The first total synthesis
of the alkyl citrate trachyspic acid
19-n-butyl ester (1) is described. A
formal [2 + 2]-cycloaddition of the silylketene acetal derived from
lactone 6 with di-n-butylacetylene dicarboxylate 7 provided the cyclobutene diester 5 with a dr
>20:1. Acid-mediated rearrangement of 5 followed by
lactone
ring-opening and ester protecting group manipulation eventually provided
orthogonally protected aldehyde 3. A Nozaki–Hiyama–Kishi
coupling between 3 and vinyl iodide 4 followed
by oxidation of the resultant allylic alcohol gave enone 16, which was converted into the triester 17 (dr 6:1)
by a spirocyclization/oxidative cleavage/elimination sequence. Removal
of the t-butyl esters then afforded trachyspic acid
19-n-butyl ester (1).
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