Intramolecular 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]. 

Treatment of alkanedials with the reagent for pinacol coupling causes intramolecular cyclization leading to 1,2-cycloalkanes. The diastereoselectivity depends on the ring size formed (Equation (16)). ... 

By choosing an appropriate titanium complex, a /ra r-isomer of 1,2-cyclohexanols can be prepared selectively. Because intramolecular pinacol coupling of hexanedials with Sml2 usually produces t-isomers of cyclohexane-1,2-diols, the titanium-mediated reaction complements the samarium-mediated cyclization (Equation (17)). In addition, when a /-butyl group fixes the conformation of the substrate, one of the diastereomers is produced selectively (Equation (18)). ... 

Intramolecular pinacol coupling of 2,2 -biaryldicarbaldehyde with samarium(ll) iodide shows that axial chirality transfer to central chirality proceeds in a stereospecific manner. ... 

Several bicyclo[2.2.1]heptane derivatives with a 1,4-dicarbonyl moiety undergo C—C bond cleavage via intramolecular pinacol coupling, promoted by samar-ium(II) iodide.190... 

Intramolecular geometries, database studies, 1, 597 Intramolecular hydroalkoxylation, and etherification, 10, 672 Intramolecular pinacol coupling to cyclic 1,2-diols, 11, 51 with samarium reagents, 11, 60 Intramolecular silylformylation, alkynes and alkenes, 11, 489 Intramolecular solvomercuration alkenes, 2, 436 alkynes, 2, 439... 

Not many catalyzed processes involving free radicals are known with these metals. Some vanadium-catalyzed pinacol coupling reactions were developed (reviews [129, 171], [172, 173] and cited ref, [174]). Niobium and tantalum complexes were applied in pinacol coupling reactions [130]. Vanadium(IV) [175-179] and vanadium(V) ([129], reviews [180-186]) complexes are known to catalyze asymmetric oxidative dimerizations of phenols and naphthols in moderate to excellent ees applying oxygen as the terminal oxidant. Biaryls are accessible by intramolecular coupling of sodium tetraarylborates, catalyzed by EtOVOCl2 in the presence of air [187]. 

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). 

Intramolecular pinacol coupling can also be used to prepare bicyclic ring systems, as demonstrated by Arseniyadis in a synthetic approach to paclitaxel (Taxol) (Scheme 5.8).19 Treatment of ketoaldehyde 5 with Sml2 furnished diol 6, a precursor to the A ring of the target, in high yield. 

The intramolecular Sml2-mediated pinacol coupling has been well exploited as a key step in many natural product syntheses for the construction of different ring sizes. For example, in 1997, Pancrazi reported a formal total synthesis of... 

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

Ti[0]-linduced intramolecular pinacol coupling and olefination-coupling of dicarbonyl compounds... 

The first total synthesis of ( )-sarcophytol B (5) from S./i-famesal (92), which was reported by McMurry et al. in 1989, used a low-temperature titanium-induced pinacol coupling reaction of 1,14-dialdehyde as the key step. They concluded that the natural sarcophytol B has the stereochemistry of a trans diol (Scheme 6-5). Li et al. reported on a concise total synthesis of ( )-sarcophytol B (5) from /i./i-famesol (91) by a low-valent titanium-mediated intramolecular McMurry... 

Scheme 39 An example of a Sml2-mediated intramolecular pinacol coupling...

The Sml2-mediated intramolecular pinacol coupling reactions of carbonyls 151 were found to afford heterocyclic diols 152 in diastereoselective... 

Intramolecular pinacol coupling reactions are known, giving cyclic 1,2-diols. Dialdehydes have been cyclized by reaction with TiCls to give cyclic 1,2-diols in good yield. ° A radical-induced coupling of an a,03-dialdehyde led to cw-l,2-cyclo-pentanediol when treated with BuaSnH and AIBN. or induced photochemically. ... 

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