Stereocenters diastereomers

Molecules Incorporating Several Stereocenters Diastereomers CHAPTER 5... 

If compounds have the same topology (constitution) but different topography (geometry), they are called stereoisomers. The configuration expresses the different positions of atoms around stereocenters, stereoaxes, and stereoplanes in 3D space, e.g., chiral structures (enantiomers, diastereomers, atropisomers, helicenes, etc.), or cisftrans (Z/E) configuration. If it is possible to interconvert stereoisomers by a rotation around a C-C single bond, they are called conformers. 

Diastereomers which differ at a single stereocenter are called epimers. 

Schemes 15 and 16 summarize the syntheses of intermediates that represent rings A and D of vitamin Bi2 by the Eschenmoser group. Treatment of lactam/lactone 51, the precursor to B-ring intermediate 8 (whose synthesis has already been described, see Scheme 8), with potassium cyanide in methanol induces cleavage of the y-lac-tone ring and furnishes intermediate 76 after esterification of the newly formed acetic acid chain with diazomethane. Intermediate 76 is produced as a mixture of diastereomers, epimeric at the newly formed stereocenter, in a yield exceeding 95%. Selective conversion of the lactam carbonyl in 76 into the corresponding thiolactam...

Scheme 17). A priori, both bicyclic isoxazoline epi-mers could be utilized in this synthesis because the newly formed stereocenter is eventually destroyed. Nevertheless, the two isoxazoline diastereomers were separated, and the subsequent stages of the synthesis were defined using the major isomer 30. 

Homoallylamines of type 4 contain two adjacent stereocenters therefore, two diastereomers, s.V)i-5 and anti-6, can be produced. 

The addition of the anion of the racemic 2-methyl-2-propenyl sulfoxides, rac-2-methyl-3-(phenyl-sulfinylpl-propene and /w-3-(rerr-butylsulfinyl)-2-methyl-l-propene to 2-cyclopentenone gives mixtures of (E)- and (Z )-y-l, 4-addition products which are a mixture of diastereomers at sulfur2. The (T )-products usually predominate, with the relative proportions of the (Z)-product increasing as the reaction temperature is increased. No asymmetric induction originating from the stereocenter at sulfur was observed when the sulfoxide substituent was phenyl however, there was a marginal improvement in the case of the (Zi)-product when the sulfoxide substituent was ferf-butyl. 

The addition of the anion of the 1,3-dimethyl-2-butenyl sulfoxides to 2-cyclopentenone was examined2. The anion of rar-2-methyl-4-(phenylsulfinyl)-2-pentene gave a 50 50 mixture of ( )- and (Z)-y-1,4-adducts which differed in the relative configuration of the new stereocenter regarding the stereocenter at sulfur. That is, for either the (Z)- or the ( )-product there is complete asymmetric induction from the stereocenter at sulfur, but in the opposite direction. When the rm-butyl analog, ruc-4-(/wt-butylsulfinyl)-2-methyl-2-pcntcne, was reacted, it gave exclusively the ( )-adduct, likewise as a single diastereomer. 

GC analysis revealed > 290 1 selectivity anti. syn) in the diethylzinc addition. The 4-acetoxy-l,3-dioxane 152 used in the above experiments was a 24 1 mixture of diastereomers, epimeric at the 2-position. This implies that the acetal stereocenter undergoes isomerization to the most stable oxocarbenium ion prior to reaction with Et2Zn. Conclusive evidence for this was obtained when submission of compound 156 to the identical conditions produced 155 as the major product (Eq. 26). 

Reactions of nitro compounds with chiral imines have only recently been described. Either chiral 1-phenylethylamine (auxiliary) or the glyceraldehyde acetonide aldehyde was used as the chiral precursors of the imines 66 and 68, which reacted with 3-mesyloxynitropropane to give the 3-nitropyrrolidines dl)-67 and 69, respectively, with good diastereoselectivity. In fact, both products were obtained (almost) exclusively as trans diastereomers with high level of asymmetric induction, but the configurations of the newly formed stereocenters were not determined [44] (Scheme 13). N-Boc imines can be formed... 

We can clearly see that they are not the same compound. In other words, they are nonsuperimposable. But, they are not mirror images of each other. The top stereocenter has the same configuration in both compounds. If they are not mirror images, then they are not enantiomers. So what is their relationship They are called di-astereomers. Diastereomers are any compounds that are nonsuperimposable stereoisomers that are not mirror images of each other. 

We use the term diastereomer very much like we used the term enantiomer (remember the brother analogy). One compound is called the diastereomer of the other, and you can have a group of diastereomers. When we were talking about enantiomers, we saw that they always come in pairs, never more than two. But diastereomers can form a much larger family. We can have 100 compounds that are all diastereomers of each other (if there are enough stereocenters to allow for that many permutations of the stereocenters). 

If you are given two stereoisomers, you should be able to determine whether they are enantiomers or diastereomers. All you need to look at are the stereocenters. They must all be of different configuration for the compounds to be enantiomers. 

The analogy goes like this when yon have a lot of stereocenters in a com-ponnd, there will be many stereoisomers (brothers and sisters). Bnt, they will be paired up into sets of enantiomers (twins). Any one molecule will have many, many diastereomers (brothers and sisters), bnt it will have only one enantiomer (its mirror image twin). For example, consider the following compound ... 

This componnd has hve stereocenters, so it will have many diastereomers (compounds where only some of the wedges have been inverted). There are many, many possible componnds that fit that description, so this compound will have many brothers and sisters. Bnt this compound will only have one twin—only one enantiomer (there is only one mirror image of the componnd above) ... 

C-Alkylations of l,4-dihydro-27/-pyrazino[2,l-A]quinazoline-3,6-diones at positions C-l and CM were studied in detail. Compounds of type 57 could be alkylated diastereoselectively at C-l, owing to the geometry of the piperazine ring, which is locked in a flat boat conformation with the R4 or R1 substituent in a pseudoaxial position to avoid steric interaction with the nearly coplanar C(6)-carbonyl group. Alkylation of 57 (R2 = Me, Bn, R4 = Me) in the presence of lithium hexamethyldisilazide (LHMDS) with benzyl and allyl halides resulted, under kinetic control, in the 1,4-trans-diastereomer 59 as the major product, with retention of the stereocenter at CM (Scheme 5). 

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