A compound of the formula: wherein: X is O or S; one of A and B is C 1 -C 6 alkyl and the other of A and B is -(CH 2) n M, the carbon atom at the 3' position to which A and B are attached thus being a quaternary carbon stereogenic center; n is 1 to 3; M is -OR 1, halo, mono- to per-halo C 1 -C 6 alkyl, -SR 1, aryl, -CO 2 R 2, -COR 3, heterocyclyl, heteroaryl, -NH R 5, -NR 6 R 6a, -CONR 4 R 4a, -NHSO 2 R 7, -CO-CH 2 OH, -SOR 8, -SO 2 NR 5 R 5a, -O R 3, -N 3, or C 2 -C 6 alkynyl, wherein each of the alkyl, heterocyclyl, aryl, and heteroaryl is optionally substituted with one or more groups selected from C 1 -C 6 alkyl, halo, -CN, -C OR 3, -C 1 -C 6 alkyl-C OR 3, C 1 -C 6 alkoxy, and mono- to per-halo C 1 -C 6 alkyl; R t is -H, -CH 2 -P 2, -P 2, -P (OR 2) 2, C 1 -C 6 alkyl, aryl, or-C 1 -C 6 alkylaryl, wherein each of the alkyl and aryl is optionally substituted with one or more groups selected from C 1 -C 6 alkyl, halo, -CN, -C OR 3, -C 1 -C 6 alkyl-C OR 3, C 1 -C 6 alkoxy, and mono- to per-halo C 1 -C 6 alkyl; R 2 is -H, aryl, -C 1 -C 6 alkylaryl, or C 1 -C 6 alkyl; R 3 is -H, C 1 -C 6 alkyl, -(CH 2) m C OR 2 wherein m is 0 to 4, mono-to per-halo C 1 -C 6 alkyl, aryl, or-C 1 -C 6 alkylaryl, wherein each of the alkyl and aryl is optionally substituted with one or more groups selected from C 1 -C 6 alkyl, halo, -CN, -C OR 2, -C 1 -C 6 alkyl-C OR 2, C 1 -C 6 alkoxy, and mono- to per-halo C 1 -C 6 alkyl; R 4 and R 4a are independently -H, C 1 -C 6 alkyl, -(CH 2) m C OR 2 wherein m is 0 to 4, mono- to per-halo C 1 -C 6 alkyl, aryl, or -C 1 -C 6 alkylaryl, wherein each of the alkyl and aryl is optionally substituted with one or more groups selected from C 1 -C 6 alkyl, halo, -CN, -C OR 3, -C 1 -C 6 alkyl-C OR 3, C 1 -C 6 alkoxy, and mono- to per-halo C 1- C 6 alkyl; or R 4 and R 4a together with the nitrogen to which they are attached form - x, wherein x is 1 to 5, and AA is a natural, non-natural, D- or L- amino acid, wherein - x comprises a terminal -COOR 3 group wherein the carbonyl is protected or unprotected; R 5 and R 5a are independently -H, aryl, C 1 -C 6 alkylaryl, C 1 -C 6 alkyl, or C 1 -C 6 alkoxy; R 6 and R 6a are independently -H, aryl, C 1 -C 6 alkylaryl, or C 1 -C 6 alkyl; R 7 is C 1 -C 6 alkyl, aryl, C 1 -C 6 alkylaryl, or mono- to per-halo C 1 -C 6 alkyl; R 8 is C 1 -C 6 alkyl, aryl, C 1 -C 6 alkylaryl, or mono- to per-halo C 1 -C 6 alkyl; C is -H, halo, azide, or-OH; D is halo, azide, or -OH; and Base is selected from: X is O or S; one of A and B is C 1 -C 6 alkyl and the other of A and B is -(CH 2) n M, the carbon atom at the 3' position to which A and B are attached thus being a quaternary carbon stereogenic center; n is 1 to 3; M is -OR 1, halo, mono- to per-halo C 1 -C 6 alkyl, -SR 1, aryl, -CO 2 R 2, -COR 3, heterocyclyl, heteroaryl, -NH R 5, -NR 6 R 6a, -CONR 4 R 4a, -NHSO 2 R 7, -CO-CH 2 OH, -SOR 8, -SO 2 NR 5 R 5a, -O R 3, -N 3, or C 2 -C 6 alkynyl, wherein each of the alkyl, heterocyclyl, aryl, and heteroaryl is optionally substituted with one or more groups selected from C 1 -C 6 alkyl, halo, -CN, -C OR 3, -C 1 -C 6 alkyl-C OR 3, C 1 -C 6 alkoxy, and mono- to per-halo C 1 -C 6 alkyl; R t is -H, -CH 2 -P 2, -P 2, -P (OR 2) 2, C 1 -C 6 alkyl, aryl, or-C 1 -C 6 alkylaryl, wherein each of the alkyl and aryl is optionally substituted with one or more groups selected from C 1 -C 6 alkyl, halo, -CN, -C OR 3, -C 1 -C 6 alkyl-C OR 3, C 1 -C 6 alkoxy, and mono- to per-halo C 1 -C 6 alkyl; R 2 is -H, aryl, -C 1 -C 6 alkylaryl, or C 1 -C 6 alkyl; R 3 is -H, C 1 -C 6 alkyl, -(CH 2) m C OR 2 wherein m is 0 to 4, mono-to per-halo C 1 -C 6 alkyl, aryl, or-C 1 -C 6 alkylaryl, wherein each of the alkyl and aryl is optionally substituted with one or more groups selected from C 1 -C 6 alkyl, halo, -CN, -C OR 2, -C 1 -C 6 alkyl-C OR 2, C 1 -C 6 alkoxy, and mono- to per-halo C 1 -C 6 alkyl; R 4 and R 4a are independently -H, C 1 -C 6 alkyl, -(CH 2) m C OR 2 wherein m is 0 to 4, mono- to per-halo C 1 -C 6 alkyl, aryl, or -C 1 -C 6 alkylaryl, wherein each of the alkyl and aryl is optionally substituted with one or more groups selected from C 1 -C 6 alkyl, halo, -CN, -C OR 3, -C 1 -C 6 alkyl-C OR 3, C 1 -C 6 alkoxy, and mono- to per-halo C 1- C 6 alkyl; or R 4 and R 4a together with the nitrogen to which they are attached form - x, wherein x is 1 to 5, and AA is a natural, non-natural, D- or L- amino acid, wherein - x comprises a terminal -COOR 3 group wherein the carbonyl is protected or unprotected; R 5 and R 5a are independently -H, aryl, C 1 -C 6 alkylaryl, C 1 -C 6 alkyl, or C 1 -C 6 alkoxy; R 6 and R 6a are independently -H, aryl, C 1 -C 6 alkylaryl, or C 1 -C 6 alkyl; R 7 is C 1 -C 6 alkyl, aryl, C 1 -C 6 alkylaryl, or mono- to per-halo C 1 -C 6 alkyl; R 8 is C 1 -C 6 alkyl, aryl, C 1 -C 6 alkylaryl, or mono- to per-halo C 1 -C 6 alkyl; C is -H, halo, azide, or-OH; D is halo, azide, or -OH; and Base is selected from: