Protecting. TPS

The complex [CrH(H20)j]2+ briefly mentioned in the introduction is obtained by pulse radiolysis from aqueous Cr +, but this is an unstable species which decomposes by protonolysis. Its spectroscopic properties are similar to those of the analogous alkyl species, known to adopt a spin quartet configuration [3]. On the other hand, the isoelectronic, sterically protected Tp B CoH complex [Tp -Bu,Me = hydrotris(3- er/-butyl-5-methylpyrazolyI)borate], also having a spin quartet ground state, could be isolated and characterized by X-ray crystallography [85b],... 

Health Effects Assessment for Hexavalent Chromium, EPA/540/1-86-019, United States Environmental Protection Agency (EPA), Sept. 1984 Toxicological Profile for Chromium, Agency for Toxic Substances and Disease Registry (ASTDR), ASTDR/TP-88/10,1989. 

In an attempt to change the electronics of the chromium atom, we are replacing the carbon based cyclopentadienyl ring with ligands containing harder donor atoms. For example, we have employed the tris(pyrazolyl)borate moiety, an isoclectronic replacement for Cp featuring tridentate N-coordination.[9] Figure 2 shows the molecular structure of Tp SU Cr-Ph, a representative Cr° alkyl. It will be noted, that this complex is mononuclear, due to the steric protection of the extremely bulky tris(pyrazolyl)borate. 

EPM > CO > TP > CB. The highly crystalline TB had an etch rate about six times that of CB, ascribable to a morphology difference, while the partially crystalline TO had an etch rate somewhat higher than that of amorphous CO. Cis/trans content had little or no effect on the etch rate of the polyalkenamers. A mechanism involving crosslinking through vinyl units is proposed to explain the unexpected protection imparted to vinylene-rich polybutadienes by the presence of 1,2 double bonds. 

Fig. 1.3. Experimental setup for electrochemical thermal desorption mass spectroscopy (ECTDMS). C = electrochemical cell, W = working electrode, El = electrolyte inlet, EO = electrolyte outlet, EH = electrode holder, V = valve, TP = turbo pump, VC = vacuum chamber, L = light source, W = window, P = protective jacket, A = aperture to analysis chamber, GI = grid ion source, S = SEM detector.

The sulfone moiety was reductively removed and the TBS ether was cleaved chemoselectively in the presence of a TPS ether to afford a primary alcohol (Scheme 13). The alcohol was transformed into the corresponding bromide that served as alkylating agent for the deprotonated ethyl 2-(di-ethylphosphono)propionate. Bromination and phosphonate alkylation were performed in a one-pot procedure [33]. The TPS protecting group was removed and the alcohol was then oxidized to afford the aldehyde 68 [42]. An intramolecular HWE reaction under Masamune-Roush conditions provided a macrocycle as a mixture of double bond isomers [43]. The ElZ isomers were separated after the reduction of the a, -unsaturated ester to the allylic alcohol 84. Deprotection of the tertiary alcohol and protection of the prima-... 

All attempts to achieve a direct transformation of the carbazomadurins A (253) and B (254), as well as the disilyl-protected carbazomadurins A (769a) and B (769b), into the epocarbazolins A (258) and B (259) were unsuccessful and resulted in complete decomposition. Therefore, prior to the epoxidation, the disilyl-protected carbazomadurins A (769a) and B (769b) were transformed to the corresponding trisilyl-protected carbazomadurins A (770) and B (771) by treatment with TPS chloride in the presence of stoichiometric amounts of 4-(dimethylamino)pyridine (DMAP). Epoxidation of the fully protected carbazomadurins A (770) and B (771) with dimethyldioxirane at — 20°C, followed by desilylation, provided racemic epocarbazolin A (258) and epocarbazolin B (259) (605) (Scheme 5.82). 

As one of the characteristics of t t[CH2NH] replacement is the introduction of a new basic site, protection of the secondary amine can be required during peptide elongation. However, it has been shown by Coy and co-workers that the tp[CH2NH] surrogate is generally resistant to further acylation, except for the incorporation of the Gly residue, probably due to a steric factor J7 8 In the case of pseudopeptide libraries, protection of the secondary amine seems to be important for affording products in better yields and purity.[9]... 

The TPS group is a sil> 1 protecting group that can be removed using TBAF as a source of fluoride ion. After removing the protecting group the alcohol function is converted into a mesylate, which is transformed in the third step via a Finkelstein reaction into an iodide. 

Deprotection of silyl ethers can be accomplished by using a variety of reagents such as TBAF, BF3-OEt2, alkali metal tetrafluoroborate and HF. Here, PPTS is used to distinguish the TBS from the TPS group. Thus, the rearranged 27 cleaves selectively the least bulky silyl group to form mono-protected 8.12... 

The resulting propargylic alcohol is protected as TIPS ether by a standard procedure using the corresponding silyl chloride and imidazole in DMF. Optionally the more reactive silyl triflate and 2,6-lutidine may be employed in order to shorten the reaction time. Under acidic conditions TIPS and TPS are nearly stable protecting groups. Therefore the TBS ether is selectively cleaved with acetic acid even in presence of the acetal moiety.13 Subsequent reaction with iodine and triphenylphosphine, known as the Appel reaction14, provides the desired iodide 4. 

Newton, M. and E.C. Cole (1997). Environmental effects of vegetation management. In Reforestation and Vegetation in Central Alaska. USDA Forest Service Forest Health Protection Special Report R10-TP-65, pp. III-1-20. 

Stephan CE. 2002. Use of species sensitivity distributions in the derivation of water quality criteria for aquatic life by the U.S. Environmental Protection Agency. In Posthuma L, Suter GW, Traas TP, editors. Species sensitivity distributions in ecoloxicology. Boca Raton (FL) CRC Press, p 211-254. 

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