Pinacol oximes

Reductions with aluminum are carried out almost exclusively with aluminum amalgam. This is prepared by immersing strips of a thin aluminum foil in a 2% aqueous solution of mercuric chloride for 15-60 seconds, decanting the solution, rinsing the strips with absolute ethanol, then with ether, and cutting them with scissors into pieces of approximately 1 cm [141,142]. In aqueous and non-polar solvents aluminum amalgam reduces cumulative double bonds [143], ketones to pinacols [144], halogen compounds [145], nitro compounds [146, 147], azo compounds [148], azides [149], oximes [150] and quinones [151], and cleaves sulfones [141, 152, 153] and phenylhydrazones [154] (Procedure 30, p. 212). 

Reduction of ketones other than 48 to dimers has been examined. Ethyl pyridyl ketone (51) gave a poor yield of the corresponding pinacol (52).79 Pinacols can also be made from crossed coupling of ketones with 3-(48).80 Finally, a study of the radical anions formed from the isomeric benzoylpyr-idines (53) was done, and the rates of rotation of the pyridyl ring were determined.81 The mechanism of reduction of the oxime and thiosemicar-bazone derivatives of 53 was determined by voltammetry techniques.82 The monoimine derivative of the pyridil 54 was reduced to the a-amino ketone 55 (Scheme 18).83 Unsaturated pyridyl ketones and heteroaryl pyridyl ketones have also been studied by voltammetry.84,85... 

Meyer (17) in the following year then expressed the view that the products of the action of nitrous acid on the ketones were not true nitroso compounds (4) but isomeric oximes (5), and he considered their reduction to be analogous to the conversion of a nitro to amino group, rather than that of ketone to pinacol. Thereafter the Treadwell formulation of pyrazines was abandoned. 

Dihydroxylation of the stilbene double bond in the trans isomers of Combretastatin A-1 and A-4 produced diols which by treatment with boron trifluoride in ethyl ether [44] or with trifluoroacetic acid [17] resulted in pinacolic rearrangement to produce an aldehyde. The aldehyde was converted in a variety of derivatives, as illustrated in the Scheme 20, via the following reaction sequence reduction with sodium borohydride to primary alcohol which was derivatized to the corresponding mesylate or tosylate, substitution with sodium azide and final reduction to amine with lithium aluminum hydride. Alternatively the aldehyde was converted to oxime which was catalitically hydrogenated to amine [17]. 

Elements of Group V Bonds to Oxygen.—Alcohol groups can be displaced from triethoxyarsine by oximes or diethylhydroxylamine in a stepwise manner to give the products (EtO)3-nAs(ON=CR1R2) .659 Two hew 1,3,2-dioxa-arsolans (92) result when tris(dimethylamino)arsine and either ethylene glycol or pinacol react, and further reaction of these compounds with a -hydroxy-acids leads... 

Reactions of allylboronates (87)-(89) and aldoximes, imines and sulfenimides have been described. These reactions are considerably slower than those of aldehydes, and consequently reagents (87) and (88) are generally used in preference to the less reactive pinacol ester (89). The reactions of imines and (87) proceed at room temperature, while those of (88) and oximes and sulfenimides generally require heating in refluxing CCU or toluene. The reaction of (88) and oximes can be performed at room temperature if a pressure of several kilobars is applied. ... 

In reactions that resemble the pinacol rearrangement, thiopyranoside sulphonate esters undergo ring contraction and expansion reactions via the intermediacy of sulphonium ions, e.g. equations 37 and 38102. Similarly, cyclic sulphonates are able to trap oxime anions at the carbon atom (equation 39)103. Such carbon alkylation of oxime ions is rare, and by suitable choice of substrate both ring-contraction and ring-expansion reactions are possible103. 

Previously, tin-ketyl radicals have been added to alkenes only in an intramolecular fashion. [9] In recent publications, however, pinacols and amino alcohols have been prepared by cyclisation of dicarbonyl compounds [10] or keto-oximes [11] with tributyltin hydride. Cyclisation of 1,5-ketoaldehydes 1 and 1,5-dialdehydes with tributyltin hydride yields cw-diols 2 with excellent stereoselectivities, whereas the keto-oxime 4 with four benzyloxy-substituents affords a 58 42 cis trans) mixture. The tran -product was transformed in two more steps to the potent glycosidase inhibitor 1-deoxynojirimycin (6). [lib] The reversibility of both the addition of the tributyltin-radical to the carbonyl group and the intramolecular radical C-C bond formation is believed to be responsible for the high selectivity in the formation of 2. In the cyclisation of 1,5-pentanedial the unhydrolyzed coupling product 3 could be isolated, therefore providing evidence for a new mechanistic variant of the pinacol reaction, in which only 1.2 equivalent of the reducing agent are necessary. 

---