Vzc-Diols

Catalytic enantioselective synthesis of vzc-diols is a challenging issue. Chiral induction using chiral ligands is difficult to achieve. The moderately enantioselective pinacolization of benzaldehyde is demonstrated to be performed by the chiral titanocene catalysts 15 and 16 [42,43]. 

The 1,5- and 1,6-dialdehydes 22 and 24 undergo the annulative pinacol coupling to give the cyclic vzc-diols 23 and 25, respectively (Scheme 13) [29]. The vanadium-catalyzed intramolecular coupling reaction of 1,5-diketone 26 also proceeds with excellent selectivity (Scheme 14) although the intermolecular coupling of ketones such as acetophenone results in low diastereoselectivity under these conditions [21]. 

Scheme 8), [CrO(czs-01,02-29)(05,06-29-furanose)] (complex III, Scheme 8), and [Cr0(05,06-29-furanose)2] (complex IV, Scheme 8). At longer reaction times, the proportion of the species at glso l decreased and the spectra were dominated by species at yiso2 and giso 3. These five-coordinated Cr(V)-bischelates were still observed 15 h after mixing. Thus, initially, the Cr(V)-bischelate is formed, with Cr(V) bound to the 1,2-czs-diol component of the pyranose form of 29 (kinetic control). However, with time, it transforms into bis-chelates with Cr(V) bound to the 5,6-vzc-diol group of the furanose form of the ligand.

Dioxetanes are converted to vzc-diols with reductants as lithium aluminum hydride [132a] or thiols [132b] (Sch. 77). 

Fig. 115 Pinacol rearrangement of polymeric vzc-diol for reverse polarity change...

Dehydration is the first step of pinacol rearrangement of vzc-diol. Tertiary alcohols can dehydrate intramolecularly with an acid as a catalyst to form olefins, which provides another mechanism of a reverse polarity change from a polar to nonpolar state [353]. 

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