Unnatural Analogues of Calcitriol and Uses Thereof

in which R1, R2, R3, R4, R5, R6, X, and a are defined herein, or a pharmaceutically acceptable salt thereof. Also disclosed are a pharmaceutical composition comprising a compound or salt thereof of Formula (I) and a method of treating a disease which benefits from the modulation of the vitamin D receptor, such as a bone disorder, cardiovascular disease, a cardiovascular complication associated with renal disease, endothelial dysfunction, hyperparathyroidism, hypocalcemia, an immune disorder, left ventricular hypertrophy, a proliferative disease, proteinuria, renal disease, and thrombosis.