this invention relates to compounds which have oral antihyperglycemic activity of the formula : ## str1 ## wherein : r 1 is c 1 - c 6 alkyl , c 3 - c 8 cycloalkyl , thienyl , furyl , pyridyl , ## str2 ## r 2 is hydrogen or c 1 - c 6 alkyl ; x os o or s ; n is 0 , 1 , or 2 ; a is ## str3 ## y is o or s ; z is n or ch when y is o and z is ch when y is s ; or a pharmaceutically acceptable salt thereof .