Ammonium nitroaldol reaction

The enantioselective nitroaldol reaction of phe-nylalaninals 45 with nitromethane was also promoted with the N-anthracenylmethyl ammonium fluorides in the presence of potassium fluoride.1411 Interestingly, as shown in Scheme 16, the major product was the (2R,3S)-isomer 46a when N,N-dibenzyl-(S)-phenylalaninal and 12 (R=benzyl, X=F) were used while the (2S,3S)-isomer 46b was major when N-tert-butoxycarbonyl derivative 45b and 12 (R=allyl, X=Br) together with potassium fluoride were used. The nitroalcohols 46a and 46b were respectively converted to amprenavir 47a, a HIV protease inhibitor, and its diastereomer 47b. The... 

Aldol and Related Condensations As an elegant extension of the PTC-alkylation reaction, quaternary ammonium catalysts have been efficiently utilized in asymmetric aldol (Scheme 11.17a)" and nitroaldol reactions (Scheme ll.lTb) for the constmction of optically active p-hydroxy-a-amino acids. In most cases, Mukaiyama-aldol-type reactions were performed, in which the coupling of sUyl enol ethers with aldehydes was catalyzed by chiral ammonium fluoride salts, thus avoiding the need of additional bases, and allowing the reaction to be performed under homogeneous conditions. " It is important to note that salts derived from cinchona alkaloids provided preferentially iyw-diastereomers, while Maruoka s catalysts afforded awh-diastereomers. 

Cerium(IV) ammonium nitrate, 67 Trifluoroacetyl nitrate, 324 Nitroaldol reaction (see Aldol-type reactions)... 

Corey and Zhang utilized chiral quaternary ammonium fluoride 4d possessing a 9-anthracenylmethyl group on nitrogen for the face-selective nitroaldol reaction of nitromethane with protected (S)-phenylalaninal. This was directed toward the... 

The nitroaldol reaction of silyl nitronates with aldehydes promoted by ammonium fluorides, which was originally introduced by Seebach and Colvin in 1978 [24], is a useful method for the preparation of 1,2-functionalized nitroalkanols. Recently, the present authors have succeeded in developing an asymmetric version of high efficiency and stereoselectivity by using a designer chiral quaternary ammonium bifluoride of type 6 as catalyst, which was readily prepared from the corresponding bromide by the modified method C in Scheme 9.5 [25]. 

Table 9.5 Asymmetric nitroaldol reactions catalyzed by chiral quaternary ammonium bifluorides 6.

During the course of studies on these asymmetric nitroaldol reactions catalyzed by chiral ammonium bifluorides 6 (X= HF2), the reaction of 23a was attempted with trans-cinnamaldehyde, a representative a,P-unsaturated aldehyde, under the influence of (R,R)-6c (X = HF2, 2mol%) in THF at — 78 °C. The starting aldehyde was consumed within 30 min at this temperature and, surprisingly, the 1,4-addition product 25 was obtained predominantly as a diastereomeric mixture with concomitant formation of the initially expected nitroaldol (1,2-addition product) 26 [72% combined yield, 25/26 = 17 1, syn/antiof25 = 85 (4% ee) l 5 (25% ee)] (Scheme 9.13). 

Aldol and Nitroaldol Reactions (Preparation of Chiral Quaternary Ammonium Fluorides)... 

Corey EJ, Zhang F. Re- and i(-face-selective nitroaldol reactions catalyzed by a rigid chiral quaternary ammonium aslt a highly stereoselective synthesis of the HIV protease inhibitor amprenavir (Vertex 478). Angew. Chem. Int. Ed. 1999 38(13-14) 1931-1934. 

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