Diastereoisomers racemization

The most common flavomannin derivative is the 6,6 -di-0-methyl ether, FDM (348), which occurs in both of its atropisomeric forms in Cortinarius. In Tricholoma auratum (=T. equestre) a mixture of FDM diastereoisomers, racemic at the biaryl linkage, is present. 

Diaryl derivatives with a hydroxyl group at another site in the alkylene chain are represented by BRL 14342 (78). Some structural relationship with (76) and (77) may be seen in this compound, which is currently undergoing tolerance and EEG studies in human volunteers. The results suggest that (78) is a potent nondepressant CNS-active drug [220]. The compound is a 60 40 mixture of the two possible diastereoisomers (racemates ), which are of approximately equal activity [220]. It is stated that the (-)-isomer shows fewer peripheral anticholinergic effects than the (+)-form. The (+)-isomer is more active than the (—)-isomer in the reserpine reversal test at low doses, but the 2 isomers have rou Iy equal effects in tests on prevention of reserpine-induced hypothermia [221]. Because (78) has 2 chiral centres it is not clear if the optically active compounds are single diastereoisomers or mixtures of 2 of them. 

Menthol can also be synthesized from optically active terpenoids such as (+)-citroneUal, (-)- P-pheUandrene, and (+)-3-carene. The synthesis from (+)-3-carene has already been discussed in the section on hydrocarbons. Such methods must avoid any racemization during the course of a usually multiple-step synthesis. One disadvantage of such methods is that the other menthol diastereoisomers must be equilibrated and recycled. 

Vitamin K compounds ate yellow solids or viscous liquids. The natural form of vitamin is a single diastereoisomer with 2 (E), 7 (R), ll (R) stereochemistry. The predominant commercial form of vitamin is the racemate and a 2 (E)j (Z) mixture. Table 1 fists some physical and spectral properties of vitamin K. ... 

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... 

Bohman and Allenmark resolved a series of sulphoxide derivatives of unsaturated malonic acids of the general structure 228. The classical method of resolution via formation of diastereoisomeric salts with cinchonine and quinine has also been used by Kapovits and coworkers " to resolve sulphoxides 229, 230, 231 and 232 which are precursors of chiral sulphuranes. Miko/ajczyk and his coworkers achieved optical resolution of sulphoxide 233 by utilizing the phosphonic acid moiety for salt formation with quinine. The racemic sulphinylacetic acid 234, which has a second centre of chirality on the a-carbon atom, was resolved into pure diastereoisomers by Holmberg. Racemic 2-hydroxy- and 4-hydroxyphenyl alkyl sulphoxides were separated via the diastereoisomeric 2- or 4-(tetra-0-acetyl-D-glucopyranosyloxy)phenyl alkyl sulphoxides 235. The optically active sulphoxides were recovered from the isolated diastereoisomers 235 by deacetylation with base and cleavage of the acetal. Racemic 1,3-dithian-l-oxide 236... 

Moreover, a-, p- and y-HBCD diastereoisomers are chiral. Thus HBCD have three pairs of enantiomers (+)a, (—)a, (+)p, (—)p, (+)y and (—)y. The enantiomers have identical physicochemical properties and abiotic degradation rates, but may have different biological and toxicological properties and therefore different biotransformation rates. These transformations may result in nonracemic mixtures of the enantiomers that were industrially synthesized as racemates [16, 19]. The rates of metabolisation process of the enantiomers of chiral environmental pollutants may be significantly different [20],... 

A series of thiazolo[2,3- ]isoquinolines 426, 3-one derivatives 427, and J-oxide derivatives 428 have been studied in detail as regard to their spectroscopic properties <2001T3499, 2002TA2329, 2003T1173>. These compounds have been prepared using previously reported chemistry. One of the 3-one derivatives 427 was prepared in enantiomeri-cally pure form and therefore gave access to optically enriched 428. Isolated diastereoisomers of this A-oxide were however found to be unstable and to epimerize to give a thermodynamic mixture of syn- and //-diastereoisomers. This epimerization was accompanied by a racemization (Scheme 110). 

Most optically active polysilanes owe their optical activity to induced main-chain chirality, as outlined above. However, backbone silicon atoms with two different side-chain substituents are chiral. Long-chain catenates, however, are effectively internally racemized by the random stereochemistry at silicon, and inherent main-chain chirality is not observed. For oligosilanes, however, inherent main-chain chirality has been demonstrated. A series of 2,3-disubstituted tetrasilanes, H3Si[Si(H)X]2SiH3 (where X = Ph, Cl, or Br), were obtained from octaphenylcyclote-trasilane and contain two chiral main-chain silicon atoms, 6.16 These give rise to four diastereoisomers the optically active S,S and R,R forms, the activity of which is equal but opposite, resulting in a racemic (and consequently optically inactive) mixture and the two meso-forms, S,R and R,S, which are optically inactive by internal compensation. It is reported that the diastereoisomers could be distinguished in NMR and GC/MS experiments. For the case of 2-phenyltetrasilane, a racemic mixture of (R)- and (A)-enantiomers was obtained. 

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