Sulfur, chiral structures

Certain chalcogen structures display the phenomenon of chirality (Chapter 10.2). As with carbon,2 chirality at sulfur can influence physiological events there are many stereoselectivities in the interactions of chiral sulfur compounds with enzymes and receptor molecules. Sulfur chirality in secondary metabolites is most commonly observed with sulfonium salts, sulfoxides and sulfoximines.3... 

Bridges between aromatic rings of calixarenes and resorcinarenes are commonly considered chemically inert. Therefore, substitution of the bridges is the least common among modifications due to synthetic difficulties. However, in some cases, the modifications can be introduced prior to the cyclization step, as in 49 (Fig. 2.13) [55, 56]. Another possibility involves using thiacalixarenes. Oxidation of the adjacent sulfide functional groups in an a/ifi-relationship leads to an inherently chiral structure 50 [57]. It should be noted however, that inherent chirality in this case is inextricably boimd to stereogenic centers at the sulfur atoms. 

The aldehyde structures and the tosylhydrazone salts were varied in an extensive study of scope and limitations, with use of both achiral and chiral sulfur ylides [73]. Aromatic aldehydes were excellent substrates in the reaction with benzaldehyde-derived ylides, whereas aliphatic aldehydes gave moderate yields and transxis ratios. 

Platinum-thiourea complexes have been extensively studied because of their biological activity [54], but few have been used in catalysis. Neutral thioureas are able to coordinate to metal centres through their sulfur atom (Scheme 9) [55,56] monomeric (I) and oligomeric (II) species are known for Rh [57], and an X-ray structure has also been determined for the chiral complex III [58]. In many complexes hydrogen bonding has been observed... 

An unprecedented stereoselective procedure to obtain enantiomerically pure transition cluster M3Q4 complexes consists of the direct excision of the M3Q7X4 n polymers using chiral diphosphanes, namely (+)-l,2-bis[(2J ,5R)-2,5-(dimethylphospholano)]ethane [(R,R)-Me-BPE] and its respective enantiomer [(S,S)-Me-BPE] to afford the trinuclear complexes (P)-[Mo3S4Cl3(J ,J -Me-BPE)3] and (Af)-[Mo3S4Cl3(S,S-Me-BPE)3] , respectively [30]. The structures of both enantiomers are shown in Fig. 7.3. The symbols (P) and (M) refer to the rotation of the chlorine atoms around the C3 axis, with the capping sulfur pointing towards the viewer. 

In addition, Rowlands has involved chiral sulfoxide-containing ligands for the catalytic addition of McsSiCN to aldehydes. " The ligand structure was based on a phenolic oxazoline scaffold with introduction of the sulfur substituent via cysteine derivatives. The best enantioselectivities of up to 61% ee were obtained with the bulkiest tert-butyl substituted ligand (Scheme 10.42). The effect of the sulfoxide configuration was studied, showing that the use of... 

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