Phosphor(n)amidatacetal and phosph(on)atalcetal compounds

A compound of Formula IVa:

• or a stereoisomer or a pharmaceutically acceptable salt thereof
• wherein:
• R1 and R2 are independently selected from the group consisting of an optionally substituted C1-C6 alkyl, an optionally substituted C6-C10 aryl, and an optionally substituted 5- to 10-membered heteroaryl;
• or alternatively, R1 and R2 together with the atoms to which they are attached form a four to ten membered heterocycle optionally substituted with a C1-C6 alkyl;
• each R3 is independently selected from the group consisting of hydrogen, an optionally substituted C1-C6 alkyl, an optionally substituted C7-C16 arylalkyl, and an optionally substituted 5- to 10-membered heteroaryl(C1-C6)alkyl;
• each R6 is independently selected from halogen, an optionally substituted C1-C6 alkyl, and an optionally substituted C1-C6 alkyloxy;
• m is 0, 1, 2, or 3;
• n is 1 or 2;
• and
• X is O (oxygen) or NH.