Constitutionally heterotopic

Constitutionally Heterotopic and Diastereotopic Groups Differ in all scalar properties and are distinguishable under any conditions, chiral or achiral. Asymmetric molecules cannot contain homotopic or enantiotopic groups, only diastereotopic or constitutionally heterotopic groups. 

Nuclear magnetic resonance chemical shift differences can serve as an indicator of molecular symmetry. If two groups have the same chemical shift, they are isochronous. Isochrony is a property of homotopic groups and of enantiotopic groups under achiral conditions. Diastereotopic or constitutionally heterotopic groups will have different chemical shifts (be anisochronous), except by accidental equivalence and/or lack of sufficient resolution. 

Analyze the stereochemical relationships between the groups of ds-2,5,dimethyl-cyclopentanone 5. Specifically, Identify at least one pair of groups for each of the following relationships homotopic, enantiotopic, diastereotopic, or constitutionally heterotopic. Note if there are no pairs of a given kind, say so. 

Constitutionally heterotopic ligands are in principle always distinguishable, just as constitutional isomers are. Diastereotopic and enantiotopic ligands or faces may be lumped together under the term stereoheterotopic just as diastereomers and enantiomers are both called stereoisomers. 

Mislow el at.301 have pointed out that the distinction between population difference and intrinsic difference is artificial nuclei are either symmetry related (i.e. interchanged by operation of a symmetry element), in which case they are homotopic or enantiotopic and thus isochronous, or they are not so related, in which case they are diastereotopic or constitutionally heterotopic and therefore anisochronous. While this is certainly correct, the present author believes that the dissection between population and intrinsic difference, like many such dissections in science, is at least pedagogically and possibly in some situations even heuristically useful. 

The constitution of the angular syn-bisdienophile 74 (see Scheme 20) maintains the bicyclic nature of the two dienophilic units. However, the [a,c]-fusion present in the phenanthrene constitution reduces the end-to-end symmetry of the rc-systems, rendering the two olefinic carbon atoms in each dienophilic unit constitutionally-heterotopic. The effect of this reduction in the symmetry of the 7t-systems in 74 upon the diastereoselectivities exhibited in its cycloadditions was of considerable interest to us. Furthermore, the incorporation of two phenanthrene units into a macropolycyclic belt could ultimately lead to the preparation of an angular isomer 70 of [12]cyclacene 50 (Fig. 20). Compared with [12]cyclacene 50, the angular isomer 70 has some appealing electronic features associated with it. 

A scheme analogous to the upper part of Fig. 2 has been presented for topic and morphic relationships [18,20], Thus, fragments of the same atomic composition may be homotopic or heterotopic, depending on whether they are superimposable or not. If the latter have the same constitution, they are stereoheterotopic, in the other case they are constitutionally heterotopic. Stereoheterotopic fragments are enantiotopic or diastereotopic. Morphic analysis yields the corresponding classification (see [19]). 

The emphasis to this point has been on the stereochemical relationship of one structure to another. However, the principles developed so far are also relevant to the consideration of s)unmetry relationships within a single molecule. Many of the labels for these relationships are based on the suffix -topic, from the Greek for "place." Terms incorporating this suffix apply both to atoms and to spaces in a molecule, although we usually think of them in terms of atoms. Identical atoms that occupy equivalent environments (both in terms of chemical properties and local or molecular S5munetry) are said to be homotopic (i.e., to have the same place). Identical atoms in nonequivalent environments are said to be heterotopic (for different place). Heterotopic substituents can be either constitutionally heterotopic or stereoheterotopic. Stereoheterotopic substituents can be either enantiotopic or diastereotopic. ... 

Any one of the methyl protons and any one of the methylene protons of butane (91) are constitutionally heterotopic because the replacements produce the constitutional isomers 92 and 93, respectively (Figure 2.37). The two protons on either one of the methylene groups of butane are enantiotopic, however, since the product of replacing one methylene hydrogen by X (94a) is the enantiomer of the product formed by replacing the other hydrogen (on the same methylene) by X (94b, Figure 2.38). Similarly, the two methylene... 

Constitutionally heterotopic. The same groups or atoms with different connectivities. 

For each structure shown, label the pair of methyls as homotopic, enantiotopic, diastereotopic, or constitutionally heterotopic. 

For each structure shown, determine whether the two methyl groups are homotopic, enantiotopic, diastereotopic, or constitutionally heterotopic, both on a time scale where ring inversion is slow and on a time scale where ring inversion is fast. 

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