Hydrazono ester

More recently, asymmetric Mannich-type reactions have been studied in aqueous conditions. Barbas and co-worker reported a direct amino acid catalyzed asymmetric aldol and Mannich-type reactions that can tolerate small amounts of water (<4 vol%).53 Kobayashi found that a diastereo- and enantioselective Mannich-type reaction of a hydrazono ester with silyl enol ethers in aqueous media has been successfully achieved with ZnF2, a chiral diamine ligand, and trifluoromethanesul-fonic acid (Eq. 11.31).54 The diastereoselective Mannich-type reaction... 

A formal enantioselective synthesis of the antibiotic L-azatyrosine was developed. The asymmetric ally lation of hydrazono esters with ally lsilanes in the presence of a catalytic amount of ZnF2-chiral diamines in aqueous media generated (benzoyl)hydrazino-4-pentenoates in high enantioselec-tivity(Eq. 11.46).77... 

In 1996, Carreira reported a catalytic enantioselective aUylation of aldehydes using BlNOL-modified Tip4. More recently, Kobayashi reported a catalytic enantioselective aUylation of hydrazono esters in aqueous media using ZnF2-chiral diamine complex, [Eq. (13.36)]. In both reactions, reaction mechanism via dual activation of the substrate by Lewis acid metals and aUylsUanes by fluoride is proposed ... 

In the presence of an excess amount of homochiral sulfoxide 23, allyltrichlorosilanes react smoothly with N-benzoylhydrazones in CH2CI2 to give allylated products with good to high enantioselectivity (Equation (31)).130 The sulfoxide-promoted crotylation also shows stereospecificity. The use of BINAP and Tol-BINAP dioxides instead of 23 achieves high enantioselectivity in the allylation of a-hydrazono esters.131... 

Catalytic enantioselective allylation of a-hydrazono esters with allyltrimethoxysilanes in aqueous media can be accomplished by ZnF2-chiral diamine complexes.132... 

Carbohydrates with a hydrazono ester at the C-2 position underwent intramolecular radical C-glycosylation upon reaction with tin hydride and AIBN, to afford the corresponding lactone in good yield (O Scheme 47). Although the stereoselectivities of the amino acid moiety were generally low, the methods have been used for the synthesis of a variety of C-glycosyl amino acids [73], The following reaction was a key step in the total synthesis of furanomycin, an antibiotic. 

Quite recently, chiral diamine ligands have been used by Kobayashi et al. for highly enantioselective Mannich-type reactions [228, 229]. The chiral complex prepared from chiral diamine 89a and Cu(OTf)2 effects highly enantioselective addition of silyl enolates to a-N-acyhmino esters (Scheme 10.82) [228]. The first example of catalytic asymmetric Mannich-type reaction in aqueous media has been achieved by the combined use of ZnF2, diamine 89b, and TfOH in the reaction of a-hydrazono esters [229]. 

Scheme 1. Formation of a-hydrazono esters by reacting arenediazonium salts with the active methylene group of acyl chlorides.204...

Epoxy-cyclohexyl)- -athylester 662 Hydrazino- 671 Hydrazono- -athylester 578 Hydrazono- -ester 706 (1-Hydroxy-cyclohexyl)- -athylester 662 (3-Hydroxy-cyclopentyl)-... 

The asymmetric version of the Mannich-type reaction has been studied in aqueous conditions. Hydrazono esters reacted with silyl enol ethers in water in the presence of zinc... 

Kobayashi et al reported for the first time the catalytic, diastereo- and enan-tioselective Mannich-type reactions of a hydrazono ester with silyl enol ethers in aqueous media (Scheme 4.38) [34]. As for substrates, they focused on acylhydra-zones such as (113), which are imine surrogates that are stable even in aqueous inedia. Substoichiometric Znp2 (50mol%) with chiral diphenylethylenediamine... 

After Kobayashi spioneering study on theabiUty of (S)-2,2 -bis(diphenylphosphanyl)-1,1 -binaphthyl dioxide (9, BINAPO) as a promoter of the enantioselective allylation of a-hydrazono esters with allyltrichlorosilanes [27], Nakajima reported the first catalytic system for the allylation of aldehydes, in which the use of N,N-diisopropylethylamine and tetrabutylammonium iodide was essential for accelerating the catalytic cycle (Scheme 7.16) [28], For this allylation, y-functionahzed nucleophiles such as cis-y-bromoallyltrichlorosilane could be employed as addressed by Maikov and Kodovsky [29], In addition, 9 could be apphed to the asymmetric ring opening of meso-epoxides with SiCh, as expected from the scope of the chiral bipyridine N,N -dioxide catalysis [30], and could also catalyze the SiCfi-mediated, enanhoselective phosphonylation of aldehydes with trialkyl phosphites [31],... 

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