Diastereomeric ratio reaction

Two- and three-component Hantzsch reactions using C-glycosylated reagents have been reported as an alternate method for conducting asymmetric syntheses of 1,4-dihydropyridines." ° Reaction of 109, 110 and 97 generate 111 with Ri = sugar. Alternatively, 112 and 113 produce 111 with Ri = sugar. While the yields were acceptable (60-90%), the diastereomeric ratio varied from 30-60%. 

Some workers avoid delay. Pai)adium-on-carbon was used effectively for the reductive amination of ethyl 2-oxo-4-phenyl butanoate with L-alanyl-L-proline in a synthesis of the antihyperlensive, enalapril maleate. SchifTs base formation and reduction were carried out in a single step as Schiff bases of a-amino acids and esters are known to be susceptible to racemization. To a solution of 4,54 g ethyl 2-oxO 4-phenylbutanoate and 1.86 g L-alanyl-L-proline was added 16 g 4A molecular sieve and 1.0 g 10% Pd-on-C The mixture was hydrogenated for 15 hr at room temperature and 40 psig H2. Excess a-keto ester was required as reduction to the a-hydroxy ester was a serious side reaction. The yield was 77% with a diastereomeric ratio of 62 38 (SSS RSS)((55). 

In addition to the boron trifluoride-diethyl ether complex, chlorotrimcthylsilanc also shows a rate accelerating effect on cuprate addition reactions this effect emerges only if tetrahydrofuran is used as the reaction solvent. No significant difference in rate and diastereoselectivity is observed in diethyl ether as reaction solvent when addition of the cuprate, prepared from butyllithium and copper(I) bromide-dimethylsulfide complex, is performed in the presence or absence of chlorotrimethylsilane17. If, however, the reaction is performed in tetrahydrofuran, the reaction rate is accelerated in the presence of chlorotrimethylsilane and the diastereofacial selectivity increases to a ratio of 88 12 17. In contrast to the reaction in diethyl ether, the O-silylated product is predominantly formed in tetrahydrofuran. The alcohol product is only formed to a low extent and showed a diastereomeric ratio of 55 45, which is similar to the result obtained in the absence of chlorotrimethylsilane. This discrepancy indicates that the selective pathway leading to the O-silylated product is totally different and several times faster than the unselective pathway" which leads to the unsilylated alcohol adduct. A slight further increase in the Cram selectivity was achieved when 18-crown-6 was used in order to increase the steric bulk of the reagent. 

Very high levels of induced diastereoselectivity are also achieved in the reaction of aldehydes with the titanium enolate of (5)-l-rerr-butyldimethylsiloxy-1-cyclohexyl-2-butanone47. This chiral ketone reagent is deprotonated with lithium diisopropylamide, transmetalated by the addition of triisopropyloxytitunium chloride, and finally added to an aldehyde. High diastereoselectivities are obtained when excess of the titanium reagent (> 2 mol equiv) is used which prevents interference by the lithium salt formed in the transmetalation procedure. Under carefully optimized conditions, diastereomeric ratios of the adducts range from 70 1 to >100 1. 

In the Mukaiyama variation of the aldol reaction, 3-benzoyloxy-2-trimethylsiloxy-l-butene adds to 2-methylpropanal in a stereoselective manner. Best results are obtained in the presence of titanium(IV) chloride, giving the adducts 9/10 in a diastereomeric ratio of 92 8. Hydrolysis of the benzoyl group and subsequent oxidative cleavage of the 1,2-diol moiety liberates / -hy-droxycarboxylic acids593. 

The lithium cnolate generated by deprotonation of 2-/m-butyl-6-methyl-l,3-dioxan-4-onc, readily available from polyhydroxybutyric acid (PHB), predominantly affords the diastereo-mers 7 when reacted with aldehydes. The diastereomeric ratios of aldol adducts 7/8, produced by reactions with aliphatic aldehydes, range from 87.5 12.5 to >99 1. Pure diastereoiners7are obtained by recrystallization in 25-74% yield116-118. Only marginal diastereoselectivities with respect to the carbinol center are obtained with aromatic aldehydes111-119. Benzoylation of the dioxanones 7, followed by reduction with lithium aluminum hydride, affords enan-tiomerically and diastereomerically pure triols 9 in >85% yield 11. ... 

Either diastereomer 2 or 3 may be preferentially produced with high selectivity depending on the nature of the enolate counterion present2642-44. Mechanistic details of the diastercofacial differentiation process are not clear in many cases the diastereomeric ratio of the products exhibits a complex dependence on stoichiometry, enolate counterion and reaction conditions26. The dependence of the d.r. on the nature of the enolate counterion is roughly outlined (vide supra) while examples of conditions employed for the reaction of the enolates 1 with prochiral aldehydes arc listed (Table 4). 

The lithium enolate 2a (M = Li ) prepared from the iron propanoyl complex 1 reacts with symmetrical ketones to produce the diastercomers 3 and 4 with moderate selectivity for diastereomer 3. The yields of the aldol adducts are poor deprotonation of the substrate ketone is reported to be the dominant reaction pathway45. However, transmetalation of the lithium enolate 2a by treatment with one equivalent of copper cyanide at —40 C generates the copper enolate 2b (M = Cu ) which reacts with symmetrical ketones at — 78 °C to selectively produce diastereomer 3 in good yield. Diastereomeric ratios in excess of 92 8 are reported with efficient stereoselection requiring the addition of exactly one equivalent of copper cyanide at the transmetalation step45. Small amounts of triphcnylphosphane, a common trace impurity remaining from the preparation of these iron-acyl complexes, appear to suppress formation of the copper enolate. Thus, the starting iron complex must be carefully purified. 

The addition of allylboronates 1 to the chiral oxime 2 results in the formation of a hydroxyl-amine. This is a general method for the subsequent reductive generation of primary homoallyl-amines, but with poor diastereoselectivity in the case of 3 and 4. A diastereomeric ratio of 90 10 is achieved in the addition reaction, using the chiral allylboronate 59 (double stcrcodifferenti-ation). 

Stereoselective Strecker reactions with galactosylamine 1 can also be achieved with sodium cyanide and acetic acid in 2-propanol. The reactions, however, proceed slowly and with a lower stereoselectivity, giving diastereomeric ratios of the products between 3 1 and 7 1. The scope of the method can be extended to other glycosylamines, e.g., 2,3,4-tri-O-pivaloyl-a-D-arabinosyl-amine which allows the stereoselective synthesis of (A )-amino nitriles61,62. 

The Lewis acid mediated reaction of a-alkoxycarbamates 1 with -/-oxygenated alkylLin compound 2 proceeds in certain cases with very high diastereoselectivity83. The yield and the diastereomeric ratio appear to depend highly on the Lewis acid used and the substituents in the carbamate. While with R1 = C6H5 and R2 = CH, sole formation of the on//-isomer was observed, for other substituents the, y -isomer is obtained either exclusively or predominantly. The reason for this variable diastereoselectivity is not clear at present. 

The diastereomeric ratio of the trimethylsilyl triflate catalyzed amidoalkylation of a number of silyl enol ethers at — 40 CC appears to be dependent on the substituents in the substrate87. At — 40 °C the diastereomeric ratio is shown to be kinetically controlled. On allowing the reaction mixture to warm to 20 "C slow epimerization, increasing the amount of the minor isomer, is observed. In the case of the naphthalene derivative, sodium methoxide catalyzed epimerization of the kinetic mixture [(antijsyn) 88 12] produces the thermodynamic mixture [(antijsyn) 9 91]. 

Optically active bicyclo[2.2,2]octanes can be obtained via diastercoselective MIMIRC reaction of lithium dienolates and a,/ -unsaturated esters of various chiral alcohols. Good yields (70-90%), high endo selectivities (> 95%) and diastereomeric ratios that depend on the auxiliary alcohol are found in these additions. The highest diastereomeric ratio reached was 18 82 using a camphor derived sulfonamide. The diastereomeric ratio could be improved (up to 9 91) by titanium(IV) chloride catalyzed addition of the corresponding silylenolates with the chiral a,/J-unsaturated esters358. 

Accordingly, there have been numerous studies on the stereochemistry of these a-sulfinyl carbanions56-77. Representative data on the reactions of a-lithiosulfoxides derived from benzyl sulfoxides with some electrophiles are listed in Table 13. Although the stereochemistry depends on the substituent on the sulfinyl function, the diastereomeric ratio remains the same regardless of the electrophile used for each sulfoxide. 

TABLE 14. Diastereomeric ratios in the reaction of 44 with various electrophiles (after Reference 71)... 

Demailly and coworkers195 found that the asymmetric induction increased markedly when optically active methyl pyridyl sulfoxide was treated with an aldehyde. They also synthesized (S)-chroman-2-carboxylaldehyde 152, which is the cyclic ring part of a-tocopherol, by aldol-type condensation of the optically active lithium salt of a,/3-unsaturated sulfoxide. Although the diastereomeric ratio of allylic alcohol 151 formed from lithium salt 149 and 150 was not determined, the reaction of 149 with salicylaldehyde gave the diastereomeric alcohol in a ratio of 28 72196. 

The reaction of chiral sulfones 161, derived from (lS)-( + )-10-camphorsulfonyl chloride, with cyclopentadiene gives predominantly the endo adduct in a diastereomeric ratio of 91 9 from which one diastereomer 162 can be isolated in pure form by recrystallization (equation 115)109. 

The inherent plane of chirality in the metal carbene-modified cyclophane 45 was also tested in the benzannulation reaction as a source for stereoselectivity [48]. The racemic pentacarbonyl(4-[2.2]metacyclophanyl(methoxy)carbene)-chromium 45 reacts with 3,3-dimethyl-1-butyne to give a single diastereomer of naphthalenophane complex 46 in 50% yield the sterically less demanding 3-hexyne affords a 2 1 mixture of two diastereomers (Scheme 30). These moderate diastereomeric ratios indicate that [2.2]metacyclophanes do not serve as efficient chiral tools in the benzannulation reaction. 

A stereoselective radical reaction of the ester of M-hydroxy-2-pyridinethione (see also Sect. 4.1) to chiral vinylphosphine oxides has also been described and moderate to good diastereomeric ratios have been obtained for the compound 112 [70] (Scheme 33). 

NHC-catalysed homoenolate generation has been applied by Bode and Struble in the formal synthesis of the natural product salinosporamide A [77], The key step in the synthesis is a late-stage NHC-catalysed intramolecular lactonisation step of intermediate 186. When this reaction was attempted with an achiral triazolium-derived NHC, a 4 1 diastereomeric ratio of products was obtained in preference for the undesired product 189. In order to circumvent this, chiral triazolium salt 187 was employed, giving an approximately 1 1 mixture of desired undesired diastereoisomers (Scheme 12.41). 

When dealing with reactions leading to stereoisomeric products we have the additional complication that descriptors such as enantiomeric (diastereomeric) excess and enantiomeric (diastereomeric) ratio are used to describe product purities. The evaluation of RME for a specific stereoisomer, say the R enantiomer, is exactly as above using the connecting relationships for the fraction of each product shown below. 

One interesting observation deduced from the data of Table 13 is that, despite the same diastereomeric ratio, the absolute configuration around the carbon atom of the major diastereomer changed with the electrophile used. In other words, the reactions of a-lithiobenzyl sulfoxides with 020 - and its carbonylation and carbonation proceeded with retention of configuration, while methylation with CHjl resulted in inversion . The stereochemical study of these reactions was conducted with optically active benzyl sulfoxides 42 and 39 as shown in Scheme i - - nd an interesting... 

Treatment of a mixture of alcohol 10 and chiral imidate 67 with catalytic TfOH only afforded a 1.2 to 1.3 1 mixture of 18 19 in a combined HPLC assay yield of 91%. Clearly, under these conditions, the reaction was proceeding under an SN1 reaction pathway. The use of other acid catalysts (TMSOTf, HC1, H2S04, TFA, MsOH) in a variety of solvent systems and under a number of reaction conditions did not improve the diastereomeric ratio of 18 19 (typically 1.2 1), or simply resulted in no reaction. 

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