Figure 7.

The synthesis of prostacyclin, the last of the ‘major' PGs to be discovered. By the mid-1970s it became clear from bioassay and other experiments that there were other routes of endoperoxide metabolism that were not accounted for by the pathways shown in Figure 4. This led to the discovery of the unstable ‘PGI₂' or ‘Prostacyclin', which decayed spontaneously to the inactive 6-keto-PGF_1α end product.