Preparation For AB Ring Synthesis
In the next phase (Scheme 2), starting from ketal 12, the cyclohexane ring was cleaved to provide two anchoring points for fusion with the A ring. Alcohol 12 was protected by a benzyl group. The acetonide protecting group was removed from the ketone. Ketone 14 was converted to silyl enol ether 15 by reaction with trimethylsilyl triflate, and a modified Rubottom oxidation using 3,3-dimethyldioxirane followed by a treatment with camphorsulfonic acid introduced a hydroxyl group alpha to the ketone. Ring opening by oxidative cleavage with lead tetraacetate in methanol gave compound 17. In the next step, the aldehyde was protected as a dimethyl acetal, and the ester was reduced to give primary alcohol 18. The hydroxyl group was converted in a Grieco elimination to the selenide (19), which on oxidation with hydrogen peroxide gave alkene 20. Ozonolysis with ozone and triphenylphosphine provided aldehyde 21.
| Scheme 2 |
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