A compound of the formula: wherein: R 1 and R 2 are each independently: alkyl (C≤12), cycloalkyl (C≤12), alkenyl (C≤12), cycloalkenyl (C≤12), alkynyl (C≤12), aryl (C≤12), aralkyl (C≤12), heteroaryl (C≤12), heteroaralkyl (C≤12), acyl (C≤12), alkoxy (C≤12), aryloxy (C≤12), aralkoxy (C≤12), heteroaryloxy (C≤12), heteroaralkoxy (C≤12), acyloxy (C≤12), alkylamino (C≤12), dialkylamino (C≤12), arylamino (C≤12), aralkylamino (C≤12), heteroarylamino (C≤12), heteroaralkylamino (C≤12), or acylamino (C≤12); or R 1 and R 2 are taken together and are alkanediyl (C≤12), cycloalkanediyl (C≤12), alkenediyl (C≤12), or cycloalkenediyl (C≤12); and R 20, R 21, and R 22 are each independently: hydrogen, hydroxy, halo, oxo, amino, hydroxyamino, nitro, imino, cyano, azido, mercapto, or thio; or alkyl (C≤12), cycloalkyl (C≤12), alkenyl (C≤12), cycloalkenyl (C≤12), alkynyl (C≤12), aryl (C≤12), aralkyl (C≤12), heteroaryl (C≤12), heteroaralkyl (C≤12), acyl (C≤12), alkylidene (C≤12), alkoxy (C≤12), alkenyloxy (C≤12), alkynyloxy (C≤12), aryloxy (C≤12), aralkoxy (C≤12), heteroaryloxy (C≤12), heteroaralkoxy (C≤12), acyloxy (C≤12), alkylamino (C≤12), dialkylamino (C≤12), alkoxyamino (C≤12), alkenylamino (C≤12), alkynylamino (C≤12), arylamino (C≤12), aralkylamino (C≤12), heteroarylamino (C≤12), heteroaralkylamino (C≤12), alkylsulfonylamino (C≤12), acylamino (C≤12), alkylimino (C≤12), alkenylimino (C≤12), alkynylimino (C≤12), arylimino (C≤12), aralkylimino (C≤12), heteroarylimino (C≤12), hetero-aralkylimino (C≤12), acylimino (C≤12), alkylthio (C≤12), alkenyl-thio (C≤12), alkynylthio (C≤12), arylthio (C≤12), aralkylthio (C≤12), heteroarylthio (C≤12), heteroaralkylthio (C≤12), acylthio (C≤12), thio-acyl (C≤12), alkylsulfonyl (C≤12), alkenylsulfonyl (C≤12), alkynyl-sulfonyl (C≤12), arylsulfonyl (C≤12), aralkylsulfonyl (C≤12), heteroarylsulfonyl (C≤12), heteroaralkylsulfonyl (C≤12), alkylammonium (C≤12), alkylsulfonium (C≤12), alkylsilyl (C≤12), or alkylsilyloxy (C≤12); wherein "acyl" refers to a monovalent group with a carbon atom of a carbonyl group as the point of attachment, further having a linear or branched, cyclic or acyclic structure, further having no additional atoms that are not carbon or hydrogen, beyond the oxygen atom of the carbonyl group; or a pharmaceutically acceptable salt, tautomer, acetal, or ketal thereof. R 1 and R 2 are each independently: alkyl (C≤12), cycloalkyl (C≤12), alkenyl (C≤12), cycloalkenyl (C≤12), alkynyl (C≤12), aryl (C≤12), aralkyl (C≤12), heteroaryl (C≤12), heteroaralkyl (C≤12), acyl (C≤12), alkoxy (C≤12), aryloxy (C≤12), aralkoxy (C≤12), heteroaryloxy (C≤12), heteroaralkoxy (C≤12), acyloxy (C≤12), alkylamino (C≤12), dialkylamino (C≤12), arylamino (C≤12), aralkylamino (C≤12), heteroarylamino (C≤12), heteroaralkylamino (C≤12), or acylamino (C≤12); or R 1 and R 2 are taken together and are alkanediyl (C≤12), cycloalkanediyl (C≤12), alkenediyl (C≤12), or cycloalkenediyl (C≤12); and R 20, R 21, and R 22 are each independently: hydrogen, hydroxy, halo, oxo, amino, hydroxyamino, nitro, imino, cyano, azido, mercapto, or thio; or alkyl (C≤12), cycloalkyl (C≤12), alkenyl (C≤12), cycloalkenyl (C≤12), alkynyl (C≤12), aryl (C≤12), aralkyl (C≤12), heteroaryl (C≤12), heteroaralkyl (C≤12), acyl (C≤12), alkylidene (C≤12), alkoxy (C≤12), alkenyloxy (C≤12), alkynyloxy (C≤12), aryloxy (C≤12), aralkoxy (C≤12), heteroaryloxy (C≤12), heteroaralkoxy (C≤12), acyloxy (C≤12), alkylamino (C≤12), dialkylamino (C≤12), alkoxyamino (C≤12), alkenylamino (C≤12), alkynylamino (C≤12), arylamino (C≤12), aralkylamino (C≤12), heteroarylamino (C≤12), heteroaralkylamino (C≤12), alkylsulfonylamino (C≤12), acylamino (C≤12), alkylimino (C≤12), alkenylimino (C≤12), alkynylimino (C≤12), arylimino (C≤12), aralkylimino (C≤12), heteroarylimino (C≤12), hetero-aralkylimino (C≤12), acylimino (C≤12), alkylthio (C≤12), alkenyl-thio (C≤12), alkynylthio (C≤12), arylthio (C≤12), aralkylthio (C≤12), heteroarylthio (C≤12), heteroaralkylthio (C≤12), acylthio (C≤12), thio-acyl (C≤12), alkylsulfonyl (C≤12), alkenylsulfonyl (C≤12), alkynyl-sulfonyl (C≤12), arylsulfonyl (C≤12), aralkylsulfonyl (C≤12), heteroarylsulfonyl (C≤12), heteroaralkylsulfonyl (C≤12), alkylammonium (C≤12), alkylsulfonium (C≤12), alkylsilyl (C≤12), or alkylsilyloxy (C≤12);