxan-5-one is attacked by nucleophiles in the gas
  phase from the equatorial direction, because of electrostatic repulsion from the fraction of negative charge
  carried by the two oxygen atoms in the ring, as Houk had predicted. The selectivity is not particularly high
  (67:33), because the usual torsional strain is still present, enhanced in this case by the shorter C—O bond
  lengths making the ring less puckered, and increasing the dihedral angle by which the carbonyl oxygen atom
  is pushed below the neighbouring C—H bonds. In solution, however, the ketone is attacked
  from the axial direction, just like cyclohexanones. Evidently the solvent insulates the charge on theincoming nucleophile from the electrostatic repulsion, the torsional strain remains high and there are no axial
  substituents hindering axial attack.The sulfur atoms in the dithia analogues are less electronegative than the oxygen atoms in the acetals but the axial lone pairs bulge further up, and are not heavily solvated. However, the carbonyl group is
  displaced even further down by the changes in bond length and bond angles from having two C—S bonds in
  the ring. The one effect makes axial attack less favourable, and the other makes equatorial attack less
  favourable. In the event both solution and gas phase reactions take place with a strong preference for
  equatorial attack.