The pilot reported that while overflying a grass airstrip to land, the left wing struck a tree after crossing the departure end of the runway at treetop level. The pilot maneuvered to a local airport for an emergency landing. During landing, shortly after the airplane crossed the runway threshold, a ground observer notified the pilot that the left landing gear was not extended. The pilot then placed the landing gear selector lever in UP position, but the right gear did not retract Subsequently, the airplane ground looped shortly after touchdown and collapsed the right landing gear. Post accident examination revealed that the left wing had substantial damage from impact with a tree. The pilot added that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.