Homotopic faces

These principles are nicely illustrated by the contrast between the serine-threonine and serine-allothreonine (allothr) systems. The relative orientation of molecular serine vis-a-vis its various crystal faces suggests that allothr can be adsorbed on the homotopic 100 faces as well as on the enantiotopic 011 faces (Figure 21). 

Figure II. Some synthetic strategies leading to chiral crown compounds with homotopic faces stalling from chiral piecuisois with C2 symmetry.

In Chapter 32 we showed that homotopic and enantiotopic protons are identical by NMR. Similarly, homotopic faces or groups are always chemically identical. Enantiotopic faces are also chemically identical, provided that all the reagents in the reaction in question are achiral or racemic. In Chapter 45. we will consider what happens to enantiotopic faces when enantiomerica ly pure reagents are used. 

Figure 4.19. Complications of pyridinium additions due to ring symmetry, (a) Homotopic faces of C-2 and C-6 (b) Equivalence of 100% selective addition to only the front face with no regio-selectivity and 100% regioselectivity with no face selectivity (c) A bulky group at C-3 simplifies the situation by blocking attack at C-2 (and coincidentally C-4) (d) Comins s conformational model favoring Re-face (back side) attack at C-6 of an acylpyridinium ion [112].

Homotopic faces are present in molecules 1-33. Achirohomotopic faces H/H are found in achiral molecules of subclasses hi (1,2), h2 (3,4), hs (5-13), whereas chirohomotopic faces H /H are present in chiral molecules belonging to subclasses h4 (14,15), hs (16,17), and h6 (18-33). Enantiotopic faces (E, 3) are exemplified by molecules 34-48 all of which are achiral no enantiotopic faces can exist in chiral molecules. Diastereotopic faces are found in molecules 49-70. Achirodiastereotopic faces D/F are observed in achiral molecules of subclasses di (49), d2 (50-52), da (53-61) while chirodiastereotopic faces D, F are present in chiral molecules of subclass d4 (62-70). 

Reactions of homotopic faces hi and h4 are always afacioselective, with... 

Figure 12.8. Examples of Facioaselectivity and Faciononselectivity at Homotopic Faces...

In sum, (a) all homotopic faces hi-he can be avectoselective (achiral and chiral reagents) (b) only h2,hs can be nonvectoselective (achiral reagents) (c) only h2,h3,h5,he can be vectostereoselective (achiral and chiral reagents) (d) only h2 and hs can be vectoastereoselective as well as vectononequiselective (achiral and chiral reagents). It should be noted that a... 

Consideration of all homotopic faces, with both sets of divalent and trivalent reactants listed above, leads to the following generalizations ... 

An anguloselective process on a given homotopic face (hl-h3 of achiral substrates, and faces h4-h6 of chiral substrates) may occur with either an achiral or chiral reagent. 

Figure 14.11. Relationship of Facioselectivity, Vectoselectivity and Anguloselectivity for 1,2-Ligogenic Processes at Homotopic Faces hi-h2...

In Figure C.3, each of molecules xvi and xvii has homotopic faces those in xviii are enantiotopic, whereas faces in xix and xx are diastereotopic. The same Re/Si descriptor is applied to enantiotopic faces of xviii, and diastereotopic faces of xx. Furthermore, different descriptors are used for diastereotopic faces of xix (re/si), and diastereotopic faces of xx (Re/Si). 

If the latter phantom point is above the plane, a clockwise l- 2 3 sequence leads to the "top" (or "bottom" face) if the phantom point is below the plane, an anticlockwise 1 2—>3 sequence points to the "bottom" face (or "top" face). If the faces are enantiotopic, a clockwise l->2->3 sequence leads to top face E, whereas an anticlockwise sequence, to bottom face 3 (Figure D.2b). Qn the other hand, if the faces of an achiral molecule are diastereotopic, a clockwise 1—>2—>3 sequence leads to top face D, anticlockwise sequence, to bottom face F (Figure D.2b). For a chiral molecule with diastereotopic faces, clockwise 1 2->3 sequence leads to top face D, anticlockwise sequence, to bottom face F (Figure D.2b). As pointed out above, only achiral molecules possess E / H enantiotopic and D/F diastereotopic faces and only chiral ones have D /F diastereotopic faces. (Clearly, no 1 2—>3 sequence exists for homotopic faces, in either achiral or achiral molecules.) One is left with the issue of selecting the proper vector Vx for each of cases A1-A12 and C1-C4. In the determination of Vx, one considers skeletal atoms first, followed by the ligands. In sum, the order of priorities is as follows ... 

At this stage, a comparison of different nomenclatures here of monocentric cases is in order. To date, no stereodescriptors have been assigned to homotopic faces, obviously because of their equivalence. However, the assignment of H or H to homotopic faces allows one to (a) ascertain the homotopic/chirohomotopic nature of the half-spaces, (b) compare homotopic faces of different moieties in a molecule - Hj vs. Hj, (c) differentiate between homotopic H faces and chirohomotopic H faces of different molecules, and (d) contrast homotopic H or H faces to E, 3,... 

A comparison of different facial stereodescriptors for bicentric cases is in order (Figure D.6). The Si/Re (or Re/Si) descriptor may indiscriminately refer to homotopic (vii), or diastereotopic (xiii) faces. Similarly, Re-Re (or Si-Si) applies equally well to homotopic (ix), enantiotopic (x, xi), and diastereotopic (xiv) faces. Furthermore, the Re/Si nomenclature falls short for xii with diastereotopic faces, and is inapplicable to homotopic faces (vi,vii,viii). In contrast, as seen in the examples of Figure D.6, in the novel HED system presented here, every face gets a descriptor. In this manner, one is able to (a) specify the stereotopicity of the faces,... 

Facioselectivity, 2-79 classification of, 2-90 conjunctive states in, 2-81, 2-92 enantiotopic faces e, at 2-91 diastereotopic faces dl-d4, at 2-91 face-face conjunctive processes in, 2-97 homotopic faces hl-h6, at 2-91 stereofacioselectivity, 2-89 vectoselectivity, and 2-133 vectoselectivity-anguloselectivity, and 2-166... 

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