ok I find out how to convert the SpatialReference, but can someone of you tell me why the Point which is given as a Parameter in the ZoomToResolution-Function is not the center of the map?
Here is the new code...
and here is the result in a screenshot....
SpatialReference sp = new SpatialReference();
sp.WKID = 4326;
MapPoint mpZoomToGPSPosition = new MapPoint();
mpZoomToGPSPosition.SpatialReference = sp;
mpZoomToGPSPosition.Y = epl.Latitude;
mpZoomToGPSPosition.X = epl.Longitude;
MyMap.ZoomToResolution(MyMap.Resolution / 5, new WebMercator().FromGeographic(mpZoomToGPSPosition) as MapPoint);
The blue marker shows the Position of the Mappoint "mpZoomToGPSPosition".