Amino acid dipolar form

Write an equation for the reaction of an amino acid (dipolar form) with strong acid or strong base. 

The enantioselective inverse electron-demand 1,3-dipolar cycloaddition reactions of nitrones with alkenes described so far were catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminum complexes. However, the glyoxylate-derived nitrone 36 favors a bidentate coordination to the catalyst. This nitrone is a very interesting substrate, since the products that are obtained from the reaction with alkenes are masked a-amino acids. One of the characteristics of nitrones such as 36, having an ester moiety in the a position, is the swift E/Z equilibrium at room temperature (Scheme 6.28). In the crystalline form nitrone 36 exists as the pure Z isomer, however, in solution nitrone 36 have been shown to exists as a mixture of the E and Z isomers. This equilibrium could however be shifted to the Z isomer in the presence of a Lewis acid [74]. 

We saw in Sections 20.3 and 24.5 that a carboxyl group is deprotonated and exists as the carboxylate anion at a physiological pH of 7.3, while an amino group is protonated and exists as the ammonium cation. Thus, amino acids exist in aqueous solution primarily in the form of a dipolar ion, or zwitterion (German zwitter, meaning "hybrid"). 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Pyranopyrrolothiazoles can be prepared in a similar way to certain pyrano- and thiopyrano-pyrrolizines and pyrrolizinopyridines as discussed earlier. Thus, thiazolidine-4-carboxylic acid reacts with the aldehyde 179 to give a 2 1 mixture of 180 and 181 (Equation 16). This reaction is a 1,3-dipolar cycloaddition of the alkene to the 1,3-dipole formed from reaction of the amino acid amine with the aldehyde <1988T4953, 1990T2213>. The alkyne analogue of 179 is similarly converted into 182 (Equation 17). 

The 1,3-dipolar cycloaddition of nitrile oxides and 2-methylfuran provides suitable precursors for a-amino acids such as L-furanomycin 448 that contains a dihydrofuran ring (495). By using a chiral nitrile oxide derived from mannitol bis(acetonide), the enantiomerically pure furoisoxazoline 449 has been obtained. Hydroboration-oxidation of the latter leads to the hydroxy-substituted annulated THF derivative 450, which is converted via dihydrofuran 451 to furanomycin 448 in enantiomerically pure form (Scheme 1.55). 

Even this representation is not completely true, because it implies that an amino acid exists in an uncharged form (R.NH2.COOH), whereas the molecule in this state carries one negative and one positive charge and as a result shows no net charge. This is known as the dipolar form or zwitterion of the amino acid (Figure 10.6). 

Figure 10.6 Dipolar or zwitterionic form of an amino acid. Amino acids exist in a charged form in aqueous solution, the carboxyl group being dissociated and the amino group associated. Some amino acids also have an extra ionizable group present in their side chain (R group). The ionization of each group is pH-dependent and for each amino acid there is a pH at which the charges are equal and opposite and the molecule bears no net charge. This is called the iso-ionic pH (p/).

A typical 1,3-dipolar cycloaddition involves the use of azomethine ylides as the reactive species. Ylides are formed in situ by thermal condensation of a-amino acids and aldehydes, which then react to form the pyrrolidine-CNT system [32]. The R substituent on the 5-membered heterocycle attached to the SWCNT can be varied by the selection of the amino acid and the aldehyde, giving access to a range of different functional groups on the nanotube sidewalls. 

The pyrrole-functional polymer (160) is actually formed by reaction of p-diethynylbenzene (159) and a 1,3-dipolar mesoionic dioxazolone (161), which is formed in situ from bis(amino acid) (158) and acetic anhydride (Scheme 55). 

Normally, amino acids exist as dipolar ions. RCH(NH,+ iCOO-. in a neutral state, where both amino and carboxyl groups are ionized. The dipolar form, RCH(NH2)COOH may be considered, but the dipolar form predominates for the usual monoamino monocarboxylic acid and it is estimated that these forms occur 10s to 106 times more frequently than the non-polar forms. Amino acids decompose thermally at what might be considered a relatively high temperature (200-300°C). The compounds are practically insoluble in organic solvents, have low vapor pressure, and do not exhibit a precisely defined melting point. 

ZWITTERION. An ion carrying charges of opposite sign, which thus constitutes an electrically neutral molecule with a dipole moment looking like a posilive ion at one end and a negative ion at the other. Most aliphatic amino acids form such dipolar ions, hence react with both strong acids and strong bases. 

An amino-acid therefore can have a number of charged forms, it is an anion at high pH and a cation at low pH at neutral pH it normally has no net charge, but exists as a dipolar ion or zwitterion with both a positive and negative charge. 

Huisgen and coworkers have also described the cycloaddition behavior of the munchnones , unstable mesoionic A2-oxazolium 5-oxides with azomethine ylide character.166 Their reactions closely parallel those of the related sydnones. These mesoionic dipoles are readily prepared by cyclodehydration of N-acyl amino acids (216) with reagents such as acetic anhydride. The reaction of munchnones with alkynic dipolarophiles constitutes a pyrrole synthesis of broad scope.158-160 1,3-Dipolar cycloaddition of alkynes to the A2-oxazolium 5-oxide (217), followed by cycloreversion of carbon dioxide from the initially formed adduct (218), gives pyrrole derivative (219 Scheme 51) in good yield. Cycloaddition studies of munchnones with other dipolarophiles have resulted in practical, unique syntheses of numerous functionalized monocyclic and ring-annulated heterocycles.167-169... 

Review Sec. 17.7 if necessary, and use Table 17.1 for the structures of the amino acids. Write the structures in neutral form, recognizing that dipolar ion structures are possible and that the exact form and degree of ionization depend on the pH of the solution. 

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