Quaternary sulfur center

Asymmetric [4 + 2] cycloadditions of an optically active diene, or dienophile with chiral sulfinyl groups, with a quaternary sulfur center have succeeded in the LiC104-CH2CI2 medium. When chiral dienophile 66 and CP were subjected to cyclization, neither the endotexo ratio nor the enantiofacial selectivity of endo adducts was high [83], In the reaction of chiral diene 67 with MA, the best catalyst was LiC104-CH2CI2 this gave only endo isomers endo-6Sa and endo-6Sb in 70 % yield in the ratio of 96 4, presumably via TS-2 (Sch. 35) [84]. 

A subsequent study by Jprgensen et al. also demonstrated the enantioselective a-sulfenylation of (S-dicarbonyl compounds 420 using l-alkylsulfanyl[l,2,4]triazole derivatives 419 in the presence of a catalytic amount of cinchona alkaloid derivative 421. The use of cyclic (S-dicarbonyl compounds ensured the introduction of a quaternary sulfur center however, the observed enantioselectivity was modest in 51-89%. In 2009, Zhu and co-workers reported that a chiral a,a-diaryl prolinol 424 efficiently catalyzed the enantioselective sulfenylation of (S-ketoesters 420 using N-(phenylthio)phthalimide 423 as a sulfur electrophile. The absence of racemizable C—H bonds led to the optically enriched a-sulfenylated products 425 in excellent enantio-selectivities. In 2010, Fu developed a method for catalytic asymmetric 7-sulfenylation of carbonyl compounds using 2,3-allenoates 426 in the presence of a chiral bisphos-phine, TangPhos 427, and a bulky carboxylic acid 428. ... 

In contrast to isocyanates, isothiocyanates have rarely been examined as cydoad-dition components, because their strong coordination of organosulfur compounds frequently deactivates catalytic species. Some organomthenium complexes, however, recently proved to be efficient catalysts for the formation of carbon-sulfur bonds [52]. The catalytic cydoaddition of diynes 20 with isothiocyanates was also successfully achieved using 17 as a precatalyst [53]. Importantly, the cydoaddition took place across the C=S double bonds of the isothiocyanates to afford thiopyran-imines 59 (Scheme 4.21). This reaction requires 10 mol% of 17, as well as the diynes possessing a quaternary carbon center at the 4-position. When excess amounts of carbon disulfide were also employed in place of the isothiocyanates, a bicyclic dithiopyrone was obtained. 

The key step in this sequence, achieved by exposure of 46 lo a mixture of sulfuric acid and acetic anhydride, involves opening of the cyclopropane ring by migration of a sigma bond from the quaternary center to one terminus of the former cyclo-l>ropane. This complex rearrangement, rather reminiscent of the i enone-phenol reaction, serves to both build the proper carbon. keleton and to provide ring C in the proper oxidation state. 

A modihcation on verapamil uses a spiro-dithiane moiety to supply the quaternary center. Reaction of veratraldehyde (64-1) with propane 1,3-dithiol leads to dithiane (64-2). Reaction with hydrogen peroxide oxidizes the ring sulfur atoms to... 

The Fenna-Matthews-Olson (FMO) protein is an unusual, water-soluble chlorophyll protein found only in green sulfur bacteria. [18] It is believed to be located between the chlorosome and the cytoplasmatic membrane and functions as an excitation transfer link between the chlorosome and the reaction center. Each subunit contains 7 BChl a molecules embedded in a primarily /3 sheet structured protein. The protein has a trimeric quaternary structure, with a three-fold axis of symmetry in the center of the complex. [55] The green nonsulfur bacteria do not contain the FMO protein. In these organisms the chlorosome transfers energy directly to the integral membrane core antenna B808-865, and then to the reaction center. 

If opportunity is given, a (thiolated) carbon atom can be transferred from a sulfur ylide to a double bond with diastereocontrol. The carbon remains on that side of the molecule which contained the sulfur initially e.g. equation 40). By similar chirality transfer, the (3-oriented phenylthio group of octalin (147) determines the -orientation of the transposed PhSCH2 unit in the reaction product (148 equation 41). The efficient formation of a quaternary center from (147) is all the more remarkable since an... 

The migration of oxygen from a quaternary center in a cyclohexadienone may be preferred to a carbon shift, when present as an ether or ester function rather than free hydroxy. Thus the p-quinol acetate (117) yields the orcinol monoacetate (118 79%) on treatment at room temperature with trifluoroacetic anhydride, and the p-quinol ether (119) forms the resorcinol diethyl ether (120 71%) in ethanolic sulfuric acid. In the second case, hemiketalization must intervene also some methyl shift (12%) is observed. With the quinol (121), treatment with acetic anhydride-sulfuric acid leads to the lactone (122) acetylation or lactonization probably precedes oxygen shift. A number of related examples can be found in the steroid area. - Thermal 1,3-shifts of p-quinol acetates can also be induced acetate (117) yields catechol acetate (123 50-60%, 45 °C) by way of isomerization of the first-formed acetate (124). In the o-quinol acetate series, 1,2-acetoxy shift is seen in (125) (126 92%) and in (127) (128 90%), both in... 

In rat liver ADH the presence of multiple molecular forms has been correlated to disulfide bridges involving the ligands to this zinc atom (Section II,B,3). These forms are active in ethanol oxidation (62). The lobe region which binds zinc is thus, in all probability, not essential for the catalytic action of alcohol oxidation. It has been suggested (133,134) that the extra zinc atom is essential for the structural stability of the enzyme. There is no evidence in the structure that this lobe region is necessary either for tertiary or quaternary structure stabilization. From the structural point of view, this region looks much more like a second catalytic center. The zinc atom is situated in one side of an obvious cleft into which the lone pair electrons of the sulfur atom of Cys-97 project. 

Most interest in biological activity has been centered on 4,5,6,7-tetra-hydrothieno 2,3-c, - or - 3,2-c pyridines. A considerable number of derivatives of these systems have been prepared by the sodium borohy-dride reduction of quaternary ammonium salts,90,91 or by reaction of the 4,5,6,7-tetrahydro base with a suitable halide,9,I°4 tosyl derivative,105 epoxide,106 or activated alkene.107 An alternative, and very convenient, synthesis of 2-amino-3,6-substituted 4,5,6,7-tetrahydrothieno[2,3-c[-pyridincs (89), used mostly by Nakanishi and his co-workers, involves reaction of an N-substituted 4-piperidone, a compound of the type 88 and sulfur, in the presence of morpholine. The group X can be CN,... 

R348 J. L. Garcia Ruano, A. M. Martin Castro, E. Torrente and A. M. Poveda, Diastereodivergent Synthesis of Benzylic Quaternary Centers Mediated by a Remote Sulfinyl Group Spectroscopic Evidence of the Structure of the Carbanionic Intermediates , Phosphorus, Sulfur Silicon Relat. Elem., [online computer file], 2011, 186, 1119. 

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