Synthesis, in vitro Biological Stability, and Anti‐HIV Activity of 5‐ Halo (or Methoxy)‐6‐alkoxy (Azido or Hydroxy)‐5,6‐dihydro‐2′,3′‐ didehydro‐3′‐deoxythymidine Diastereomers as Potential Prodrugs of 2′, 3′‐Didehydro‐3′‐deoxythymidine (D4T).