Alane esters

Analogously prepared were the l-O-(N-acetyl) alanate ester of 2,3,5-tri-0-benzyl-N-e<, -L-arablnofuranose, and the 1-0-(N-tert-butoxy-... 

Alane is also a useful group for reducing amides and it, too, can be used to reduce amides to amines in the presence of ester groups. 

Alternatively, bromo trienyne 66, prepared by the Wittig reaction of TMS-capped propargyl ylide with , -5-bromo-2,4-pentadienal, could be coupled with dienyl zinc reagent 67, as illustrated in equation 3657. Subsequent desilylation followed by treatment with trimethyl aluminum in the presence of catalytic Cp2ZrCl2 afforded the alane of tetraenyne 68 which, on exposure to chloroformate, gave essentially all- polyene ester 69. 

Decomposition of the reaction mixtures with water followed by dilute acids applies also to the reductions with boranes and alanes. Modifications are occasionally needed, for example hydrolysis of esters of boric acid and the alcohols formed in the reduction. Heating of the mixture with dilute mineral acid or dilute alkali is sometimes necessary. 

The reason why the carbonyl group in -santonin remained intact may be that, after the reduction of the less hindered double bond, the ketone was enolized by lithium amide and was thus protected from further reduction. Indeed, treatment of ethyl l-methyl-2-cyclopentanone-l-carboxylate with lithium diisopropylamide in tetrahydrofuran at — 78° enolized the ketone and prevented its reduction with lithium aluminum hydride and with diisobutyl-alane (DIBAL ). Reduction by these two reagents in tetrahydrofuran at — 78° to —40° or —78° to —20°, respectively, afforded keto alcohols from several keto esters in 46-95% yields. Ketones whose enols are unstable failed to give keto alcohols [1092]. 

In dicarboxylic acids with one free and one esterified carboxyl group the free carboxyl only may be reduced with hydrides (alanes, boranes). The monoethyl ester or adipic acid was converted by an equimolar amount of borane in tetrahydrofuran at —18° to 25° to ethyl 6-hydroxyhexanoate in 88% yield [971] Procedure 19, p. 209). 

Alane (AIH3) and its derivatives have also been utilized in the reduction of carboxylic acids to primary alcohols. It rapidly reduces aldehydes, ketones, acid chlorides, lactones, esters, carboxylic acids and salts, tertiary amides, nitriles and epoxides. In contrast, nitro compounds and alkenes are slow to react. AIH3 is particularly useful for the chemoselective reduction of carboxylic acids containing halogen or nitro substituents, to produce the corresponding primary alcohols. DIBAL-H reduces aliphatic or aromatic carboxylic acids to produce either aldehydes (-75 °C) or primary alcohols (25 C) Aminoalu-minum hydrides are less reactive reagents and are superior for aldehyde synthesis. ... 

AIH3 and alkyl derivatives are powerful Lewis acidic reducing agents that rapidly convert carboxylic esters into primary alcohols. In many cases, the reduction may be stopped at the intermediate aldehyde stage, providing that the stoichiometry and temperature are carefully controlled. However, complete reduction is experimentally easier to carry out. DIBAL-H is the most useful and widely used alane. 

Complex alkenyl-copper and -cuprate derivatives, and also alanates, have been generated with high stereochemical purity, and upon treatment with iV-chloromethyl-iV-methylformamide or iV-chloromethyl-phthalimide provide access to highly functionalized allylic amides (entries 1 and 2, Table 7). Addition to acyliminomalonic esters or acyliminophosphonates provides acylaminomalonates and acylamino-alkylphosphonates in modest yield (entries 3 and 4, Table 7). 

Alkynoic esters, 493-494 Alkynyldiethyl alanes, 144 Allene cyclo-oligomerization, 41 Allenes, 161,315-316,438 Allenic adds, 149-150 Allenic amides, 255... 

Alane must be prepared in situ. It is an excellent reagent for the 1,2-reduction of a,p-unsaturated esters. It selectively reduces esters in the presence of halogens and nitro groups. Reviews (a) Seyden-Penne, J. Reductions by the Alumina- and Borohydrides in Organic Synthesis, Wiley-VCH New York, 1997, 2" edition, (b) Brown, H. C. Krishnamurthy, S. Tetrahedron 1979, 35, 567-607. 

Aromatic and aliphatic A,iV-dialkyl carboxylic acid amides, carboxylic esters, and free carboxylic acids are converted into aldehydes by diisobutyl-alane if reduction is carried out at low temperatures (0°C or — 70°C) 269, 279, 280, 286) ... 

Several new aspects of organoalane chemistry have been described this year. Orthoesters react with alanes, prepared from a variety of allyl and propargyl halides, to give the corresponding 0,y-unsaturated acetals (Scheme 45). Interestingly reactions involving acetals rather than ortho esters lead exclusively to the formation of allenic ethers (160). ... 

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