Thioacetal monoxides Michael addition

The terminally oxygenated two-carbon side chain was inserted by Michael addition of ketene thioacetal monoxide to the now familiar vinyl-ogous amide 428 followed by deacetalization to give the aldehyde 447 in 75% yield. In fact, the intermediate 448 existed as two separable isomers, but because each was converted to 447, they were assumed to be isomers at C-18. 

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