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(O)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(O)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(O)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(O)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(O)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(O)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. <EOS>