Methods auxiliary

The chiral auxiliary method was not commercially ideal for large scale preparation because of the required introduction of two additional steps (protection and deprotection) and because of its poor atom economy. 

Chiral Auxiliary A chiral auxiliary is an intermediate formed by the attachment of a pure enantiomer to an achiral substrate. The attachment, called a chiral auxiliary, restricts the approach of reactants to react in specihc ways to produce the chiral molecule. The antibacterial drug aztreonam is synthesized using the chiral auxiliary method. 

Scheme 5 Examples of the zeolite chiral inductor and chiral auxiliary methods for photochemical asymmetric synthesis...

Comparison of the Solid-State Ionic Chiral Auxiliary Method of Asymmetric Synthesis with the Pasteur Resolution Procedure... 

Fig. 4 Comparison of the solid-state ionic chiral auxiliary method of asymmetric synthesis with the Pasteur resolution procedure...

An acyclic methoxyamine, 0-methylephedrine 333, plays a similar role in the diastereoselective Uthiation of 334. Substitution with 335 gives 336 from 322, and lithiation of 334 typically gives products 335 with >90 10 diastereoselectivity (Scheme 149) . Auxiliary removal is possible by quaternization and substitution with dimethylamine (giving 336) or other nucleophiles, producing potential ligands with planar chirality only. Usefully, the products formed by this method are enantiomeric with those formed by most other auxiliary methods when the more readily available enantiomers of the starting materials are used. 

Selected ion monitoring can be used for the determination of the relative amount of each component of a mixture, introduced into the mass spectrometer by the direct inlet probe However, such a determination requires reference mixtures of known composition for calibration. In the present experiment, since the monochloro pentaziridino derivative had not yet been isolated in the pure form, it was necessary to determine its concentration, by an auxiliary method, in a sample which could then be utilized as a reference mixture for further experiments. In order to do this we titrated chlorine in the toxic sample of MYKO 63 (B) by the classical method. The results indicated that the amount of N3P3AZJCI was between 0.5-1.5 %. The large statistical error is due to the low chlorine content in the sample examined. Thus, we used the remarkable possibilities provided by neutron activation analysis when the impurity to be quantified is a chlorinated moiety. It is well-known indeed that the C1 -f 2n peak is amongst the most easily detectable by neutron... 

According to Jolles et al. [56, 57] the colorimetric determination of the quantity of sodium nitrite in powder (p. 555) can be an auxiliary method of estimating the stability of powders. Results agree well with the results of the heat test. 

Although the pot process should be considered as obsolete and not competitive with more modem mechanical nitrators, it may still be considered as an auxiliary method, useful when it is necessary to organize production quickly and for a low outlay or when cellulose of low grade must be nitrated. 

When this diastereoselective cyclisation was incorporated into the synthesis of 2, several drawbacks became apparent (Scheme 19). A number of additional steps were required to incorporate the auxiliary into the cyclisation starting material 76, and to remove it from the product, and purification of the only 2 1 mixture of diastereoisomers of 77 obtained from the cyclisation meant that overall yields were poor. So although we were able to make 59 in enantiomerically pure form and therefore complete a formal asymmetric synthesis of 2, chiral auxiliary methods never appeared again in our cyclisation work. 

In recent years, a few stereoselective methods for the asymmetric hydrogenation of pyridines and related heterocycles have been developed <20050BC4171>. A chiral auxiliary method starts with an oxazolidinone-substituted pyridine which on reduction with H2/Pd(OH)2 in acetic acid affords the corresponding piperidine in good yield and high enantiopurity. The chiral auxiliary is cleaved during the reaction and can be recovered (Equation 100) <2004AGE2850>. 

Molecular Modeling as an Auxiliary Method in Solving Crystal Structures Based on Diffraction Techniques... 

III) Chiral Inductor + Chiral Auxiliary Method (CIAM)... 

Obtaining high asymmetric induction therefore requires the placement of every reactant molecule next to a chiral inductor molecule (type B situation) i.e., enhancement of the ratio of type B cages to the sum of types A and C. In the chiral auxiliary method (CAM) the chiral perturber is connected to the reactant via a covalent bond [186,187,197-205]. In this... 

Another valuable process which is somewhat obscure is called Ysopaica—the art of washing things white by fire. It does not really fit in as a specifically wet or dry method and is more an auxiliary method of purifying extracts from plant or mineral sources. 

For a variety of conceptual and practical reasons, we have chosen to carry out most of our asymmetric synthesis work employing the ionic chiral auxiliary method. Conceptually, the idea was attractive because, as far as we were aware, i it represents a new approach to asymmetric synthesis [8]. Furthermore, from the practical point of view, ionic solids have a number of advantages over purely... 

III. EXAMPLES OF THE SOLID-STATE IONIC CHIRAL AUXILIARY METHOD... 

Once again the ionic chiral auxiliary method leads to respectable ees at reasonably high conversions. As in every case studied to date, photolysis of the salts in solution leads to a racemic product, a result that emphasizes the critical role... 

With these points in mind, we summarize in Table 4 all of the published examples of the use of the solid-state ionic chiral auxiliary method of which we are aware, including, for comparison purposes, the three reactions discussed above (entries 17-19). In order to conserve space, results are reported only for the chiral auxiliaries that gave the best results. The interested reader may wish to consult the original papers for a more detailed account of the work and to see what other chiral auxiliaries were tried. 

In principle, any of the photoproducts shown in Table 4 could have been prepared in enantiomerically pure form by irradiating their achiral precursors in solution to form a racemate and then separating the enantiomers by means of the classical Pasteur resolution procedure [36]. This sequence is shown in the lower half of Fig. 3. The top half of Fig. 3 depicts the steps involved in the solid-state ionic chiral auxiliary method of asymmetric synthesis. The difference between this approach and the Pasteur method is one of timing. In the ionic chiral auxiliary method, salt formation between the achiral reactant and an optically pure amine precedes the photochemical step, whereas in the Pasteur procedure, the photochemical step comes first and is followed by treatment of the racemate with an optically pure amine to form a pair of diastereomeric salts. The two methods are similar in that the crystalline state is crucial to their success. The Pasteur resolution procedure relies on fractional crystallization for the separation of the diastereomeric salts, and the ionic chiral auxiliary approach only gives good ees when the photochemistry is carried out in the crystalline state. 

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