Hdac6 inhibitors and uses thereof

A compound of Formula (II):

• or a pharmaceutically acceptable salt thereof, wherein:
• X1 is hydrogen or fluoro;
• X2 is hydrogen or fluoro;
• Yf is nitrogen or CRx;
• each A is independently hydrogen, substituted or unsubstituted alkyl, substituted or unsubstituted carbocyclyl, substituted or unsubstituted heterocyclyl, substituted or unsubstituted heteroaryl, or substituted or unsubstituted aryl;
• each R1 is independently hydrogen or substituted or unsubstituted alkyl;
• each R2 is independently hydrogen or substituted or unsubstituted alkyl;
• or R1 and R2 together form a substituted or unsubstituted heterocyclyl, or a substituted or unsubstituted cycloalkyl;
• Rx is hydrogen or substituted or unsubstituted alkyl;
• Ra is hydrogen or is joined with Rc to form a substituted or unsubstituted bridged ring;
• Rb is hydrogen, substituted or unsubstituted alkyl, or A(CR1R2)n—, or is joined with Rc to form a substituted or unsubstituted bridged ring;
• Rc is hydrogen or substituted or unsubstituted alkyl or is joined with at least one of Ra and Rb to form a substituted or unsubstituted bridged ring;
• and
• each n is independently 0 or 1.

A compound of Formula (III):

• or a pharmaceutically acceptable salt thereof, wherein:
• X1 is hydrogen or fluoro;
• X2 is hydrogen or fluoro;
• R1 is hydrogen or substituted or unsubstituted alkyl;
• R2 is hydrogen or substituted or unsubstituted alkyl;
• or R1 and R2 together form a substituted or unsubstituted heterocyclyl, or a substituted or unsubstituted cycloalkyl;
• and
• B is a substituted or unsubstituted polycyclic spiro ring system, a substituted or unsubstituted bridged ring system,

A compound of Formula (V):

• or a pharmaceutically acceptable salt thereof, wherein:
• X1 is hydrogen or fluoro;
• X2 is hydrogen or fluoro;
• Y1 is nitrogen or CRx;
• Y2 is nitrogen, CRd, a bond, —CH2—, or —NH—;
• A1 is joined with one of A2, Ra, and Rc to form a substituted or unsubstituted ring;
• A2 is hydrogen or joined with A1 to form a substituted or unsubstituted ring;
• R1 is hydrogen or substituted or unsubstituted alkyl, or R1 is joined with Rd, R3, or R4 to form a substituted or unsubstituted ring;
• R2 is hydrogen or substituted or unsubstituted alkyl, or R2 is joined with Rd, R3, or R4 to form a substituted or unsubstituted ring;
• or R1 and R2 together form a carbonyl;
• R3 is hydrogen or substituted or unsubstituted alkyl, or R3 is joined with R1 or R2 to form a substituted or unsubstituted ring;
• R4 is hydrogen or substituted or unsubstituted alkyl, or R4 is joined with R1 or R2 to form a substituted or unsubstituted ring;
• or R3 and R4 together form a carbonyl;
• Rx is hydrogen or substituted or unsubstituted alkyl;
• Ra is hydrogen or is joined with A1 to form a substituted or unsubstituted ring;
• Rc is hydrogen or is joined with A1 to form a substituted or unsubstituted ring;
• Rd is hydrogen or is joined with R3 or R4 to form a substituted or unsubstituted ring;
• and
• t is 0 or 1.

A compound of Formula (VI):

• or a pharmaceutically acceptable salt thereof, wherein:
• X1 is hydrogen or fluoro;
• X2 is hydrogen or fluoro;
• R1 is hydrogen or substituted or unsubstituted alkyl;
• R2 is hydrogen or substituted or unsubstituted alkyl;
• or R1 and R2 together form a substituted or unsubstituted heterocyclyl, or a substituted or unsubstituted cycloalkyl;
• and
• B is a substituted or unsubstituted heterocyclyl, substituted or unsubstituted carbocyclyl, a substituted or unsubstituted polycyclic spiro ring system, or a substituted or unsubstituted bridged ring system.