certain trans - 6 - pyrimidin - 5 - yl ]- ethyl ]- and ethenyl ] tetrahydro - 4 - hydroxypyran - 2 - ones and the corresponding dihydroxy ring - opened acids derived therefrom are potent inhibitors of the enzyme 3 - hydroxy - 3 - methylglutaryl - coenzyme a reductase and are useful as hypocholesterolemic and hypolipidemic agents .