A compound of formula II: wherein: n is 1, 2, 3, 4, 5, 6, 7, 8, or 9; m is 0, 1, 2, 3, or 4; R 1 is selected from the group consisting of: C 1 -C 12 alkyl, C 1 -C 12 substituted alkyl, C 1 -C 12 alkenyl, C 1 -C 12 substituted alkenyl, C 1 -C 12 alkynyl, and C 1 -C 12 substituted alkynyl; R 2 is selected from the group consisting of: hydrogen, PO 2, PO 2, -C(=O)-Y and -CO-U; Y is -Z-(CH 2) q -W-R b, or alternatively -C(=O)-Y forms an amide bond through a nitrogen atom on Y in which case Y is selected from the group consisting of: glycine, sarcosine, N,N-dimethyl glycine, alanine, valine, leucine, isoleucine, lysine, ornithine, arginine, serine, and threonine; q is 0-4; Z and W are independently selected from the group consisting of: CH 2, O, S, NR c and R b, wherein optionally Z and W are taken together to form a C 3 -C 8 heterocycloalkyl or C 4 -C 10 heteroaryl or bicyclic ring system in which one of the rings is a C 4 -C 10 heteroaryl; R c is selected from the group consisting of: hydrogen, C 1 -C 4 alkyl, and C 3 -C 6 cycloalkyl; R b is selected from the group consisting of: hydrogen, C 1 -C 12 alkyl, C 1 -C 12 substituted alkyl, C 3 -C 8 cycloalkyl, C 3 -C 8 substituted cycloalkyl, C 3 -C 8 heterocycloalkyl, and C 3 -C 8 substituted heterocycloalkyl; U is selected from the group consisting of: pyridine, 1,4-dihydropyridine, N-alkyl-1,4-dihydropyridine, and C-imidazole, or U is selected from aryl, heteroaryl or heterocycloalkyl; R 3 and R 4 together are bonded to form a spirocyclic ring system; each R 6 is independently selected from the group consisting of: hydrogen, deuterium, CH 3, F, 19 F, and 18 F; and wherein each heterocyclic and heteroaryl ring contains up to four heteroatoms selected from the group consisting of: O, N, and S; with the proviso that when both Z and W are heteroatoms, the value of q cannot be 1; or a tautomer and/or a pharmaceutically acceptable salt thereof. n is 1, 2, 3, 4, 5, 6, 7, 8, or 9; m is 0, 1, 2, 3, or 4; R 1 is selected from the group consisting of: C 1 -C 12 alkyl, C 1 -C 12 substituted alkyl, C 1 -C 12 alkenyl, C 1 -C 12 substituted alkenyl, C 1 -C 12 alkynyl, and C 1 -C 12 substituted alkynyl; R 2 is selected from the group consisting of: hydrogen, PO 2, PO 2, -C(=O)-Y and -CO-U; Y is -Z-(CH 2) q -W-R b, or alternatively -C(=O)-Y forms an amide bond through a nitrogen atom on Y in which case Y is selected from the group consisting of: glycine, sarcosine, N,N-dimethyl glycine, alanine, valine, leucine, isoleucine, lysine, ornithine, arginine, serine, and threonine; q is 0-4; Z and W are independently selected from the group consisting of: CH 2, O, S, NR c and R b, wherein optionally Z and W are taken together to form a C 3 -C 8 heterocycloalkyl or C 4 -C 10 heteroaryl or bicyclic ring system in which one of the rings is a C 4 -C 10 heteroaryl; R c is selected from the group consisting of: hydrogen, C 1 -C 4 alkyl, and C 3 -C 6 cycloalkyl; R b is selected from the group consisting of: hydrogen, C 1 -C 12 alkyl, C 1 -C 12 substituted alkyl, C 3 -C 8 cycloalkyl, C 3 -C 8 substituted cycloalkyl, C 3 -C 8 heterocycloalkyl, and C 3 -C 8 substituted heterocycloalkyl; U is selected from the group consisting of: pyridine, 1,4-dihydropyridine, N-alkyl-1,4-dihydropyridine, and C-imidazole, or U is selected from aryl, heteroaryl or heterocycloalkyl; R 3 and R 4 together are bonded to form a spirocyclic ring system; each R 6 is independently selected from the group consisting of: hydrogen, deuterium, CH 3, F, 19 F, and 18 F; and wherein each heterocyclic and heteroaryl ring contains up to four heteroatoms selected from the group consisting of: O, N, and S; with the proviso that when both Z and W are heteroatoms, the value of q cannot be 1; or a tautomer and/or a pharmaceutically acceptable salt thereof.