Isocyanide chiral

The complexes bearing one chiral substituent display a smectic A mesophase when the non-chiral chain is long, or an enantiotropic cholesteric and a monotropic SmA phase for shorter alkoxy chains. A TGBA phase is observed for the derivative which contains the chiral isocyanide combined with the diethyloxy, when the SmA to cholesteric transition is studied. The compound with two chiral ligands shows a monotropic chiral nematic transition. When this compound is cooled very slowly from the isotropic liquid it exhibits blue phases BP-III, BP-II, and BP-I. 

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The mechanistic analogy to the Streckcr synthesis becomes obvious in the addition of the isocyanide to the imine to produce the a-amino nitrilium intermediate. Since all four components are involved in this step, it might be expected that every chiral component (chiral groups R1, R2, R3, R4) contributes to diastereofacial differentiation in the nucleophilic attack on the imine. However, in peptide syntheses by four-component condensation5, the chiral isocyanide or a chiral carboxylic acid component has only limited influence on the diastereoselectivity of the a-amino amide formation5. 

Hie first helical SmC mesophase for a gold(I) mesogen has been prepared by means of complexation to an enantiomerically pure chiral isocyanide (Figure 7.32) [30]. This... 

While chiral isocyanides such as a-substituted isocyanoacetates also usually react with low stereoselectivity, the specially designed, camphor-derived, isonitrile 11... 

Mechanism B may explain why in many cases chiral isocyanides (e.g. 11) give no asymmetric induction at all [21]. Indeed, the isocyanide is not involved in the transition state. In mechanism C the substitution by the isocyanide is rate-limiting and reversible formation of 20 originates a pre-equilibrium. Although (R)-20 should be kinetically favored, (S)-20 may be more stable because of the destabilizing interac-... 

Chiral Isocyanides, Carboxylic Acids and Carbonyl Compounds... 

As already mentioned in Section 1.4.1, chiral isocyanides usually give no induction at all in Ugi reactions. For example, when using chiral a-substituted or a,a-disubstituted isocyanoacetates [7, 27, 44], the two resulting diastereoisomers are... 

Chiral isocyanides such as 198 have also been used for the synthesis of chiral 4,5-disubstituted oxazoles such as 199, which are potentially useful in fluorescence-detected circular dichroism for on-column capillary electrophoresis (Equation 12) <1996JOC8750>. 

The nitrogen atom in a-ferrocenylalkylamines generally shows the same reaction pattern as that in other amines alkylation and acylation do not provide synthetic problems. Due to the high stability of the a-ferrocenylalkyl carbocations, ammonium salts readily lose amine and are, therefore, important synthetic intermediates. Acylation of primary amines with esters of formic acid gives the formamides, which can be dehydrated to isocyanides by the standard POClj/diisopropylamine technique (Fig. 4-16) [92]. Chiral isocyanides are obtained from chiral amines without any racemization during the reaction sequence. The isocyanides undergo normal a-addition at the isocyanide carbon, but could not be deprotonated at the a-carbon by even strong bases. This deviation from the normal reactivity of isocyanides prompted us to study the electrochemistry of these compounds, but no abnormal redox behaviour, compared with that of other ferrocene derivatives, was detected [93]. The isocyanides form chromium pentacarbonyl complexes on treatment with Cr(CO)s(THF) (Fig. 4-16) and electrochemistry demonstrated that there is no electronic interaction between the two metal centres. 

Furthermore, stereoselective U-4CRs can form an even greater variety of products. Occasionally, such reactions can act stereospecifically, this being accomplished efficiently if chiral alkylamine components 2 can participate under suitable reaction conditions [10]. Only in a few exceptional cases may some P-3CRs of chiral isocyanides proceed with efficient stereoselectivity [80]. 

The bis-silylation reaction has found application in the synthesis of the antifungal metabolite (-)-avenaciolide [139] and in the enantioselective preparation of ( )-allylsi-lanes [140]. The use of chiral isocyanides prepared from the terpenoid (-H)-ketopinic acid has also been investigated in an enantioselective bis-silylation of olefins, although the levels of enantioselectivity are, in most cases, modest [141]. 

There are many examples of Passerini-tj e reactions mediated by Lewis acids with TiCU having the most widespread use. Passerini-type reactions using TiCU are compatible with a variety of functional groups as can be seen in the nearly quantitative reaction of a-isocyano phosphonate ester 25 with benzaldehyde. An examination of TiCU-mediated diastereoselective Passerini reaction failed to show any clear stereoselection with chiral isocyanide 26 among many other cases. Weaker Lewis acids such as SiCL can also mediate Passerini-twe reactions, but effectively do so only in the presence of a Lewis base. Demnark demonstrated that a-hydroxyamides, such as 27, could be formed in good yields, by using Lewis bases such as pyridine-7V-oxide to activate the SiCU-... 

The above synthesis gives an example of the high synthetic value of 1-substituted TosMICs (p-toluenesulfonyl methylisocyanides). TosMIC 1600 has been created by van Leusen, who describes syntheses of some chiral sulfonylmethyl isocyanides as TosMIC analogues [1201]. Most of the isocyanides 1600-1605 are prepared by dehydration of the corresponding formamides with phosphoryl chloride, whereby yields of 60-85% are obtained. The chiral isocyanides are compared in terms of their ability to achieve asymmetric induction in base-mediated reactions with... 

They further specify rules governing a catalyst-induced stereoregulation for the incorporation of chiral isocyanide monomers into the polymers during the chain-propagation process (21). The controlling features are a consequence of substituent relative size, and, where it occurs, substituent coordination to nickel (II). This catalyst-induced stereoregulation probably does not apply to the sulfuric acid polymerization of isocyanides. 

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