Stereochemistry diastereoisomers

Vitamin K compounds ate yellow solids or viscous liquids. The natural form of vitamin is a single diastereoisomer with 2 (E), 7 (R), ll (R) stereochemistry. The predominant commercial form of vitamin is the racemate and a 2 (E)j (Z) mixture. Table 1 fists some physical and spectral properties of vitamin K. ... 

The validity of the model was demonstrated by reacting 35 under the same reaction conditions as expected, only one diastereoisomer 41 was formed, the structure of which was confirmed by X-ray analysis. When the vinylation was carried out on the isothiazolinone 42 followed by oxidation to 40, the dimeric compound 43 was obtained, showing that the endo-anti transition state is the preferred one. To confirm the result, the vinyl derivative 42 was oxidized and the intermediate 40 trapped in situ with N-phenylmaleimide. The reaction appeared to be completely diastereoselective and a single diastereomer endo-anti 44 was obtained. In addition, calculations modelling the reactivity of the dienes indicated that the stereochemistry of the cycloaddition may be altered by variation of the reaction solvent. 

The cycloadditions of the C-2 vinyl glicals with maleic anhydride are an interesting example of facial stereocontrol. The allylic methoxy group in dienes 55a and 55b exerts an nnh -stereodirecting effect as shown by the stereochemistry of the endo-cycloadducts 56 and 57 obtained as the sole products from 55a and 55b, respectively, and by the fact that 55c produces [51] a mixture of the diastereoisomers 56c and 57c (Scheme 2.22). When linear acetylenic dienophiles were used, the degree of facial diastereoselectivity decreased, which indicates its dependence on steric effects. 

Thus, as expected from earlier work 36,37), the dioxabicyclization proceeds with inversion of configuration at the 3-position of the m-2-frans-3-dibromide 30, the stereochemistry at the 2-position being unaffected. However, experiments with individual diastereoisomers unexpectedly showed that the franj-2-m-3-dibromide 31, also reacts with silver trifluoroacetate, albeit less efficiently, to give the same bicyclic peroxide. We feel that this probably proceeds via an isomerisation (Eq. 25). 

Eliminations have also been carried out on a number of compounds of the form HalCH2CH2Y, where Y = OH, OR, OCOR, NH2, etc. these eliminations normally require conditions more drastic than for 1,2-dihalides, and metals or metal cations are found to be more effective than Ie. These eliminations are often found to be somewhat indiscriminate in their stereochemistry. The elimination of CO Br6 from the diastereoisomer (72) of 2,3-dibromo-3-phenylpropanoate in Me2CO is, however, found to proceed 100% ANTI, and under extremely mild conditions ... 

Most optically active polysilanes owe their optical activity to induced main-chain chirality, as outlined above. However, backbone silicon atoms with two different side-chain substituents are chiral. Long-chain catenates, however, are effectively internally racemized by the random stereochemistry at silicon, and inherent main-chain chirality is not observed. For oligosilanes, however, inherent main-chain chirality has been demonstrated. A series of 2,3-disubstituted tetrasilanes, H3Si[Si(H)X]2SiH3 (where X = Ph, Cl, or Br), were obtained from octaphenylcyclote-trasilane and contain two chiral main-chain silicon atoms, 6.16 These give rise to four diastereoisomers the optically active S,S and R,R forms, the activity of which is equal but opposite, resulting in a racemic (and consequently optically inactive) mixture and the two meso-forms, S,R and R,S, which are optically inactive by internal compensation. It is reported that the diastereoisomers could be distinguished in NMR and GC/MS experiments. For the case of 2-phenyltetrasilane, a racemic mixture of (R)- and (A)-enantiomers was obtained. 

Hydroxyacyl analogues were prepared as a mixture of two diastereomers. In the case of 3-hydroxyhexanoyl (R=C3H7), the two diastereoisomers were successfully separated by HPLC. The absolute stereochemistry at the 3-hydroxy centre in either of the isomer was not established [15]. 

In all cases, mixtures of cis and trans diastereoisomers are reported. In terms of experimental observation, it is now possible to prepare 1,3-disubstituted tetrahydro-j8-carbolines and 1-substituted tetrahydro-/3-carbolines of known absolute stereochemistry. For example, if optically active A -benzyltryptophan methyl ester of known chirality reacts with aldehydes in refluxing benzene, the trans derivative is obtained. Arylglycidate was also used in the synthesis instead of aldehydes (78CPB2305). 

Without exception, (3-diastereoisomers are substantially more potent than the a-forms in spite of their stereochemistry differing from that of morphine... 

The cyclic form of glucose is termed glucopyra-nose, since the new ring system is a reduced form of the oxygen heterocycle pyran. Nucleophilic attack onto the planar carbonyl may occur from either of its two faces, generating two different stereochemistries at this new chiral centre, designated as a or p. This new chiral centre is termed the anomeric centre. Since there are other chiral centres in the molecule, the mixture of a- and -anomeric forms is not a racemate, but a mixture of diastereoisomers (see Section 3.4.4). The mixture does not contain 50% of each anomer (see below). Although both forms are produced, the form with the equatorial hydroxyl is thermodynamically favoured (see Section 3.3.2). 

Neomenthol differs from menthol in having different stereochemistry at a single position. This makes it a configurational isomer, a diastereoisomer (there are two other chiral centres that are unchanged), and an epimer. The other terms are not applicable. 

Bis-oxazoline ligands can also be produced by oxidative coupling of the copper derivative of diastereoisomerically pure 306 (Scheme 145) . Further lithiations of the product 317, which was produced as single diastereoisomer, occur (as in Scheme 143) at the second site adjacent to the oxazoline, giving, for example, 318, despite the (presumably) less favourable stereochemistry of the lithiation step. Bisoxazolines 318 direct the asymmetric copper-catalysed cyclopropanation of styrene using diazoacetate. 

An advantage of these enzymes is that they are stereocomplementary, in that they can synthesize the four possible diastereoisomers of vicinal diols from achiral aldehyde acceptors and DHAP (Scheme 4.2). Although this statement is generally used and accepted, it is not completely true since tagatose-l,6-bisphosphate aldolase (TBPA) from Escherichia coli-the only TBPA that has been investigated in terms of its use in synthesis-does not seems to control the stereochemistry of the aldol reaction when aldehydes different from the natural substrate were used as acceptors [7]. However, this situation could be modified soon since it has been demonstrated that the stereochemical course of TBPA-catalyzed C—C bond formation may be modified by enzyme-directed evolution [8]. 

Two 4-methylene-l,3-dioxane diastereoisomers, isomeric at C-6, were subjected to the rhodium-catalyzed hydro-formylation. The stereochemistry of the newly formed stereogenic carbon was guided solely by the acetal stereocenter (not by C-6) (Scheme 56) <1997JA11118, 1998TL6423>. 

The Diels-Alder reaction is stereo specific. The stereochemistry of the dienophile is retained in the product i.e., cis and trans dienophiles produce different diastereoisomers in the product. For example, freshly distilled cyclopentadiene, having s-cis configuration, reacts with maleic anhydride to give c/ -norbornene-Sjh-endo-dicarboxylic anhydride. 

The reaction of phenyl-substituted alkenes (2-phenylprop-l-ene, ( )-l-phenylprop-l-ene, 1,1-diphenylethene, 1,1-diphenylprop-l-ene) with F-Teda BF4 (6) in the presence of various alcohols results in the formation of vicinal fluoro alkoxy adducts with Markovnikov-type regioselec-tivity.89,94 The stereochemistry of the fluorination-methoxylation addition reaction is slightly syn predominant in the case of (Z)-stilbene, indene, and dibenzosuberenone, while equal amounts of both diastereoisomers are formed in the case of ( )-l-phenylprop-l-ene and acenaphthylene. 

The stereochemistry of the other pyrrolizidine alcohols and related compounds was determined on the basis of comparison of their configurations with those of the above basic compounds.2,45,81 For example, platynecine can be converted into (— )-isoretronecanol (134) under conditions which cannot affect the configuration at C-l, and 134 can then be transformed into (— )-heliotridane (135). On the basis of these data, the compounds can be named, respectively, as 1 -endo-hydroxymethylpyrrolizidine and 1 -ewdo-methylpyrrolizidine, and their diastereoisomers (- )-trachelanthamidine (136) and (— )-pseudoheliotridane (137) as I -exo-hydroxymethylpyrrolizidine and 1 -eso-methylpyrrolizidine. 

The synthesis of sugars from noncarbohydrate substrates (total synthesis or de novo synthesis) has attracted the attention of organic chemists for more than 130 years [1], However, only the last 20 years have witnessed a rapid development of methods leading to structures of desired constitution and stereochemistry. Initially, sugars have been obtained as pure diastereoisomers in racemic form and, more recently, as pure enantiomers of desired configuration. This development is certainly correlated with the discovery of highly chemo- and stereoselective methods of modem organic synthesis. The subject of total... 

Effect of the Stereochemistry. With rhodium, the geometry of molecules such as (E)- and (Z)-2-pheny 1-2-butenes influences both the distribution of diastereoisomers formed by addition of CHO group on the ft carbon and the distribution of aldehydic isomers. Depending on the starting (E) or (Z) stereoisomer, one of the two diastereoisomers predominates this implies some degree of stereospecificity in the hydroformylation reaction. On the other hand, when Co2(CO)8 is used, no stereospecificity in the hydroformyaltion reaction is observed. It is thus possible to get information about the stereochemistry of the hydro-... 

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