Silylnitronate, Henry reaction

In the presence of a catalytic amount of tetrabutylammonium fluoride, either freshly dried over molecular sieves22 or as the trihydrate16, silylnitronates 2 derived from primary nitroalkanes react readily at — 78 C or below, via their in situ generated nitronates. with aromatic and aliphatic aldehydes to give the silyl-protected (/J, S )-nitroaldol adducts 3 in excellent yield4,22-24-26,27. Silylnitronates, derived from secondary nitroalkanes. afford the adducts in 30 40% overall yield24. In contrast to the classical Henry reaction (vide supra), the addition of silylnitronates to aldehydes is irreversible. Ketones are unreaetive under such conditions. 

Asymmetric Aza-Henry Reactions of Nitronates with Imines. Although the Henry reaction and its aza-analogs are powerful C-C bond-forming reactions, there are few reports of catalytic asymmetric versions of these reactions. The cir-DiPh-Box copper complexes are excellent catalysts for highly diastereo and enantioselective aza-Henry reactions of a variety of trimethyl-silylnitronates with Af-(p-methoxyphenyl)-a-imino-esters (eq 3). The use of an A-(/ -methoxyphenyl) group for protection prevents undesirable side reactions and can be easily removed. The aza-Henry reaction products can be further derivatized to the corresponding a,3-diamino acids whose syntheses have rarely been reported. 

Various modifications have been made to the original Henry reaction to either expand its synthetic application (aza-Henry reaction ), or to improve its yield and selectivity (silylnitronates and lithionitronates. condensations ). 

Sterochemical control in the Henry reaction had been essentially neglected until lithionitronates, doubly-lithiated nitronates and silylnitronates methodologies were explored. Treatment of nitropropane with -BuLi furnishes nitronate 57, which then reacts with 4-nitrobenzaldehyde in the presence of Ti(/-OPr)Cl3 in THF to provide nitroalcohol 55 in high erythro anti) selectivity. Silyl nitronates in the presence of fluorides also prefers erythro (anti) products " while dilithiated nitronates reacts with aldehydes to afford 2-nitroalcohols in high threo syn) selectivity. ... 

A typical Henry reaction procedure involving silylnitronate and aldehyde... 

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See also in sourсe #XX -- [ ]

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