Propanal aldimine from

Formation of aldimines from propanal and octanal. To a stirred solution (0°C) of 10 mmol of the methoxyamine dissolved in 30 ml of benzene (CAUTION) (previously washed with concentrated sulphuric acid and distilled) is added 10 mmol of the pure aldehyde. An immediate cloudiness usually results. The mixture is allowed to warm to room temperature and c. 15 g of anhydrous sodium sulphate added. After stirring an additional 30-40 minutes, it is filtered and the sodium sulphate washed thoroughly with dry ether. The solvent is removed first with aspirator pressure and then with a vacuum pump (0.5 mm) to generally furnish 9.5-10 mmol of the aldimine as a colourless oil. The aldimines are dissolved in tetrahydrofuran (0.4 m) and stored at —20 to — 30 °C. Attempts to store the aldimines as neat liquids result in deterioration. The solutions of aldimines are conveniently transferred via a syringe to reaction vessels. 

The literature authors prepared mixtures of enantiomerically pure 2-methyl-octanal (Md5-8.90°C) (isolated by preparative g.l.c. from the sample prepared by the above procedure from the aldimine of propanal and hexyl iodide) containing w/w amounts of 12.7, 25.7 and 44.6 per cent of hexyl iodide. The [a]D values of these mixtures were plotted against the weight per cent to give a linear relationship. The g.l.c. composition of the reaction product and its extrapolated specific rotation thus allowed an ee per cent value to be calculated. In a similar way the ee per cent of the reaction of the aldimine of octanal and methyl iodide was calculated from the mixtures of octanal with the optically pure 2-methyloctanal of 55.4,74.3 and 87.3 per cent and the plot weight per cent v. [a]D was again linear. 

Some of the problems associated with the synthesis of a-dicarbonyl starting materials have been alleviated by the use of propane-1,3-dithiol, which reacts with aldehydes to give cyclic thioacetals. With butyllithium the resulting stable dithiane anions (134) can be transformed into a-diketones or a-hydroxy ketones (Scheme 73). A further approach to such compounds is found in the reaction of a-ketonitrate esters with sodium acetate (Scheme 73), while aryl a-diketones are also available from a-ketoanils (prepared from the cyanide-ion-catalyzed transformation of aromatic aldimines) (70AHC(12)103). 

Although 1,2-dicarbonyl substrates (especially unsymmetrical benzils) are often difficult to make, there are a number of approaches which may be appropriate. Propane-1,3-dithiol reacts with aldehydes to give cyclic thioacetals (in 52-91% yields) which form stable dithiane anions when treated with butyllithium. Subsequent quenching with an acid chloride followed by mercury(ll) chloride treatment gives a 1,2-dicarbonyl species. Alternatively, substitution of an aldehyde for the acid chloride gives rise eventually to an a-hydroxycarbonyl derivative (Scheme 5.1.2) [16j. An alternative approach to a-ketoaldehydes (82-86% yields) reacts an a-ketonitrate ester with sodium acetate in DMSO [17]. Aryl a-diketones can be made from a-ketoanils, which are in turn made by cyanide ion-catalysed transformation of aromatic aldimines [18], and the range of unsymmetrical benzils has been increased by... 

Secondary Amines.—The reduction of imines to the corresponding secondary amines can be effected by various methodologies. New additions are the sodium triacyloxyborohydrides (easily obtainable from sodium borohydride and AT-acyl derivatives of optically active amino-acids), which are used for the asymmetric reduction of cyclic imines. Also now available is a highly stereoselective reduction of N-benzylimines derived from substituted cyclohexanones, with alkali-metal borohydrides, in particular L-selectride. A fiuther addition is the first report of the reduction of aldimines by hydrogen transfer from propan-2-ol,... 

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