Other Imino Substrates

2 Other Imino Substrates 6.3.2.2.1 Reactions with iminium Salts 

Chiral acyliminiums were used for the preparation of enantiopure piperidines [267]. Recently, the use of enantio-enriched y-alkoxyallyltins onto chiral acyl iminiums [268] provided a new entry into the synthesis of potential precursors of a-amino-/3-hydroxy adds or aminosugars, with a total control of the stereochemistry. 

These reagents are used as protected imines, which upon allylation and subsequent deprotection give an access to primary homoallylic amines. For instance, the use of benzoyl- and acylhydrazones as stable surrogates of imines were exploited in allylation reactions with tetraallyltin [269]. Finally, nitrones can be used as substrates for allylation reactions giving access to homoallylic hydroxyl-amines [270]. 

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Bis-acceptor-substituted diazomethanes are most conveniently prepared by diazo group transfer to CH acidic compounds either with sulfonyl azides under basic conditions [949,950] or with l-alkyl-2-azidopyridinium salts [951] under neutral or acidic conditions [952-954]. Diazo group transfer with both types of reagents usually proceeds in high yield with malonic acid derivatives, 3-keto esters and amides, 1,3-diketones, or p, y-unsaturated carbonyl compounds [955,956]. Cyano-, sulfonyl, or nitrodiazomethanes, which can be unstable or sensitive to bases, can often only be prepared with 2-azidopyridinium salts, which accomplish diazo group transfer under neutral or slightly acidic reaction conditions. Other problematic substrates include amides of the type Z-CHj-CONHR and P-imino esters or the tautomeric 3-amino-2-propenoic esters, which upon diazo group transfer cyclize to 1,2,3-triazoles [957-959]. 

Several Lewis acid-base interactions between alkali metal cations and heteroatom-containing molecules are indispensable in the promotion of reactions involved in critically important and fundamental transformations—deprotonation with lithium amides at the a-hydrogens of carbonyl or imino compounds and the addition of organolithium compounds to such electrophilic substrates. Because it is impossible to cover the multitude of these and other closely related subjects, this chapter describes only briefly general aspects of current interest. 

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