Imine derivatives, formation

The formation of oximes, hydrazones, and related imine derivatives is usually catalyzed by both general acids and general bases. General base catalysis of dehydration of the tetrahedral intermediate involves nitrogen deprotonation concerted with elimination of hydroxide ion. ... 

Dipolar [3 + 2] cycloadditions are one of the most important reactions for the formation of five-membered rings [68]. The 1,3-dipolar cycloaddition reaction is frequently utihzed to obtain highly substituted pyrroHdines starting from imines and alkenes. Imines 98, obtained from a-amino esters and nitroalkenes 99, are mixed together in an open vessel microwave reactor to undergo 1,3-dipolar cycloaddition to produce highly substituted nitroprolines esters 101 (Scheme 35) [69]. Imines derived from a-aminoesters are thermally isomerized by microwave irradiation to azomethine yhdes 100,... 

Amines can react with various carbonyl compounds and their derivatives in aqueous media to give the corresponding imine derivatives. These reactions have been discussed in related chapters. The synthetically most useful reaction of this type is the formation of imines and imine derivatives from the condensation of amines with aldehydes and ketones. Water is an excellent solvent for such condensation reactions. For example, water was found to be an ideal solvent for a high-yield, fast preparation of easily hydrolyzable 2-pyrrolecarbaldimines.23 In the presence of Cu2+, the reaction afforded the corresponding Cu(II) chelates (Eq. 11.19). 

The heteroatom version of the vinylcyclopropane rearrangement serves to facilitate alkaloid construction. Scheme 13 outlines a strategy for the pyrrolizidine alkaloid isoretronecanol 211 90). Use of a carboxaldehyde (i.e. 213) as a synthon for the primary alcohol provides an ability to adjust stereochemistry. It also sets up formation of the pyrrolidine ring bearing the aldehyde by an aldol-type condensation of an enol of the aldehyde onto an imine derived from 214. Because of the lability of such systems, introduction of X=PhS imparts stability. The resultant azacyclopentene translates to an imine 215 using the iminocyclopropane rearrangement methodology. Simple condensation of the primary amine 216 with aldehyde 37a then initiates this... 

Pathway 2 of Scheme 9 corresponds to one of the most interesting developments in the Beckmann rearrangement chemistry. By trapping of the electrophilic intermediate with a nucleophile (Nu ) other than water, an imine derivative 227 is produced that may be used for further transformations. Carbon or heteroatom nucleophiles have been used to trap the nitrilium intermediate. Reducing agents promote the amine formation. More than one nucleophile may be added (for example, two different Grignard reagents can be introduced at the electrophilic carbon atom). Some of the most used transformations are condensed in Scheme 11. 

In contrast to earlier known imines, those imines derived from a-(methoxymethyl)benzene-ethanamine, which allow formation of a rigid chelate by additional coordination of the lithium with the methoxy group, enabled the preparation of a-alkylated cyclic ketones in very high enantiomeric excesses (90-99% ee)7,8. However, alkylations of imines derived from medium ring ketones were accomplished in 30-82% ee9. The alkylation of acyclic ketones was performed with enantiomeric excesses of more than 75 % and, in the case of the imine derived from 4-heptanone, proceeded with complete asymmetric induction10. 

The use of chiral complexes gives rise to enantioselectivity of carbon—carbon bond formation and this phenomenon has also been applied to resolution54 and enantioselective deuteration55 56 of amino acids. Both the kinetic acidity of the a-methylene protons and the enantioselectivity of bond formation are greatly enhanced by the formation of chiral complexes (32) and (33) of imines derived from the amino acid and salicylaldehyde or pyridoxal respectively.57-59 Similar use has been made of inline complexes (34) derived from pyruvic acid and the amino acid.60 61 Very recently, an asymmetric synthesis of threonine has been achieved using the chiral imine complex (35).62... 

The potential substrates for the Strecker reaction fall into two categories ald-imines (derived from aldehydes, for which cyanide addition results in formation of a tertiary stereocenter) and ketoimines (derived from ketones, for which addition results in a quaternary stereocenter). As in the case of carbonyl cyanation, significant differences are observed between the substrate subclasses. To date, while a few catalyst systems have been found to display broad substrate scope with respect to aldimine substrates, successful Strecker reactions of ketoimines have been reported in only two cases. As is the case for all asymmetric catalytic methodologies, the breadth of the substrate scope constitutes a crucial criterion for the application of the Strecker reaction to a previously unexplored substrate. 

Aldehyde groups can be converted into terminal amines by a reductive amination process with ammonia or a diamine compound. The reaction proceeds by initial formation of a Schiff base interaction—a dehydration step yielding an imine derivative. Reduction of the Schiff base with sodium cyanoborohydride or sodium bor-ohydride produces the primary amine (in the case of ammonia) or a secondary amine derivative terminating in a primary amine (for a diamine compound) (Fig. 88). 

In Yb(OTf)3-catalyzed Mannich-type reaction of the imine with silicon enolate conducted in SCCO2, the desired product is obtained in only 10 % yield after 3 h due to the low solubility of reactants in scC02 (Scheme 3.11, R1, R2, R3, R4, Rs=Ph, Bn, Me, Me, OMe) [57]. Addition of PEG is found to improve the yield to 72 %. The formation of emulsions can be observed in the presence of PEG. The highest yield (72 %) can be reached at 15 MPa CO2 pressure using PEG400 (MW = 400). This system has been applicable to various substrates including imines derived from aromatic and heterocyclic as well as aliphatic aldehydes and silicon enolates derived from esters, thioesters, and a ketone as depicted in Scheme 3.11. 

Recently, Pelletier, Nowacki, and Mody 155) reported that treatment of alkaloid imine derivatives with ethylene oxide in acetic acid or methanol afforded the oxazolidine ring-containing alkaloids in excellent yield. Treatment of lindheimerine (7) with ethylene oxide in acetic acid afforded ovatine (6) in 98% yield. Similarly, veatchine azomethine acetate (10) afforded veat-chine acetate (210) in 97% yield. Formation of the oxazolidine ring in these... 

Ketones and aldehydes also condense with other ammonia derivatives, such as hydroxyl amine and substituted hydrazines, to give imine derivatives. The equilibrium constants for these reactions are usually more favorable than for reactions with simple amines. Hydroxylamine reacts with ketones and aldehydes to form oximes hydrazine and its derivatives react to form hydrazones and semicarbazide reacts to form semicarbazones. The mechanisms of these reactions are similar to the mechanism of imine formation. 

Thus tryptophan synthetase, which catalyzes the addition of serine to indole via an QE.P-unsaturated imine derivative, is inactivated by QE-cyanoglycine (33). In this case, QE-cyanoglycine, an analogue of the substrate serine, undergoes Schiff base formation. Proton abstraction then occurs and the resultant QE-cyano carbanlon is apparently reprotonated to generate a reactive keteneimlne which can alkylate a nucleophilic active site residue. 

Azidouracils. photolysis, 55, 197, 200 Azimines, pyrazole-derived, formation from azomethine imines and nitroso compounds, 60, 34 Azinium azolates, definitions, 60, 200 Aziridine... 

Dipolar cycloaddition of iV-imines, derived from the mono-7V-amino salt of 2-phenyl-quinoxaline, with dimethyl acetylenedicarboxylate results in formation of a tricyclic heterocyclic ring system with bridgehead nitrogen. 

Preparation of imines and enamines from carbonyl compounds and amines can be achieved with a dehydrating agent under acid/base catalysis [563]. Basically, primary amines afford imines unless isomerization to an enamine is favored as a result of conjugation, etc (see Eq. 252), and secondary amines afford iminium salts or enamines. These transformations can be conducted efficiently with a catalytic or stoichiometric amount of a titanium salt such as TiCU or Ti(0-/-Pr)4. Equation (247) illustrates an advantageous feature of this method in the imination of a hindered ketone. f-Butyl propyl ketone resisted the formation of the imine even by some methods reported useful for sterically hindered ketones [564,565]. The TiCU-based method works well, however, for this compound, giving the desired imine in high yield within a relatively short reaction period [566]. Imine derivatives such as iV-sulfonylimines could be... 

In this context, it is worthwhile to note that the use of alcohols in catalytic hydrogenations may lead to related aldehydes or ketones which in turn are capable of producing stable imidazolid-4-one derivatives with the N-terminal amine group.Moreover, when carrying out hydrogenations in an alcohol, oxygen has to be removed meticulously from the system to avoid as a serious side reaction N-alkylation via aldehyde and related imine derivatives which has been observed to occur readily, particularly with methanol as the solvent.f l An additional inconvenience observed when air is not rigorously excluded, is the formation of palladium complexes with the peptides. 

The regiochemistry of deprotonation of imines derived from unsymmetrical ketones is of special significance for the synthetic applications of these anions for carbon-carbon bond formation. This selectivity is sensitive to both the amine moiety and the base used. With imines derived from cyclohexyl- or r-butyl-amine, deprotonation with either Grignard reagents or lithium dialkylamide bases will result in high selectivity (>98 2) for removal of the proton on the less substituted a-carbon as in equations (39) and (40). 3i... 

The two major synthetic applications of imine derivatives of ketones and aldehydes are (i) for reduction to amines and (ii) for formation of carbon-carbon bonds at the a-position to the original carbonyl group. Both topics are covered extensively in other volumes in this series and thus will be treated only briefly here. 

---