Imines, alkylation comparison with

The SAMP/RAMP Method As early as 1976, azaenolates derived from A,A-dialkyl hydrazones were studied as an alternative to direct ketone and aldehyde enolate alkylations. These species were found to exhibit higher reactivity toward electrophiles, as well as better regioselectivity for C-alkylation than their parent carbonyl compounds. A,A-diaIkyl hydrazones are stable and are relatively easy to prepare, making them appealing from a practical point of view in comparison with imines and enamines, which can be difficult to form quantitatively and are hydrolytically unstable. Given these desirable attributes, Enders undertook the development of chiral nonrace-mic A,A-diaIkyl hydrazine auxiliaries for the asymmetric a-alkylation of ketones. The result of his efforts were (5)-and (R)-l-amino-2-methoxypyrrohdine hydrazine (1 and 2, respectively), now commonly known as the SAMP and RAMP auxiliaries, respectively (Figure 7.1). Over the years, the SAMP/RAMP method has come to be considered the state-of-the-art approach to asymmetric ketone... 

A problem that is not entirely avoidable is the formation of imine byproducts via reaction of the released amine with the aldehydic group in the photoproduct. This occurrence could be suppressed with alkyl or aryl substitution at the benzylic position, leading to the formation of a less reactive ketone in comparison with the nitroso aldehyde formed with no substitution at the benzylic position. Imine byproduct formation is also less likely to occur in relatively nonpolar solvent systems, such as THF, which ultimately limits the application of the o-nitrobenzyl carbamate photoprotecting group to nonaqueous systems in this regard. 

The 2-amino-selenazoles are crystalline compounds, and sometimes unstable, for example, the parent compound on heating in water undergoes complete decomposition. A few of these selenazoles which are substituted in the amino group are oily liquids. The basic character is more pronounced than in the alkyl- and aryl-selenazoles. The hydrochloride salts are, therefore, not so easily hydrolyzed in aqueous solution. 2-Amino- and 2-methylamino-4-methylselenazole have been considered to exist partly in the tautomeric selenazol-2-one imine form from comparison of their ultraviolet spectrum with that of 2-diethylamino-4-methylselenazole. ... 

Table3. MEA imine hydrogenation with Ir-ferrocenyldiphoshine complexes Comparison of catalyst performances of (a) homogeneous catalysts, (b) immobilized on silica gel, and (c) for reductive alkylation in a two-phase system (formulas see Fig. 7)...

This reaction usually works for nonenolizable imines (e.g., A -aryl aldimines) and has the features of readily accessible starting materials, and simple preparation of 3-substituted or unsubstituted 2-azetidinones (i.e., 3-lactams) with direct control of stereoselectivities. It has been reported that the yield of this reaction depends on the activation and the type of zinc. The stereochemistry of /3-lactams depends on the a-substituent of bromoacetates, the solvent, and the alkyl portion of the esters. For example, when the a-substituent is an alkyl group (e.g.. Me, Et, /-Pr, cyclohexyl, r-Bu), the major product has cis geometry, such a trend is especially prevailing for the reaction of acetate with a branched a-substituent (e.g., j-Pr, cyclohexyl, r-Bu) in THF. s For comparison, the reaction of isopropyl acetate in toluene tends to form jS-lactams of trans geometry. In addition, phenyl acetates favor the trans isomers regardless of the solvents. ... 

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