Racemic host

Enantiomer distribution constant (EDC). Estimates in CDClj-rich layer in the partitioning of an equivalent of racemic host (H) and optically pure L-valine as guest (G) and their complexes between two liquid phases composed of RCO,D(H). CDC1 and 020"... 

A method for determining optical purities of less than one percent for secondary alcohols has been published, and Mislow has observed a partial resolution which may serve to distinguish between a meso and a racemic host product when a derived inclusion compound is recrystallized from an enantiomerically enriched guest solvent. ... 

In the case of known racemic host systems, enantiomeric resolution or chiral synthesis can provide samples of the pure (+)- and/or (—/-compound. There is no guarantee that the chirally pure material will behave in a similar manner to the racemic mixture, or even that it will still show inclusion behaviour. Pre-resolution has, however, recently been demonstrated to play an important role in inclusion chemistry. 

All yields of the chiral hosts obtained by the resolution are calculated based on the amount of enantiomers containing in the racemic host compounds used for the resolution. 

Section 2.3.4 deals with a seeding process in the solid state. Racemic 2,2 -dihydroxy-1,T-binaphthyl forms two different kinds of inclusion complex crystals with ether in the host guest ratio, namely, 3 1 and 1 2. Selective inclusion complexation by contact of the racemic host crystal with ether vapor in the solid state can be controlled by adding a seed crystal. 

Whereas the separation of racemates in the case of urea and TOT was achieved only by a chiral crystal lattice of the achiral or racemic host, respectively, the optically active cyclodextrins, available from the chiral pool, are able to differentiate a chiral guest within their intramolecular cavity. Therefore, they do not necessarily need the crystal lattice to form inclusion compounds. The guest is encapsulated, while is is in solution, too, if the guest by size and shape fits into the cavity of the specific cyclodextrin molecule (a- (26), P- (27), or y-cyclodextrins). 

Scheme 7.7 Enantioenrichment of racemic host components of a dynamic combinatorial library through the addition of an enantiopure guest.

A racemic host in a complex with a chiral guest prefers a particular sense of dynamic helicity through supramolecular transmission of chirality (Section 14.3.2 Scheme 14.1). Similar helicity biasing can also be attained by the attachment of point chirality to the nitrogen through intramolecular transmission of chirality (Scheme 14.3 and below). 

Specimen results for SCE13(R), a commercial racemic host, are given in Fig. 11 [28]. 

Optically active thiiranes have been obtained by resolution of racemic mixtures by chiral tri-o-thymotide. The dextrorotatory thymotide prefers the (5,5)-enantiomer of 2,3-dimethylthiirane which forms a 2 1 host guest complex. A 30% enantiomeric excess of (5,5)-(—)-2,3-dimethylthiirane is obtained (80JA1157). 

The analytical capability of these matrices has been demonstrated for chiral amines [12, 13]. The procedure is illustrated in Fig. 8-4 for the separation of NapEtNH " CIO . Concentrated methanol/dichloromethane solutions of the racemic mixture were placed on a column containing the chiral macrocycle host. The enantiomers of the ammonium salts were resolved chromatographically with mixtures of methanol and dichloromethane as the mobile phase. The amounts of R and S salts in each fraction were determined by polarimetry. Because the chiral supported macrocycle interacts more strongly with S salts, the R salt passes through the column first and the S salt last, as seen in Fig. 8-4. 

A different non-classical approach to the resolution of sulphoxides was reported by Mikolajczyk and Drabowicz269-281. It is based on the fact that sulphinyl compounds very easily form inclusion complexes with /1-cyclodextrin. Since /1-cyclodextrin as the host molecule is chiral, its inclusion complexes with racemic guest substances used in an excess are mixtures of diastereoisomers that should be formed in unequal amounts. In this way a series of alkyl phenyl, alkyl p-tolyl and alkyl benzyl sulphoxides has been resolved. However, the optical purities of the partially resolved sulphoxides do not exceed 22% after... 

Chiral Recognition. The use of chiral hosts to form diastereomeric inclusion compounds was mentioned above. But in some cases it is possible for a host to form an inclusion compound with one enantiomer of a racemic guest, but not the other. This is caUed chiral recognition. One enantiomer fits into the chiral host cavity, the other does not. More often, both diastereomers are formed, but one forms more rapidly than the other, so that if the guest is removed it is already partially resolved (this is a form of kinetic resolution, see category 6). An example is use of the chiral crown ether (53) partially to resolve the racemic amine salt (54). " When an aqueous solution of 54 was... 

A regio- and stereoselective Beckmann rearrangement utilized diastereose-lective host guest interactions of the inclusion complexes 225 and 228 in a solid state reaction. Initially, a 1 1 mixture of the chiral host 223 and the racemic oximes 224 and 227, respectively, was treated with ultra sound in the solid state to induce the optical resolution. Then H2SO4 was added to start the Beckmann rearrangement, the corresponding c-caprolactams 226 and 229 were isolated in 68 % and 64 % yields and ee of about 80 % and 69 % (determined by HPLC analysis on chiracel OC) (Scheme 43) [46]. 

Some solid-solid reactions were shown to proceed efficiently in a water suspension medium in Sect. 2.1. When this reaction, which gives a racemic product, is combined with an enantioselective inclusion complexation with a chiral host in a water suspension medium, a unique one-pot preparative method of optically active product in a water medium can be constructed. Some such successful examples are described. 

Optically active Diels-Alder adducts were also prepared by using a one-pot preparative method and enantioselective formation of inclusion complex with optically active hosts in a water suspension medium.68 For example, A-ethylmaleimide reacts with 2-methyl-1,3-butadiene in water to give the racemic adduct 1. Racemic 1 and the optically active host 2 form enantioselectively a 1 1 inclusion complex of 2 with (+)-l in a water suspension. The inclusion complex can be filtered and heated to release (+)-l with 94% ee (Eq. 12.23). 

The structure of the parent compound 43 of the 1,1 -binaphthyl host family has been determined many times in independent laboratories and in different (racemic and resolved) crystal forms 60,61,72,73). A common feature of both types of structure is that molecules of 43 form helices. Lateral contacts between such helices play an important role in the respective crystals. These spatial arrangements also emphasize the importance of the question concerning the presence of alpha or beta substituents positioning as mentioned earlier 28 ... 

Inclusion complexation of racemic 63 with the meso isomer of 2 was also examined. The host compound included racemic 63 as did 2a, and photoreaction of the complex in the solid state gave racemic 65 again 23). 

Euproctis pseudoconspersa Host sex pheromone [attraction] Racemic 10,14-dimethylpentadec-1-yl isobutyrate 25 [75]... 

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