An efficient and short degradation of the cholic acid side chain: A new method for the preparation and dehydrogenation of 4,5-dihydro-oxazoles

11-Oxolithocholic acid (2) and other unhindered aliphatic carboxylic acids undergo an efficient, boric acid mediated, condensation with 2-amino-2-methylpropan-1-ol (3) to give the corresponding 4,5-dihydro-oxazoles. The latter can be dehydrogenated in high yield to the α,β-unsaturated derivatives (e.g. 18) with benzeneseleninic acid or anhydride. Acylation with trichloroacetyl chloride in the case of (18) followed by ozonolysis and saponification furnishes the 20-oxopregnane derivative (21) in over 80% overall yield. The side chain can also be cleaved in one step by benzeneseleninic acid albeit in relatively low yield [10-40% from (4) or (5)].