A stereocontrolled synthesis of the enantiomerically pure epoxide 7b from propargyl ether 15 has been realized in 15 steps. Epoxide 7b represents a building block for the “eastern” moieties of the title compounds. Key steps in our approach were a desymmetrizing Sharpless epoxidation (→ anti,cis‐16), the selective processing of the bis‐enolate of the bis(tert‐butyl alkoxyacetate) 11 through a diastereoselective [2,3]‐Wittig rearrangement (→ syn,syn‐9), and a stereo‐ and chemoselective iodolactonization (→ 35). The CO2H groups of dicarboxylic acid 37 were differentiated in a one‐pot bis‐oxidation reaction. The latter entailed the novel transformation of HO2CCH2‐O‐alkyl into AcOCH2‐O‐alkyl.
A stereoselective synthesis of a C1–C11 building block for the polyol‐polyene antibiotic rimocidin has been developed. Its functional groups originate from a disubstituted indane, which underwent Birch reduction, and oxidative cleavage. This provided a dihydropyranone with a β‐keto ester side‐chain. The latter was subjected to a Noyori hydrogenation (ds > 97:3). An oxy‐Michael addition gave a mixture of two spiroketals. Luche reduction then led to three spiroketals in an 80:10:10 ratio. The major spiroketal became isolable by separating one of the by‐products chromatographically and the other by a diastereomer‐selective thioketalization. The remaining spiroketal was ring‐opened to give a dithiolane. Its CO2Me group was converted into the 1,3‐dithiane unit of the target compound (i.e., 47) using an odor‐reducing work‐up procedure, which should prove to be generally useful.
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