A compound of the formula: wherein: R 1 and R 6 are each independently selected from hydrogen, alkyl (C≤8), substituted alkyl (C≤8), acyl (C≤8), or substituted acyl (C≤8); R 2 and R 5 are each independently selected from hydrogen, hydroxy, alkyl (C≤8), substituted alkyl (C≤8), cycloalkyl (C≤12), substituted cycloalkyl (C≤12), aryl (C≤8), substituted aryl (C≤8), heteroaryl (C≤8), substituted heteroaryl (C≤8), alkoxy (C≤8), substituted alkoxy (C≤8), acyl (C≤8), or substituted acyl (C≤8); and R 3 is cycloalkyl (C≤12), substituted cycloalkyl (C≤12), aryl (C7-18), substituted aryl (C7-18), heteroaryl (C≤12), substituted heteroaryl (C≤12), or -C R a, wherein: R a is aryl (C≤18), heteroaryl (C≤18); or a substituted version of any of these groups; and R 4 is hydrogen, hydroxy, alkyl (C≤12), substituted alkyl (C≤12), cycloalkyl (C≤12), substituted cycloalkyl (C≤12), aryl (C≤18), substituted aryl (C≤18), heteroaryl (C≤12), substituted heteroaryl (C≤12), acyl (C≤12), substituted acyl (C≤12), or -C R a, wherein: R a is alkyl (C≤18), cycloalkyl (C≤18), aryl (C≤18), heteroaryl (C≤18), alkoxy (C≤12), aryloxy (C≤18), -alkanediyl (C≤6) -cycloalkyl (C≤12); or a substituted version of any of these groups; R 3 and R 4 are taken together and are a group of the formula: -(CH 2) m C A(CH 2) n-, wherein: A is O or NR b, wherein: R b is hydrogen, alkyl (C≤6), or substituted alkyl (C≤6); and m and n are each independently selected from 0, 1, 2, or 3; provided that the compound is not a compound of the formula: or a pharmaceutically acceptable salt or tautomer thereof. R 1 and R 6 are each independently selected from hydrogen, alkyl (C≤8), substituted alkyl (C≤8), acyl (C≤8), or substituted acyl (C≤8); R 2 and R 5 are each independently selected from hydrogen, hydroxy, alkyl (C≤8), substituted alkyl (C≤8), cycloalkyl (C≤12), substituted cycloalkyl (C≤12), aryl (C≤8), substituted aryl (C≤8), heteroaryl (C≤8), substituted heteroaryl (C≤8), alkoxy (C≤8), substituted alkoxy (C≤8), acyl (C≤8), or substituted acyl (C≤8); and R 3 is cycloalkyl (C≤12), substituted cycloalkyl (C≤12), aryl (C7-18), substituted aryl (C7-18), heteroaryl (C≤12), substituted heteroaryl (C≤12), or -C R a, wherein: R a is aryl (C≤18), heteroaryl (C≤18); or a substituted version of any of these groups; and R 4 is hydrogen, hydroxy, alkyl (C≤12), substituted alkyl (C≤12), cycloalkyl (C≤12), substituted cycloalkyl (C≤12), aryl (C≤18), substituted aryl (C≤18), heteroaryl (C≤12), substituted heteroaryl (C≤12), acyl (C≤12), substituted acyl (C≤12), or -C R a, wherein: R a is alkyl (C≤18), cycloalkyl (C≤18), aryl (C≤18), heteroaryl (C≤18), alkoxy (C≤12), aryloxy (C≤18), -alkanediyl (C≤6) -cycloalkyl (C≤12); or a substituted version of any of these groups; R 3 and R 4 are taken together and are a group of the formula: -(CH 2) m C A(CH 2) n-, wherein: A is O or NR b, wherein: R b is hydrogen, alkyl (C≤6), or substituted alkyl (C≤6); and m and n are each independently selected from 0, 1, 2, or 3; A is O or NR b, wherein: R b is hydrogen, alkyl (C≤6), or substituted alkyl (C≤6); and m and n are each independently selected from 0, 1, 2, or 3; provided that the compound is not a compound of the formula: or a pharmaceutically acceptable salt or tautomer thereof.