Pinacol-type derivatives

Similar stereoselectivities are achieved in the allylation of enantiomerically pure proline-derived a-oxoamides47. l-Bromo-3-methy]-2-butcne reacts with clean allylic inversion. Since pinacol-type coupling products are also produced under the reaction conditions, this was taken as evidence for a radical addition mechanism47. 

Racemic aldehyde 2 under similar reaction conditions yielded two diastereo-meric pinacols the configuration of all the products was assigned by X-ray diffraction. On the basis of these results a mechanistic scheme could be proposed to explain the stereochemical course of the coupling reactions [19]. It is assumed that the reaction proceeds via an intermediate titanaoxirane by insertion of a molecule of aldehyde 2 into the Ti—C bond - a proposal that has been put forward recently on the basis of DFT calculations [20]. The pinacol-type coupling products could be employed as bidentate P,P chelate ligands to Mo(CO)4 fragments however analogous experiments carried out with the olefinic derivatives 18 were unsuccessful [19]. 

Moreover, no aldol or pinacol-type dimerization of the carbonyl compound was observed, even in the case of easily reducible benzophenone or benzaldehyde derivatives. As observed previously, the nature of the solvent is an important factor for the success of the reaction. Indeed, no homoallylic alcohols were formed in DMF. 

The diterpene derivative 1 was treated with BF3-OEt2 and AC2O in an attempt to induce a "pinacol-type rearrangement leading to a ring B-homo derivative. No such reaction was observed. Instead, the only product which could be isolated (33%) was shown to be 2, formed by "a profound backbone rearrangement" of 1. 

Reduction of nitro compound 269 with stannous chloride did not give the expected amino compound but rather the tricyclic 1,4-thiazepine derivative 270 after a semi-pinacol-type rearrangement (Equation 22) <2002JOC8662>. 

Pinacols (61) derived from a variety of aromatic aldehydes have been employed in enantio- and diastereo-selective allylations of aliphatic aldehydes.174 Their allyl-boronate derivatives react under Lewis acid conditions (SnCU) with a variety of aldehyde types, in good yield and ee. Even better results are obtained by addition of (61) as a I irons led acid (auto)catalyst, via coordination/activation of the tin catalyst. 

Ail carbcne rearrangements (as well as carbenium ion rearrangements, such as pinacol type isomerisations) require that the o-bond of the migrating group and the empty orbital of the carbene should be parallel. TNs has been dcmunstiated, for example, by the isotope labeling of hydrogens in norbornane derivatives (33] ... 

Monotosylates of c/s-l,2-diols, such as that derived from o -pmene, undergo pinacol-type rearrangements upon treatment with r-BuOK (eq 40). The trans isomer is converted to the epoxide under the same conditions (eq 41). 4-Benzoyloxycyclo-hexanone undergoes a mechanistically interesting rearrangement to benzoylcyclopropanepropionic acid when treated with r-BuOK/ t-BuOH (eq 42). ... 

The first total synthesis of ( )-furoscrobiculin B (18.9) was accomplished by K. Kanematsu 160) through a novel construction of the azuleno [6,7-c] furan ring system by base catalysed pinacol-type rearrangement of the isonaphthofuran derivatives 26.122 (Schemes 37... 

High stereoselectivity for the d,l product diastereomer has been found in the photochemically induced pinacol-type coupling of aldehydes by tristannanes (Scheme 35). The stannadioxolans (55) produced may be worked-up with acetyl chloride (or trimethylsilyl chloride) to give derivatives of 1,2-diols. 

The formation of a-terpineol as the major product from deamination of fenchyl amine (43-NH2) (98) is remarkable in that it requires first rearrangement to the more strained pinane skeleton. Small amounts (3—11%) of pinane-type products have been isolated from the bornyl and fenchyl diazonium ions under alkaline conditions (98 a). The contra-thermodynamic rearrangement of apobornyl and camphenilyl derivatives to nopinone may be effected in good yield with the driving force of the pinacol-type rearrangement (98 b, 99). 

The classic precursors used for generating diradicals are cyclic, bicychc and polycyclic diazenes. However, diradicals have also been made by Norrish type I photochemical extrusion of CO from cychc ketones, by thermal cleavage of vinyl and divinylcyclopropanes, by pinacol reactions of diketones, by Bergman-type cyclisations of endiynes, by several types of photoelectron transfer reactions and in other ways. Most synthetic applications have started with a derivative of 2,3-diazabicyclo[2.2.1]hept-2-ene which on heat-... 

Since boronates derived from pinacol are air stable, we decided to prepare olefins of type (4) according to the following sequence ... 

More recently the introduction of low-valent transition metal and lanthanoid based reducing systems, especially those based on titanium, has provided dramatic advances in efficiency and selectivity. It is now possible to select appropriate conditions for efficient coupling of all types of carbonyl compounds, often with high chemo-, regio- and stereo-selectivity. Moreover, imino- and thio-carbonyl derivatives are also coupled via pinacolic methodology. The coupling of imines to 1,2-diamines is particularly effective, with excellent control of vicinal stereochemistry. 

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