Abstract: A process for preparing a vitamin D derivative including providing an ultraviolet irradiation apparatus for photochemical reactions, which has an ultraviolet radiation-emitting lamp, an optical system which receives light from the ultraviolet radiation-emitting lamp and emits ultraviolet rays having a specific wavelength, and a quartz rod which receives the ultraviolet rays having the specific wavelength from the optical system and emits the ultraviolet rays, irradiating a solution of a provitamin D derivative with the ultraviolet rays emitted from the quartz rod to cause a photochemical reaction to the provitamin D derivative solution, thereby forming a previtamin D derivative, and subjecting the previtamin D derivative to a thermal isomerization reaction to prepare the vitamin D derivative.

Abstract: Disclosed herein are an ultraviolet irradiation apparatus for photochemical reactions which can irradiate the photo-reactive solution with ultraviolet rays having a specific wavelength suitable for the intended photochemical reaction at a high efficiency, and a process by which a provitamin D derivative can be converted into a provitamin D derivative at a high efficiency by means of a photochemical reaction by one-step process of light irradiation, thereby preparing a vitamin D derivative at a high efficiency.

Abstract: Highly crystalline vitamin D derivatives are represented by the formula: ##STR1## (where R.sub.1 is a straight or branched C.sub.4 -C.sub.9 alkyl group optionally having a hydroxyl group; R.sub.2 and R.sub.3 which may be the same or different are each an acyl group), 22-oxavitamin D derivatives of high purity are produced by hydrolyzing said compounds which derivatives are useful as pharmaceuticals.

Abstract: A 22-oxacholecalciferol derivative represented by formula (I): ##STR1## wherein R.sub.1 represents a hydrogen atom or a hydroxyl group; and R.sub.2 and R.sub.3, which may be the same or different, each represent a lower alkyl group having from 1 to 5 carbon atoms,and a process for preparing the same are disclosed. The compound of the present invention has potent differentiation inducing activity and cell proliferation inhibitory activity and is expected to be useful as an anti-tumor agent, an antirheumatic, a treating agent for psoriasis, and a treating agent for hyperparathyreosis. The process of the present invention makes it possible to efficiently produce a desired 22-oxacholecalciferol derivative while reducing by-production.