Stereoselective preparation of Vitamin D precursors using the intramolecular coupling of alkynes and cyclopropylcarbene-chromium complexes: a formal total synthesis of (+/-)-Vitamin D3

Cyclopentenones attached to tetrahydropyran rings are the products of the intramolecular coupling of alkyne-cyclopropylcarbene-chromium complexes with a propargylic stereocenter in high stereoselectivity. The trans heterocycle is preferentially generated, which is explained by thermodynamic control during the reduction of a cyclopentadienone intermediate. These reaction processes are used for the synthesis of the Vitamin D precursor de-ABC-cholestan-14-one.