Abstract: The invention relates to a method for the photochemical isomerization of organic compounds, in particular for the photochemical conversion of tachysterol compounds into previtamin D compounds and of trans-vitamin D compounds into cis-vitamin D compounds, under the influence of radiation, by exposing a solution of the organic compound to be converted in the presence of a non-polymeric photosensitizer to light with approx. 300-1,000 nm wavelength, and by then isolating the resulting product, wherein a substituted thiophene derivative having a substantial absorption in said wavelength region is used as the photosensitizer. The invention also relates to a new photosensitizer to be used for said isomerization reaction.

Abstract: The invention relates to a method of preparing .DELTA.5,7-steroids, in particular 7-dehydrocholesterol compounds, of the general formula: ##STR1## wherein R is a hydrogen atom, a hydroxy group, optionally etherified with a C.sub.1 -C.sub.4 alkanol or esterified with a C.sub.1 -C.sub.4 alkane carboxylic acid, a C.sub.2 -C.sub.5 alkanoyl group, optionally ketalized with a C.sub.1 -C.sub.4 alkanol, or a branched or non-branched, saturated or unsaturated C.sub.1 -C.sub.10 alkyl group, optionally substituted with one or more fluorine atoms or hydroxy groups, which hydroxygroup or groups is (are) optionally etherified with C.sub.1 -C.sub.4 alkanol(s) or esterified with C.sub.1 -C.sub.4 alkane carboxylic acid(s), andR.sup.1, R.sup.2 and R.sup.3 are equal or different and represent hydrogen atoms or hydroxy groups, optionally etherified with C.sub.1 -C.sub.4 alkanols or esterified with C.sub.1 -C.sub.4 alkane carboxylic acids, or wherein R.sup.1 and R.sup.2 together constitute a C.sub.2 -C.sub.