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Chiral auxiliaries recovery

The resolution process developed by Syntex is almost ideal (Pope Peachy resolution), with an efficient racemization and recycling of the unwanted (R) -enantiomer (yield >95% of (S)-naproxen from the racemate) and the chiral auxiliary (recovery >98%). [Pg.1296]

SCHEME 13.12 Large-scale asymmetric aldol reaction with chiral auxiliary recovery. [Pg.482]

Clearly it is advantageous to be able to use achiral starting materials and a chiral reagent to induce an asymmetric reaction, thus obviating the need to attach and remove a chiral auxiliary and permitting the recovery and reuse of the chiral reagent. [Pg.20]

Cleavage of the oxathiane moiety can be carried out with iV-chlorosuccinimide/silver nitrate and leads to the a-hydroxy aldehyde 21 along with a diastereomeric mixture of sultines 20. The sultines can be reduced to the hydroxy thiol 22 which can be reconverted to the chiral auxiliary 16 in ail overall recovery of about 70%39. [Pg.113]

In most cases homogeneous chiral catalysts afford higher enantioselectivities than heterogenous catalysts. Nevertheless, the development of heterogeneous chiral catalysts has attracted increasing interest because workup of the reaction, and recovery of often valuable chiral auxiliaries by simple filtration, is more convenient than in the case of homogeneous catalysts. [Pg.174]

As with the acyl oxazolidinone auxiliaries, each of these systems permits hydrolytic removal and recovery of the chiral auxiliary. [Pg.43]

It has been reported that the cleavage of SAMP hydrazones can proceed smoothly with a saturated aqueous oxalic acid, and this allows the efficient recovery of the expensive and acid-sensitive chiral auxiliaries SAMP and RAMP. No racemization of the chiral ketones occurs during the weak acid oxalic acid treatment, so this method is essential for compounds sensitive to oxidative cleavage.393... [Pg.89]

The search for new enantiopure molecules in industry have recently become more and more important especially in the pharmaceutical area. Until now there are only a few examples of asymmetric synthesis on solid supports. However, as the recovery of a polymer-supported chiral auxiliary by simple filtration techniques... [Pg.159]

Chiral auxiliaries are very often applied for induction of asymmetry in 1,3-dipolar cycloadditions. For most of the reactions described in this section, recovery of the chiral auxiliary has been demonstrated, but for some reactions the chiral moiety has the potential to be recovered, although it was not performed. Most frequently, the chiral auxiliary is connected to the dipolarophile, very often as a,p-unsaturated esters or amides. In a few cases, auxiliaries have been attached to the 1,3-dipole. [Pg.851]

The use of chiral vinyl ethers in 1,3-dipolar cycloadditions with nitrones allows for the subsequent removal and recovery of the chiral group. Using the chiral vinyl ether 197 and the cyclic nitrone 77, the cycloaddition proceeded with high diastereoselectivity (Scheme 12.56). The endo/exo-selectivity was not given in this communication by Carmthers et al. (313), but this is of minor importance for the final outcome of this work, since one of the chiral centers was destroyed in the conversion of 198 into the final product 199. The chiral auxiliary can by recovered in this reaction sequence, and 199 was obtained with an optical purity of >95% ee. [Pg.858]

Hydrolysis of the phosphonodiamides 2 (R1 = CH3) under mild conditions gave a-substi-tuted alkylphosphonic acids 3 (R1 = CH3) with excellent yields, with concomitant recovery of the chiral auxiliary. For further examples see Table 1. [Pg.658]

The highly selective alkylation reaction of chiral imines, which in some cases occurs under complete asymmetric induction, as well as the simple introduction and recovery of the chiral auxiliaries, ensures that asymmetric alkylation of carbonyl compounds via their corresponding imines is a valuable tool in organic chemistry. [Pg.973]

Alkylated Carbonyl Compounds and Recovery of the Chiral Auxiliary by Cleavage of Alkylated SAMP/ RAMP-Hydraznnes General Procedure for Ozonolysis3 ... [Pg.1010]

From an industrial chemist s point of view the use of proline, phenylalanine, valine, and other commercially available amino acids, is fine. To date, however, tert.-(S)-leucine is still an exotic compound. It should also be noted that the recycling of the chiral amino acid moiety is of importance for possible technical processes. On the other hand, the recovery of the chiral auxiliary sometimes does not make sense, especially in syntheses which the require the use of stochiometric amounts of expensive reagents, e.g. LDA. [Pg.235]

Various chiral auxiliaries have been linked to insoluble supports and used to prepare enantiomerically enriched carboxylic acid derivatives (Entries 2 and 3, Table 13.8). Although the recovery of support-bound chiral auxiliaries is certainly easier than in solution reactions, there have been reports of variable results regarding the stereoselectivity of C-alkylations on insoluble supports [131], It seems that the optimi-... [Pg.340]

Although desirable, the recovery of a chiral auxiliary is not always crucial. Some techniques employ a, -unsaturated carbonyl compounds containing a disposable stereogenic center, which is removed after the (asymmetric) conjugate addition step has been performed. This concept appears to be useful only if the chiral substrate is easily accessible in high enantiomeric purity. [Pg.211]

Tartaric acid has been used as a chiral auxiliary in a patented route to (S)-naproxen (20) (Scheme 23.3).45" 9 In initial studies, the acetal 21 was used to allow a stereoselective bromination that resulted in a 91 9 ratio of the (RRS)- and (RRR)-bromo derivatives 22 and 23. The bromo acetal diesters could be completely separated. Debromination of 22, followed by acid hydrolysis, led to formation of (S)-naproxen (20) in 80% yield, >99% ee, and recovery of the auxiliary. Conversely, debromination and hydrolysis of 23 gave only 12% yield of (R)-naproxen and 86% ee. In this case, the hydroxy acetal 24 was the major product (68%). However, the auxiliary was recovered in enantiomerically pure form. [Pg.447]

Overall recovery of the auxiliary was 64%. On cost estimates for scale up work it was found that the previously mentioned chiral auxiliary route exceeded the desired cost by a factor of six, primarily as a result of the cost of the chiral oxazolidinone and a yield of 33% over 10 steps. The route was ultimately replaced with a classical resolution protocol using mandelic acid, and this has been superseded by asymmetric approaches (see Chapter 12). [Pg.449]

Amides of (S)-lactic acid have been used as chiral auxiliaries in the dynamic kinetic resolution of racemic ibuprofen (Scheme 23.9).56 The therapeutically effective (S)-isomer 33 was obtained in 80% yield, with complete recovery of the pyrrolidine-derived (S)-lactamide auxiliary 34. [Pg.451]

Conformationally constrained m-aminoindanols and its derivatives have been used as chiral auxiliaries in a number of asymmetric reactions. The availability of both cis enantiomers, the high levels of asymmetric induction attained, and the ease of recovery are all assets to the development of efficient and practical processes with those auxiliaries. [Pg.466]

In 1992 Ghosh and co-workers provided the first example of the utility of rigid cis-1 -amino-2-indanol-derived oxazolidinone 36 as the chiral auxiliary in the asymmetric. vv//-aldol reaction.60-61 Aldol condensation of the boron enolate of 37 with various aldehydes proceeded with complete diastereofacial selectivity. Effective removal and recovery of the chiral auxiliary was carried out under mild hydrolysis conditions (Scheme 24.6). As both enantiomers of the chiral auxiliary were readily available, both enantiomers of the. yyn-aldol could be prepared with equal asymmetric induction. [Pg.466]

Cyclic nitrones generated by [4+ 2]-cycloaddition of nitroalkenes undergo various, synthetically very valuable reactions. Thus, Denmark et al. have developed an elegant access to different enantiopure, 3- and 3,4-substituted pyrrolidine derivatives by reductive ring contraction of the cyclic nitrone resulting from a hetero Diels-Alder reaction [389,390]. Upon reaction of -2-nitrostyrene 4-51 with the chiral enol ether 4-52 in the presence of the bulky Lewis acid MAPh (4-53), three diastereomeric cycloadducts 4-54, 4-55 and 4-56 were formed. Hydrogenolysis of the main product 4-54 yielded the desired pyrrolidine 4-57 in excellent optical purity and allowed nearly quantitative recovery of the chiral auxiliary (Fig. 4-12) [391]. It is noteworthy that the nature of the Lewis acid catalyst, especially its steric demand, decisively influences the stereochemical course of such cycloadditions [392]. [Pg.71]


See other pages where Chiral auxiliaries recovery is mentioned: [Pg.123]    [Pg.411]    [Pg.123]    [Pg.411]    [Pg.260]    [Pg.299]    [Pg.464]    [Pg.278]    [Pg.58]    [Pg.856]    [Pg.105]    [Pg.571]    [Pg.894]    [Pg.969]    [Pg.991]    [Pg.991]    [Pg.994]    [Pg.1007]    [Pg.1024]    [Pg.1028]    [Pg.1028]    [Pg.702]    [Pg.61]    [Pg.204]    [Pg.221]    [Pg.101]    [Pg.114]    [Pg.483]    [Pg.133]   
See also in sourсe #XX -- [ Pg.918 ]




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Chirality auxiliaries

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