Synthesis of 7-Substituted 5-Oxo-5H-thiazolo[3,2-a]pyrimidine-6-carboxylic Acids, 2-Substituted 4-Oxo-4H-pyrido [1,2-a]pyrimidine-3-carboxylic Acids, and 2,6-Disubstituted 4-Quinolones from Meldrum's Acid Derivatives