Method for producing 3, 4-dihydro-1,2,3-oxathiazin-4-one-2,2-dioxide compound or salt thereof

A method for producing a 3,4-dihydro-1,2,3-oxathiazin-4-one-2,2-dioxide compound represented by the following formula (2):

• wherein R1 and R2 are the same as or different from each other and are each a hydrogen atom, alkyl group, alkenyl group, alkynyl group, cycloalicyl group, acyl group, aralkyl group or aryl group;
• and R3 is a hydrogen atom, alkyl group, alkenyl group, alkynyl group, cycloalkyl group, acyl group, aralkyl group or aryl group, or a salt thereof,
• the method comprising the step of subjecting a beta-ketoainide-N-sulfonic acid represented by the following Formula (1):
• wherein R1, R2 and R3 are as defined above;
• and X is a hydrogen atom, or a salt thereof, to cyclization in the presence of an acid anhydride with or without further subjecting the cyclized product to hydrolysis,
• wherein the cyclization is continuously carried out using a continuous flow reactor selected from a tubular reactor or a motionless mixer having a tube in the tubular reactor or the motionless mixer having an inner diameter of from 0.2 to 30 millimeters,
• wherein the beta-ketoamide-N-sulfonic acid or the salt thereof and the acid anhydride are cooled to −100° C. to 10° C. before the cyclization, an amount of the acid anhydride for use in the cyclization is 1 to 20 moles per 1 mole of the beta-ketoamide-N-sulfonic acid or the salt thereof, and
• wherein a premixer selected from the group consisting of a stirred mixer, an ultrasonic mixer, a static mixer and a piping joint is optionally employed with the continuous flow reactor, and when a premixer is employed, a residence time in the premixer ranges from 0.1 to 10 second, and the residence time in the continuous flow reactor ranges from 0.001 to 60 seconds, and
• when the continuous flow reactor is used without a premixer, the residence time in the continuous flow reactor ranges from 0.1 to 10 seconds.