Synthesis of Thiazinoimidazoles by Lewis Acid-Catalyzed [3+3] Cycloaddition Reactions of Propargyl Alcohols with 2-Mercaptoimidazoles: Synthesis of Thiazinoimidazoles by Lewis Acid-Catalyzed [3+3] Cycloaddition Reactions of Propargyl Alcohols with 2-Mercaptoimidazoles