5yn-l,3-Diols

An unexpected result was obtained in the coupling of bis(3-alkoxypropyl)zincs with 152 [56] (Scheme 25). A substantial erosion in 1,3-anti selectivity was observed. The lower diastereoselectivity observed in these couplings may be a consequence of an equilibrium between the open-chain form (167) and the intramolecular chelated form (168) of the dialkylzinc, which increases the steric bulk about the C-Zn bond [57,58] (Fig. 6). As the steric bulk of the chelated form increases, addition from the less hindered equatorial trajectory begins to become competitive, and substantial amounts of the 5yn-l,3-diol synthon are generated. 

The enolate, generated by TBAF cleavage of the enoxysilane derived from phenylselanylacetone, was trapped with benzaldehyde [39] (Scheme 33). The antilsyn ratio of diastereoisomers, easily identified by H and Se NMR, was dependent on the temperature and the E or Z structure of the substrate. The borane reduction followed by a conventional acetonization led to a protected syn, 5yn-l,3-diol. 

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