Borche reduction

FORSTER - DECKER Amine Synthesis Selective monoalkylation of primary amines via imines An altemativa method is the reaction ol 1 and 2 in the preserve of NaCNBH4 or tnaceloxyborohydride (Borch reduction)/... 

Geissoschizine has also been synthesized by Wenkert and co-workers (161). The previously prepared intermediate keto ester 290 (130) was converted by Borch reduction to 291 in moderate yield. Further selective reduction of 291 resulted in ( )-3-epigeissoschizine (292), which was finally epimerized to ( )-geissoschizine (161). 

See also the Eschweiler-Clarke Methvlation and the Borche Reduction. 

Reduction of N-benzylimines.2 Borch reduction of imines (4, 450) to amines is not stereoselective, although reduction to equatorial amines is somewhat favored. Reductions with 1 and a number of dialkylcyanqborohydrides are much more stereoselective. Thus substituted cycloalkanone imines are reduced mainly to axial amines. However, highest selectivities are obtained with lithium tri-sec-butylborohy-dride (4, 312). 

Reductive amination. Conversion of ketones or aldehydes to amines is usually accomplished by reduction of the carbonyl compound with sodium cyanoborohydride in the presence of an amine (Borch reduction, 4, 448-449). However, yields are generally poor in reactions of hindered or acid-sensitive ketones, aromatic amines, or trifluoromethyl ketones. Yields can be improved markedly by treatment of the ketone and amine first with TiCl4 or Ti(0-i -Pr)42 in CH2C12 or benzene to form the imine or enamine and then with NaCNBH3 in CH3OH to effect reduction. Note that primary amines can be obtained by use of hexamethyldisilazane as a substitute for ammonia (last example). 

The dilactam (69), obtainable as described above (see Scheme 10) from (68a) or its (Z)-isomer, has been used in an improved synthesis of geissoschizine (80).54a Methanolysis of (69) gave a quantitative yield of the tetracyclic ester (68b), which, on Borch reduction and formylation, gave geissoschizine (80). 

BOGER Heterocycle synthesis 40 BOORO Enol ether synthesis 4i BORCH Reduction 42 Bormann 404 Borodin 183 Borsche 123... 

The strained S-lactam was opened smoothly to yield fS-ketoester 324, which on treatment with sodium methylate provided lactam 323. A highly selective Borch reduction finally led to (+)-eburnamonine. Olefination of both enantiomers followed by allylic bromination and oxidation eventlessly afforded apovincamine, which had been converted into vincamine already. 

The cycloaddition proceeded smoothly, when promoted by acid, to provide ketones 75 and 76. Whereas 76 was surely the kinetic product, acid promoted enolization-isomerization provide 75 as well. Treatment of the mixture with hydroxylamine gave the desired oxime (77) and Borch reduction from the convex face provided 72 The rest of the synthesis proceeded as expected. The intramolecular nitrone cycloaddition gave 70. Methylation of the amine and reduction of the weak N-O bond provided 78. Jones oxidation (Cr in acid) of 78 completed the synthesis of luciduline (55). When a single enantiomer of 74 was used, a single enantiomer of luciduline was obtained. 

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