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Enantiomeric heterodimers

Tlie second example of such an asymmetric synthesis involves a (2+2)-photocyclo-addition reaction in a two-component single crystal. A mixed crystal of the diphenyl-butadiene shown in Scheme 5 with its thiophene analog was irradiated with light absorbed only by the latter. One of the enantiomeric heterodimers was produced in excess [8]. [Pg.188]

Kinetic resolution can also be accomplished via eliminative pathways. Thus, the enantiomerically enriched allylic alcohol 102 can be prepared from the meso epoxide 96 with up to 96% ee by the action of LDA in the presence of the chiral diamine 101 and 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU). The DBU is believed to function as an aggregation modifier, and the active catalyst is theorized to be a heterodimer of the lithium amide (deprotonated 101) and DBU, although some nonlinear effects have been observed at low DBU concentrations <00JA6610>. Dipyrrolidino derivatives (e.g., 104) have also demonstrated utility with regard to kinetic resolution <00H1029>. [Pg.63]

Bottom Pictorial representation of (a) the achiral, S8 symmetric homodimer 16-16 and of (b) the two enantiomeric forms of a calixarene capsule heterodimer. The head-to-tail directionality of the urea groups (arrow) defines the chirality. [Pg.204]

Inclusion of a chiral guest (R) in a chiral heterodimer of type I should lead to two diastereomeric host-guest complexes AP-(R)-BM and AM-(R)-BP indicated by the same doubling of the NMR signals (for both tetraurea parts A and B) as before. However, the diastereomeric host-guest pairs (R)-AP/(R)-AM and (R)-BP/(R)-BM exist now in different (enantiomeric) capsules and must not appear in the ratio 1 1. (In an extreme case, one of the two diastereomeric complexes AP-(R)-BM and AM-(R)-BP could be (strongly) favored, or in other words, the guest could induce the chirality of the reversibly formed capsule.)... [Pg.212]

Heterodimers and heterotrimers are formed in water if planar cations and anions are mixed and if polymerization is impeded by ethyl groups or larger alkyl substituents (Fig. 1.5.4). Other possibilities are the isolation of hydrophobic C-G or A-T pairs in small micelles (Fig. 2.5.4), formation of racemates (Figs. 4.2.7, 8.6.1, and 9.5.3), and formation of pseudo-racemates (Fig. 4.5.10) between enantiomeric amphiphiles with chiral head groups and the same or similar hydrophobic skeletons. [Pg.44]

An Interesting example of the role of the exclmer as Intermediate comes from the system l-(2,6-dlchlorophenyl)-4-phenylbuta-dlene. Irradiation of a mixed crystal of this compound together with the corresponding 4-thlenyl derivative gives dimers by a (2n + 2tr) photocycloaddltlon reaction. The products are homo-dlmers of the phenyl and of the thienyl derivatives, and hetero-dimers. The two heterodimers formed are enantiomeric and, because the crystal Is of chiral structure, they are formed In different yields (54). Thus Irradiation of a single crystal has been found (55) to lead to a 70% enantiomeric excess. [Pg.517]

Later, Wheeler and coworkers demonstrated that chiral sulfonamidecinnamic acids self-assemble into hydrogen-bonded dimers wherein the olefins lie parallel, with the C=C bonds separated by 3.68 The approach was expanded to cross-photoreactions that generate heterodimers. On UV irradiation, the quasiracemate cocrystals underwent asymmetric photodimerizations to afford an enantiomerically pure a-truxillic acid photoproduct (Figure 21). [Pg.2467]

At sep>arations based on the distribution between solid and another (liquid or melt) phase formation of a crystalline material is needed. The chiral discrittiiiiating effect can be explained by the presence and reactions of homo- and heterochiral aggregates (the simplest members of them are the dimers), and by their different physical and chemical properties. Thus in the solution or in the melt of racemic mixtures of enantiomers (S and R) two homodimers (SS and RR) and one heterodimer (SR) should be present, while in a hypothetic enantiomeric mixture (S>R, ee = 50%) ptresence of two dimmers, namely SS and SR, is expected. Considering that homo- and heterodimers are in diastereoisomeiic relation with each other, their solubility and reactivity are different. ... [Pg.5]


See other pages where Enantiomeric heterodimers is mentioned: [Pg.268]    [Pg.503]    [Pg.1498]    [Pg.268]    [Pg.503]    [Pg.1498]    [Pg.18]    [Pg.16]    [Pg.5236]    [Pg.504]    [Pg.13]    [Pg.5235]    [Pg.151]    [Pg.271]   
See also in sourсe #XX -- [ Pg.503 ]




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