Synthesis of a smoothened cholesterol: 18,19-di-nor-cholesterol

Herein, we report the first synthesis of a demethylated form of cholesterol (18,19-di-nor-cholesterol), in which the C18 and C19 methyl groups of the β-face were eliminated. Recent molecular simulations modeling 18,19-di-nor-cholesterol have suggested that cholesterol's opposing rough β-face and smooth α-face play necessary roles in cholesterol's membrane condensing abilities and, additionally, that specific facial preferences are displayed as cholesterol interacts with different neighboring lipids and transmembrane proteins. Inspired by these poorly characterized biochemical interactions, an extensive 18-step synthesis was completed as part of a collaborative effort, wherein synthesizing a "smoothened" cholesterol analogue would provide a direct way to experimentally measure the significance of the β-face methyl groups. Starting from known perhydrochrysenone A, the synthesis of 18,19-di-nor-cholesterol was accomplished with an excellent overall yield of 3.5%. The use of the highly stereoselective Dieckmann condensation and the employment of Evans' chiral auxiliary were both key to ensuring the success of this synthesis.