Diastereotopic, and the

In 39 the two aryloxy groups are diastereotopic and the configuration of the auxiliary moiety was known. Reaction with sodium isopropoxide17 yielded 40 (see also p434). 

The addition criterion may similarly be applied to recognize diastereotopic faces. Methyl a-phenethyl ketone, 58 in Fig. 19 has a chiral center addition clearly gives rise to diastereomers (59a, 59b) the faces of the carbonyl carbon are diastereotopic and the C = 0 group is prochiral. This case is of importance in conjunction with Cram s rule 10). Compounds 60, 62 and 64 also display diastereotopic faces even though the products 61, 63 and 65 are not chiral 60, 62 and 64 have prostereogenic rather than prochiral faces. The C=0 group in 60 is propseudoasymmetric, since C(3) in 61 is a pseudoasymmetric center. a-Phenethyl methyl sulfide (66) displays diastereotopic sides of a molecular plane not due to a double bond 5,24> and may alternatively be considered a case of diastereotopic phantom ligands (unshared pairs on sulfur). This case does involve chirality and the sulfur atom is prochiral. 

When the OH group is on a chiral centre the faces of the alkene become diastereotopic and the face syn to the OH group is preferred. These two diastereoisomers anti-106 and. svn-108 show clearly that this is chelation control and not mere steric hindrance as the cyclopropane ring is formed on the same side as the OH group regardless of the position of the benzene ring.20... 

The top and bottom faces of the five-membered ring in 82 are diastereotopic and the intramolecular reaction selects the syn face. But the two ketone groups in 82 are enantiotopic. If the enol attacks the right hand ketone one enantiomer of 83 is formed while if it attacks the left hand ketone, the other enantiomer is formed. 

The two faces of the double bond of the dienophile are now different because of the chiral centre they are diastereotopic, and the diene can distinguish between them. If we now do the Diels-Alder reaction in the presence of a Lewis acid, Et2AlCl, the aluminium chelates the oxygen atoms of the dienophile to form the rigid and reactive structure shown below. The isopropyl group is held in such a way that its steric bulk prevents the diene attacking that face of the prochiral alkene. The diene has no choice but to attack from above, and only one of the possible diastereoisomeric products is formed. 

In case the groups R or Z in VIII are chiral, the double bond plane gets diastereotopic and the approaches of Y from one and the other side of the bond become inequivalent (asymmetrical induction)—the choice of the approach will depend on the conformation of R. For the reactions of nucleophilic addition to alkenes and carbonyl compounds the sterical models represented by the Newman projection formulas XVII are usually invoked for the rationalization of stereoselectivities ... 

In a chiral aldehyde or ketone the tv o carbonyl faces are diastereotopic and the products resulting from either Re or Si face attack of the enolate are... 

If the amount of the sample is sufficient, then the carbon skeleton is best traced out from the two-dimensional INADEQUATE experiment. If the absolute configuration of particular C atoms is needed, the empirical applications of diastereotopism and chiral shift reagents are useful (Section 2.4). Anisotropic and ring current effects supply information about conformation and aromaticity (Section 2.5), and pH effects can indicate the site of protonation (problem 24). Temperature-dependent NMR spectra and C spin-lattice relaxation times (Section 2.6) provide insight into molecular dynamics (problems 13 and 14). 

The concept of heterotopic atoms, groups, and faces can be extended from enantiotopic to diastereotopic types. If each of two nominally equivalent ligands in a molecule is replaced by a test group and the molecules that are generated are diaster-eomeric, then the ligands are diastereotopic. Similarly, if reaction at one face of a trigonal atom generates a molecule diastereomeric with that produced at the alternate face, the faces are diastereotopic. 

When the aromatic group of the sulfoxide is replaced by a heteroaromatic group (e.g., N-methylimidazole), the internal coordination between Li—N to form a five-membered metallocycle apparently predominates over Li—O coordination to form a four-membered metallocycle . Reaction of imidazole (S)-sulfoxide 16 with benzaldehyde produces aldol 17 as the major product in which the a-H and the sulfoxide lone pair are syn (equation 14) imidazole (R)-sulfoxide 18 reacts similarly (equation 15). The stereochemical outcome of these reactions is rationalized in terms of a-lithiosulfoxides in which the reactive diastereomer (i.e., 20 and 21) is that having one diastereotopic face of the five-membered Li—N metallocycle carrying both H and sulfoxide lone pair. 

In the discussion of the stereochemistry of aldol and Mukaiyama reactions, the most important factors in determining the syn or anti diastereoselectivity were identified as the nature of the TS (cyclic, open, or chelated) and the configuration (E or Z) of the enolate. If either the aldehyde or enolate is chiral, an additional factor enters the picture. The aldehyde or enolate then has two nonidentical faces and the stereochemical outcome will depend on facial selectivity. In principle, this applies to any stereocenter in the molecule, but the strongest and most studied effects are those of a- and (3-substituents. If the aldehyde is chiral, particularly when the stereogenic center is adjacent to the carbonyl group, the competition between the two diastereotopic faces of the carbonyl group determines the stereochemical outcome of the reaction. 

The CHF carbon is chiral in 1,2,2-trilluorocthyl compounds. Thus, the fluorines of the CF2H group are diastereotopic and appear as an AB system, and each of the fluorines can potentially couple with... 

The two faces of the borabenzene ring of this borabenzene-oxazoline adduct are inequivalent (diastereotopic), and complexation to Cr(CO)3 occurs on the less hindered face with high diastereoselectivity (Scheme 3). This work provided the first description of an enantiopure borabenzene and of an enantiopure planar-chiral Lewis acid complex. 

An unusual temperature dependence on the regioselective behavior of an allylbarium species has been demonstrated in asymmetric allylation with the optically active imine SAMP-hydrazone [SAMP = (A)-(—)-l-amino-2-meth-oxymethylpyrrolidine] (Scheme 10).322 Its reaction with prenylbarium chloride at 0°C produced an a-allylated hydrazine in 60% diastereotopic excess, but at — 78 °C, the y-adduct was generated with 98% diastereotopic excess. The temperature dependence of the ct/y ratio may reflect competition between a kinetically favored y-adduct at low temperature and a thermodynamically preferred ct-form at higher temperatures. 

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