Tethered acceptor

In a different version of the same concept. Stork [67,68] and Bols [69-72] introduced silylene type tethering with glycosyl sulfoxide [67,68] or thioglycoside [69-72] donors. The silicon-tethered acceptor approach was also extended to the synthesis of a-D-gluco- [70,72] and a-D-galactopyranosyl linkages in oligosaccharides [70,72]. 

Scheme 18. Preparation of branched-chain sugars by intramolecular addition of sugar-based radicals to O-tethered acceptors...

Scheme 10-78 Proposed mechanism for intramolecular glycosylation from a tethered acceptor.

Intramolecular [3-1-2] cycloadditions, i.e., having the TMM moiety and the acceptor linked by a tether, have great synthetic utility in polycarbocycle construction. The construction of 5.5, 6.5, and 7.5 ring systems has been demonstrated with this methodology [21-25]. A number of efficient routes to acyclic precursors were developed (Scheme 2.11). The organometallic reagent (31), generated from 2-bro-mo-3-(trimethylsilyl)propene (32) [26], is a key component in the construction of... 

Ratio imaging is particularly suited for single-polypeptide FRET sensors. In these constructs FRET changes are due to altered distance and/or orientation of the donor and acceptor, and since the fluorophores are tethered their stoichiometry is always fixed. Thus, the filterFRET problems are easier to address and, assuming full maturation of both FPs [4], it can in fact be shown that under these circumstances two images suffice to calculate FRET quantitatively (see Textbox 1 and Appendix 7.A.6). 

In this appendix, FRET in tethered constructs (where thus donor acceptor stoichiometry is exactly 1) is calculated from just two images. Provided that independent estimates of cross talk magnitude are available, and the excitation power in both images is the same, full correction of leak through is possible from images ... 

Ziegler and coworkers prearranged a glycoside by employing a succinyl tether between C-6 of a mannosyl donor and C-3 of glucosyl acceptor [151,152]. They found that the nature of the glycosyl acceptor and the length of the tether affected the anomeric selectivity of the intramolecular mannosylation (Scheme 4.4d) [153]. 

Yet another approach uses peptides as tethers for intramolecular glycosylations via prearranged glycosides (Scheme 5.113) [326,327]. The regio- and anomeric selectivity of the intramolecular glycosylation depends on the amino acid sequence of the peptide, which links glycosyl donor and acceptor. 

In this approach, the glycosyl donor and the glycosyl acceptor are linked by the 2-OH of the donor and the free OH of the acceptor. It is one of the most predictable and reliable methods for achieving 1,2-cis stereocontrol. Acetals, mixed p-methoxybenzylacetals and silicon tethering have been widely used as well as iodonium mediated tethering acetals derived from vinyl, allyl and allenyl ethers. These methodologies have been revised.6,76... 

Allenes as versatile synthons including Diels-Alder reactions and especially intramolecular cycloadditions of this type were reviewed by Aso and Kanematsu [338], In some cases of intramolecular Diels-Alder reactions of open-chain starting materials such as 340 [339], 342 [339] and similar acceptor-substituted allenes [156], the formation of two new six-membered rings seems to be favorable if possible (Scheme 7.48). The non-activated cumulated C=C bond of 340 takes part in the [4+ 2]-cycloaddition and hence the necessary reaction temperature is high. On the other hand, the progressive truncation of the tether and the electron deficiency of the allenic C=C bond involved give rise to a remarkable Diels-Alder reactivity of the sulfone 346 generated in situ from sulfoxide 345 [339]. 

The early studies of Risser, Beratan, and Meade [13] suggested that, at short distances (especially for backbone-tethered donor-acceptor pairs), backbone mediation could dominate the coupling interactions. Later studies by Priyadarshy, Tong, Kurnikov, Risser, and Beratan showed that—for t-stacked donor-acceptor pairs—base interactions clearly dominate the bridge-mediated coupling interactions [12,16]. 

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