Stereocontrol in organic synthesis using silicon-containing compounds. A synthesis of the (±)-Prelog–Djerassi lactone

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Each of the relative stereochemical relationships present in the Prelog–Djerassi lactone 34 was set up by a stereocontrolled reaction based on the presence of a silyl group. These were the enolate protonation 3→4 of a β-silyl ester, the enolate alkylation 11→12 of a β-silyl ester, silyl-to-hydroxy conversion with retention of configuration 13→14, and stereospecifically anti protodesilylation of the allylsilanes 26 and 27 giving largely the alkene 28. These allylsilanes had themselves been prepared in a stereocontrolled, convergent synthesis from the allylic acetates 24 and 25, providing thereby a general solution to the controlled synthesis of a new stereogenic centre relative to a resident centre without regard to their distance apart, except insofar as it influences a necessary separation of diastereoisomers (18 and 19 in this case). Using the opposite double bond geometries, the allylic acetates 29 and 30 gave the complementary pair of allylsilanes 31 and 32, which underwent stereospecifically anti protodesilylation to give largely the alkene 33 diastereoisomeric to 28 at C-6. The alkenes 28 and 33 were converted into the Prelog–Djerassi lactonic acid 34 and its C-6 epimer 35, respectively.