Sulfide tether

Scheme 10-35 A sulfide tether allows [5+2] cycloaddition between an unactivated alkene and a pyrone.

Scheme 10-124 A sulfide tether was used in the synthesis of j3-cuparenone.

A possible reaction mechanism shown in Scheme 7-10 includes (a) oxidative addition of the S-H bond to Pd(0), (b) insertion of the allene into the Pd-H bond to form the tt-allyl palladium 38, (c) reductive elimination of allyl sulfide, (d) oxidative addition of the I-aryl bond into the Pd(0), (e) insertion of CO into the Pd-C bond, (f) insertion of the tethered C=C into the Pd-C(O) bond, and (g) P-elimination to form 37 followed by the formation of [baseHjI and Pd(0). 

The effect of the complexation is to stabilize the persulfoxide, PS. This stabilization can potentially manifest itself in a more rapid rate of formation, or alternatively, in suppression of physical quenching (kq in Fig. 25). Evidence that the latter phenomenon is of paramount importance was provided by examination of a series of sulfides with tethered olefinic linkages.96 In these cases the enhancement of reaction at the sulfur at the expense of the ene reaction at the olefinic linkages could be quantitatively predicted based upon the assumption that kq — 0. 

Catalysts 75c and 76a also indnce cyclization of a variety of aliphatic snbstrates for the construction of tetrasubstitnted carbon centers in good yields and high enantioselectivities (Scheme 12). Despite the snccess of carbon, nitrogen and oxygen tethers, sulfide side chains have proven resistant to cyclization nnder optimized conditions. By changing the linker to a sulfone 87, cyclization was accomplished in 98% yield, albeit 80% ee. 

Pericas and Jeong demonstrated independently that sulfur-tethered substrates, when subjected to the PKR conditions, afforded the desired bicyclic products. The sulfur tether is removed cleanly by Pummerer reaction after oxidation of sulfur to sulfoxide or 1,4-addition of bisalkyl cuprate followed by hydrogenolysis of sulfide with Raney nickel. It is worth mentioning that the regioselectivity regarding the acetylene part is opposite to that of the intermolecular version (Equation (30)). 

BTIB oxidations of aromatic ethers in which sulfido groups are tethered to the ring provide access to heterocyclic sulfides [64], while related oxidations of azido-tethered analogs lead to quinone imines and/or their dimethyl ketals... 

Table 3 shows the application of these intramolecular biaryl-forming conditions to the indirect synthesis of acyclic biaryls using a temporary tether strategy (Scheme 9). Silaketal tethers (Y = SiR2 and X = O, 31a-f) react efficiently to give the biaryl unit. A sulfide derivative (Y — S, X = CH2, 31g) led to many oxidized products, but sulfoxides (Y = SO,... 

Annulation of furans via electrochemical oxidation at the anode has become an important process for the synthesis of complex polycycles, and was covered in a review <2000T9527>. Furans tethered at the 3-position to electron-rich alkenes, enol ethers, or vinyl sulfides were converted to [6,5] and [7,5]-fused ring systems <1996JOC1578, 2002OL3763, 2004JOG3726, 2005JA8034>, as illustrated in Scheme 20. Analysis of crude reaction mixtures and side... 

Treatment of 2,3 Cpoxy-l-amines with Lewis acid induces a rearrangement to aziridinium ions that react efficiently with a nucleophiles to give functionalized hydroxy sulfides or hydroxy amines (Equation 23) <1997SL11>. Under the influence of ethylaluminium chloride, an epoxide tethered to an azide undergoes Lewis acid-assisted cyclization followed by an intramolecular Schmidt reaction and subsequent in situ reduction of the intermediate iminium species upon addition of sodium borohydride (Scheme 8). This protocol was used as a key step in a novel synthesis of indolizidine alkaloids of pharmaceutical interest <20030L583, 2004JOC3093>. 

Farmer and Patten have modified the composition of the colloidal initiators to encapsulate cadmium sulfide particles in a shell of silica [340]. The ATRP initiating groups were introduced by condensing functional monoalkoxysilanes containing 2-bromopropionate groups to the silica surface. The ATRP of St from these core-shell colloidal initiators yielded an array of luminescent particles in a matrix of tethered pSt. 

There was a hidden agenda here too. A second cyclisation onto an electron-rich benzene ring was planned to build up the skeleton of the Erythrina alkaloids and it occurred to Tamura17 that a sulfide (X = SMe below) might control both cyclisations. Therefore the anhydride 106 was combined with an imine 105 already having the aromatic ring tethered to the nitrogen atom. 

Vinylsamarium species are generated from vinyl sulfides and sulfones. Protonation or alkylation may then be performed. Formation of C-disaccharides from a 5i-tethered glycosyl sulfone has been effected with Sml2. Intervention of a glycosyl radical intermediate is implicated. 

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