Pentacyclic Compound

A compound represented by the following formula (Ia) or a salt thereof:

• wherein:
• R1 represents a phenyl group, which may have one or more substituent(s) selected from the group consisting of a halogen atom, an alkyl group and an alkoxyl group;
• R2 represents a cycloalkyl group, and this cycloalkyl group may have one or more substituents selected from the group consisting of a halogen atom, a hydroxyl group, a carboxyl group, an alkoxyl group, an aryloxy group, a phenyl group, an amino group, an alkylamino group, an alkoxycarbonyl group, an aryloxycarbonyl group, an acyl group, an acylamino group and an acyloxy group;
• R3 represents a hydrogen atom, a hydroxyl group, a halogen atom, an alkoxyl group, a group —O—R31, an acyloxy group or a group —O—CO—R31, in which the alkoxyl and acyloxy groups may have one or more substituent(s) selected from the group consisting of a halogen atom, a hydroxyl group, a carboxyl group, a cycloalkyl group, an alkoxyl group, an aryl group, an aryloxy group, an amino group, an alkylamino group, an alkoxycarbonyl group, an aryloxycarbonyl group, an acyl group, an acylamino group, an acyloxy group and a heterocyclic group wherein the heterocyclic group may have one or more alkyl group(s) on the constituent atoms of its ring,
• wherein R31 represents an alkylamino group, an alkenyl group, an alkynyl group, a cycloalkyl group, an aryl group or a heterocyclic group, in which these alkylamino, alkenyl, alkynyl, cycloalkyl, aryl and heterocyclic groups may have one or more substituent(s) selected from the group consisting of a halogen atom, a hydroxyl group, a carboxyl group, an alkyl group, an alkoxyl group, an aryloxy group, a phenyl group, an amino group, an alkylamino group, an aminoalkyl group, an alkylaminoalkyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, aii acyl group, an acylamino group, an a carbon atom acyloxy group and a nitrogen-containing heterocyclic group having a size of a three- to eight-membered ring wherein the nitrogen-containing heterocyclic group may have one or more alkyl group(s) on the constituent atoms of its ring;
• or R3 may form a three-membered ring together with the methyl group linked to adjacent to the carbon atom to which R3 is linked;
• R4 and R5 each represents a hydrogen atom, an alkyl group, an alkenyl group, an alkynyl group, an aryl group or a heterocyclic group, in which these alkyl, alkenyl, alkynyl, aryl and heterocyclic groups may have one or more substituent(s) selected from the group consisting of an alkoxyl group, an amino group, an alkylamino group, an aminoalkyl group, an alkylaminoalkyl group and a nitrogen-containing saturated heterocyclic group having a size of a five- or six-membered ring represented by the following formula:
• wherein X represents an oxygen atom, a sulfur atom, CH2, CH—Y, NH or N—Y, in which
• Y is an alkyl group,
• wherein said heterocyclic group may have one or more alkyl group(s) on a constituent atom of its ring;
• or R4 and R5 may form a thiocarbonyl group or a carbonyl group together with the carbon atom liked thereto;
• Z′ represents a hydrogen atom, a hydroxyl group, a halogen atom or an alkyl group;
• Z2 represents a hydrogen atom, a hydroxyl group, a halogen atom or an alkyl group;
• Z3 represents an alkyl group, an alkenyl group, an alkynyl group, a cycloalkyl group, an aryl group or a heterocyclic group, in which these alkyl, alkenyl, alkynyl, cycloalkyl, aryl and heterocyclic groups may have one or more substituent(s) selected from the group consisting of a halogen atom, a hydroxyl group, a carboxyl group, an alkyl group, an alkoxyl group, a phenyl group, an amino group, an alkylamino group, an aminoalkyl group, an alkylaminoalkyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an acyl group, an acylamino group and an acyloxy group;
• and
• Z4 represents an alkyl group, an aryl group or an alkoxyl group, in which these alkyl, aryl and alkoxyl groups may have one or more substituent(s) selected from the group consisting of a halogen atom, a hydroxyl group, a carboxyl group, an alkyl group, an alkoxyl group, a phenyl group, an amino group, an alkylamino group, an aminoalkyl group, an alkylaminoalkyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, an acyl group, an acylamino group and an acyloxy group;
• with the proviso that the dotted line of the following moiety:
• means that the corresponding bonding of the moiety may be a double bond, but R3 is not a hydroxyl group in that case.