Intermolecular Pinacol Couplings

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]. 

The ketyl radical anion intermediates can be exploited in carbon-carbon bond-forming reactions. Intermolecular and intramolecular pinacol couplings between the carbonyl groups of ketones and aldehydes are well known (Chapter 5, Section 5.1), as are intermolecular and intramolecular carbonyl-alkene couplings (Chapter 5, Section 5.2). 

The stereoselectivity of Sml2-mediated pinacol couplings depends highly on the substrates used. In contrast to the intermolecular cases, high levels of diastereoselectivity can be attained in the cyclisation of a variety of 1,5- and... 

The use of this protocol provided good to excellent diastereoselectivities in a range of intermolecular and intramolecular pinacol coupling reactions. 

The addition of HMPA was found to be crucial for efficient cyclisation, as intermolecular pinacol coupling proved to be the major pathway when the aldehyde hydrazone was subjected to Sml2 alone. Skrydstrup proposed that a... 

NbCl3(DME) is found to be a useful reagent for the intermolecular pinacol coupling of aliphatic aldehydes... 

As expected with a reagent that is csq)able of generating ketyls, intermolecular pinacolic coupling reactions can also be carried out with considerable efficiency using Smh. Treatment of aldehydes or... 

Dicyclopentadienylsamarium also promotes intermolecular pinacolic coupling reactions with exceptional efficiency.Both benzaldehyde and acetophenone are reported to undergo coupling very rapi y at room temperature in the presence of this reductant. After hydrolysis, pinacols are isolated in virtually quantitative yields. 

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