Compounds of Formula I, ##STR00001## wherein Y.sub.1, Y.sub.2, R.sub.1 R.sub.6, Q and Z are as defined herein, are vitamin D derivatives. These compounds are useful for, e.g., the treatment hyperprolifierative diseases of the skin, tumor diseases and precancerous stages, auto-immune diseases, rejection reactions in the case of autologous, allogenic or xenogenic transplants, AIDS, atopic skin conditions, secondary hyperparathyroidism, renal osteodystrophia, senile and postmenopausal osteoporosis, diabetes mellitus type II, degenerative diseases of the peripheral and central nervous system, hypercalcemias, granulomatous diseases, paraneoplastic hypercalcemias, hypercalcemias in hyperparathyroidism, hirsutism, ateriosclerosis, and/or inflammatory diseases.