Stereocontrolled formation of spiro enones by radical cyclization of bromo acetals

Enantioselective Synthesis of a Key “A-Ring” Intermediate for the Preparation of 1α-Fluoro Vitamin D3 Analogues

1Alpha-fluoro A-ring dienol 2, a useful building block for the preparation of fluorinated vitamin D3 analogues, was synthesized in eight steps from 4-{[tert-butyldimethylsilyl]oxy}cyclohexanone. The most distinctive synthetic development to emerge from this new synthesis is an unprecedented substrate-controlled diastereoselective fluorodesilylation of an advanced dienylsilane intermediate. This is the first enantioselective route to compound 2 relying on the use of an electrophilic fluorinating reagent.