Mukaiyama and Torgov Chemistry in the Synthesis of (D-homo) Steroid Skeletons

Three new, short, and efficient procedures have been developed for syntheses of steroid and D-homo steroid skeletons by application of Mukaiyama and Torgov chemistry. An important element in the first and in the second route is a Mukaiyama-Michael reaction with transfer of the silyl group from the starting silyl enol ether to the carbonyl group of the receiving enone. In this way a new silyl enol ether is obtained which enables either a selective reaction with the silyl enol ether in ring D as in route 1, or a selective reaction with the unprotected carbonyl group in ring B as in route 2. In all three approaches the C12-C13 bond is constructed using a Mukaiyama reaction of a Torgov type carbocation precursor with a silyl enol ether as the key transformation. In route 1, Zieglers triketone, which has been used before in the synthesis of 9,11-dehydroestrone methyl ether, has been prepared in four easy steps and in 70% overall yield, using the reactions mentioned above. In the second route a selective Grignard reaction of vinyl magnesium bromide with the unprotected carbonyl group of methoxy tetralone leads to a Torgov type intermediate. This can be converted easily into a carbocation, which then reacts intramolecularly with the silyl enol ether in ring D, under formation of the C12-C13 bond to complete the synthesis of cis (D-homo) steroid skeletons. In the third route, C17 substituted C,Dtrans coupled (D-homo) steroid skeletons have been prepared via an intermolecular addition of a carbocation, generated with ZnBr2 from a Torgov reagent, to a silyl enol ether containing ring D precursor. The adducts have been cyclized under formation of the C8-C14 bond by treatment with acid and the double bonds in the cyclized products have been reduced to all trans steroid skeletons. A chiral five membered silyl enol ether containing ring D precursor has been synthesized from carvone, and used as starting compound in the synthesis of a chiral C17 functionalized steroid.