Validation API was reworked in 5.0 and this configuration items was removed. Entities validates on transaction committing, on Session.SaveChanges() and on your demand by session.Validate(). If you need validate some fields immediately then you can set property IsImmediate to true for all constraints of this field.

For example,

  
  [HierarchyRoot]
  public sealed class Customer : Entity
  {
    [Field, Key]
    public int Id { get; set; }
    [Field(Length = 50), NotEmptyConstraint(IsImmediate = true)]
    public string FirstName { get; set; }
    [Field(Length = 50), NotEmptyConstraint(IsImmediate = true)]
    public string LastName { get; set; }
    [Field, PastConstraint(IsImmediate = true)]
    public DateTime Birthday { get; set; }
    [Field(Length = 128), EmailConstraint(IsImmediate = true)]
    public string Email { get; set; }
    [Field(Length = 50), RegexConstraint(@"^((8|\+7)[\- ]?)?(\(?\d{3}\)?[\- ]?)?[\d\- ]{7,10}$", IsImmediate = true)]
    public string PhoneNumber { get; set; }
  }

All constraints in this example was marked as immediate and DO will validate them immediately after changing.