Pinacol intermolecular reaction

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

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. 

Shikonin. Torii has accomplished an efficient intermolecular pinacol coupling reaction, en route to the total synthesis of shikonin, a compound with antiinflammatory, antibacterial, and antitumor activity (Eq. 3.11) [29], Because the aromatic aldehyde possessed a substituent capable of chelating to vanadium, Torii anticipated, based on precedent, that selective cross-coupling would be possible. The pinacol reaction proceeded in 73% yield and with good diastereoselectivity (5.5 1). 

The pinacol coupling reaction, traditionally carried out with active metals such as sodium, magnesium, or aluminum, can also be accomplished with SmU [29]. Since the initial contribution describing intermolecular pinacolization reactions, several variants have been described [30]. Although many of these papers outline adaptations of the reaction in which additives and catalytic methods have been introduced to improve the reactivity and the economics of the reaction, few of these have adequately addressed the lack of stereoselectivity typically observed in these processes [30a, 31]. 

From a synthetic standpoint the intermolecular pinacolic coupling reaction is limited because only homocoupling reactions are generally practical. Cross-coupling reactions mediated by SmF are restricted to specialized, matched partners [35]. Thus a-dicarbonyl compounds can be heterocoupled with aldehydes, providing facile entry to 2,3-dihydroxy ketones. Although selectivities vary, in some cases the diastereoselectivity of the process can be quite high (Eq. 25). 

In analogy to intermolecular pinacolic coupling reactions, organometallic dialdehydes have served as precursors for the intramolecular version of the reaction, affording cyclic 1,2-diols [39]. Both arene chromium tricarbonyl (Eq. 29) and ferrocene complexes (Eq. 30) afford excellent yields of the desired products. 

---