Ester derivatives

A compound which is represented by the following general formula (I):

• wherein A is a 6-membered single nitrogen-containing ring having:
• (i) the formula (a1):
• wherein B11 and B12 signify, independently of each other, a C1—C6 saturated or unsaturated aliphatic hydrocarbon group;
• k signifies 0, 1 or 2;
• R2 and R3 signify, either independently of each other, an optionally phenyl-substituted lower alkyl;
• and X− signifies an anion, provided that the sum of carbon atoms of B11 and B12 and k does not exceed 13;
• or
• (ii) the formula (ap1):
• wherein B11 and B12 signify, independently of each other, C1-C6 saturated or unsaturated aliphatic hydrocarbon;
• k signifies 0, 1 or 2;
• R20 signifies hydrogen or a lower alkyl optionally having substituent(s) selected from a group consisting of phenyl and cycloalkyl, provided that the sum of carbon atoms of B11 and B12 and k does not exceed 13;
• or
• (iii) the formula (b1):
• wherein B21 and B22 signify, independently of each other, a C1-C6 saturated or unsaturated aliphatic hydrocarbon group;
• m signifies 0, 1 or 2;
• R6 signifies hydrogen, lower alkyl or a group represented by —N(R8)R9;
• R8 and R9 signify, independently of each other, hydrogen or lower alkyl;
• and R71 signifies hydrogen or lower alkyl, provided that the sum of carbon atoms of B21 and B22 and m does not exceed 13;
• or
• (iv) the formula (bp1):
• wherein B21 and B22 signify, independently of each other, C1-C6 saturated or unsaturated aliphatic hydrocarbon;
• m signifies 0, 1 or 2, provided that the sum of carbon atoms of B21 and B22 and m does not exceed 13;
• Ar is phenyl optionally having substituent(s) selected from the group consisting of halogen, lower alkyl, lower alkenyl and lower alkoxy;
• and
• R1 is a fluorine-substituted C4-C6 cycloalkyl optionally having hydroxyl group(s);
• or a pharmaceutically acceptable salt thereof.