Biomimetic polyene cyclizations.† Synthesis of racemic 19-norprogesterone‡

The aim of this study was to apply biomimetic cyclization strategy to a total synthesis of racemic 19-norprogesterone (8). Treatment of enone 1 with lithium aluminium hydride in ether afforded trienynol 2 which was cyclized with trifluoroacetic acid in methylene chloride containing ethylene carbonate to give ketone 4 as a 76/24 mixture of the 17β/17α epimers in 52% yield. Conversion of 4 to DL-19-norprogesterone (8) was effected in 68% yield by ozonolysis (to give 6) followed by cyclodehydration.