Aldimines epoxidation

Reaction of metalated aldimines with aldehydes or epoxides... 

Snapper and Hoveyda reported a catalytic enantioselective Strecker reaction of aldimines using peptide-based chiral titanium complex [Eq. (13.11)]. Rapid and combinatorial tuning of the catalyst structure is possible in their approach. Based on kinetic studies, bifunctional transition state model 24 was proposed, in which titanium acts as a Lewis acid to activate an imine and an amide carbonyl oxygen acts as a Bronsted base to deprotonate HCN. Related catalyst is also effective in an enantioselective epoxide opening by cyanide "... 

In contrast to the epoxides, preparative routes to the aziridines are fairly evenly split between the [C=N + C] and the [C=C + N] routes. Among contributions in the former category, aziridine carboxylate derivatives 110 can be prepared through the lanthanide-catalyzed reaction of imines with diazo compounds, such as ethyl diazoacetate (EDA). In this protocol, iV-benzyl aryl aldimines and imines derived from aromatic amines and hindered aliphatic aldehydes are appropriate substrates <99T12929>. An intramolecular variant of this reaction (e.g.. Ill —> 112) has also been reported <990L667>. 

Epoxidation of both aldimines and ketimines is possible. Most oxaziridines formed are stable compounds, especially aldimines containing aromatic substituents, and 2-sulfonyl-and 2-sulfamyl oxaziridines5. Generally, /V-sulfonyloxaziridines are isolated as stable crystalline solids. Certain compounds are widely used in synthetic organic chemistry as oxygen-transfer reagents (15-17). 

Analogous to epoxides, aziridines can be prepared by the methylenation of imines. In this case, ethyl diazoacetate is the most common source of carbenes. For example, the imine derived from p-chlorobenzaldehyde 148 is converted to the c/j-aziridinyl ester 149 upon treatment with ethyl diazoacetate in the presence of lithium perchlorate <03TL5275>. These conditions have also been applied to a reaction medium of the ionic liquid l-n-butyl-3-methylimidazolium hexafluorophosphate (bmimPFe) with excellent results <03TL2409>. An interesting enantioselective twist to this protocol has been reported, in which a diazoacetate derived from (TJ)-pantolactone 150 is used. This system was applied to the aziridination of trifluoromethyl-substituted aldimines, which were prepared in situ from the corresponding aminals under the catalysis of boron trifluoride etherate <03TL4011>. 

Whitesell and Whitesell" have tabulated some of the many types of electrophilic reagents that C-al-kylate metallated imines. These are potent nucleophiles and undergo substitution reactions even with weakly electrophilic species such as epoxides and oxetanes. Lithiated ketimines and aldimines have been frequently used in reactions with alkylating agents containing latent 2-keto (or aldehydo) groupsor 3-keto (or aldehydo) groups. ... 

Just as epoxides can be prepared from carbonyl compounds, aziridines can be accessed through the addition of amphophilic carbon centers onto imines, and sulfur ylides are frequently used as carbon donors in this regard. Thus, S-allyl tetrahydrothiophenium bromide 139 is smoothly deprotonated with strong base to provide an ylide which adds to a variety of N-protected imines. For the A-tosyl aldimine 140 derived from isovaleraldehyde, the corresponding vinyl aziridine 141 is formed in fair yield as a mixture of stereoisomers <04TL1589>. 

Aldimines add 2-(RS)allyl silyl ethers under acid condidons, 3-ketoesters result using trimethylsilyl malonates, chiral epoxides ring open to the a-siloxyketone and hence andfungal Sch42427/SM 9164451. 

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