Mechanisms ether derivative formation

The current-voltage curve was interpreted on the basis of the mechanism illustrated in Figure 17.15a, which is derived from the behavior of the same catenane 134+ in solution.116,117 Conformation I is the switch open state and conformation IV the switch closed state of the device. When 134+ is oxidized (+2 V), the TTF unit is ionized in state II and experiences a Coulombic repulsion inside the tetra-cationic cyclophane component, resulting in circumrotation of the crown ether and formation of conformation III (note that in solution at +2 V TTF undergoes two-electron oxidation and the dioxynaphthalene unit is also oxidized).116 When the voltage is reduced to near-zero bias, a metastable conformation IV is obtained... 

The introduction and cleavage of the trityl ether proceeds through a very well-stabilised triphenylmethyl carbocation. In the case of trityl ether bond formation, the reaction is performed under anhydrous conditions and the carbocation, which is formed by an SN1 mechanism, reacts with an alcohol. In the case of cleavage, the triphenylmethyl carbocation ion is formed by treatment with acid, which is then trapped by water or a nucleophilic solvent to give trityl alcohol or other derivatives, respectively. Trityl ethers have also been used to protect thiols. 

When the thioacetal (128) was treated with allylmagnesium bromide and subsequently with BFs etherate, nucleophilic addition on the carbonyl carbon and a subsequent cationic cyclization took place the product (129) was obtained by aromatization with loss of methanethiol (Scheme 18) <84TL5095>. The reaction of (128) under the Simmons-Smith reaction conditions gave the thienothiepine derivative (130). The proposed mechanism for the formation of (130) involves a nucleophilic attack of the initially formed sulfonium ylide intermediate, intramolecular aldol type condensation, aromatization and demethylation (Scheme 19) <89TL3093>. 

Rearrangement of propargyl aryl ether 19 smoothly proceeds under thermal conditions to afford chromene derivatives 22 [23, 24]. The mechanism involves the formation of ortho allenyl phenol 20 followed by a 1,5-hydrogen shift and elec-trocyclic ring closure sequence via 21. For example, the Claisen rearrangement of propargyl aryl ether 23 prepared by the Mitsunobu reaction smoothly took place at 180 °C to give a flav-3-ene derivative 24 in excellent yield [25]. 

Other recent examples of inverse electron-demand Diels-Alder reactions in water are the cycloadditions of ( )-3-diazenylbut-2-enes 72 with a variety of vinyl ethers. The results of cycloaddition of 72 with ethyl vinyl ether (61) are reported in Table 5.5. The reactions were always faster in heterogeneous aqueous medium than in organic solvent and the endo adduct was the prevalent reaction product. Pyrrole derivatives such as ethyl-2-methyl-1-ureido-lH-pyrrole-3-carboxylates, derived from zwitterionic [3 + 2] cycloaddition reactions, were sometimes observed and a reaction mechanism of their formation has been proposed. In water, as well as in DCM, 72 (R = OEt, Ri = H) behaves like an electron-acceptor heterodiene even with a highly reactive diene such as cyclopentadiene, giving quickly, at 15°C, only the endo adduct. The cycloaddition of 72 (R = OEt, Ri = H) with the chiral vinyl ether (-F)-2-(ethenyloxy)-3,7,7-trimethylbicyclo[4.1.0]heptane (62x) was complete in water in 68 h at 15°C and gave a mixture of 83 17 endo/exo adducts with modest enantioselectivity. This is the first example of an asymmetric inverse electron-demand Diels-Alder reaction performed... 

The oxidation of a ( )-flavanone with Tl(ni) nitrate, Pb tetracetate, phenyliodonium diacetate (PIDA), or [hydroxyl(tosyloxy)iodo]benzene in trimethyl orthofonnate affords the corresponding ( )-2,3-dihydrobenzo[h]furan derivative as a major product. The structures, including the relative stereochemistry, and a plausible mechanism of formation are reported. The preferred formation of a flavone from the ( )-flavanone by PIDA is explained by quantum-chemical calculations on the intermediate formed by the addition of this reagent to the enol ether derivative of the ( )-flavanone." Formation of mixed anhydrides by rapid oxidation of aldehydes, activated by pivalic acid, Bu OCl in presence of pyridine and MeCN is catalysed by TEMPO (2,2,6,6-tetramethylpiperidin-l-oxyl). The anhydrides can be converted in situ to esters, secondary, tertiary or Weinreb amides in high yield. Oxidation of the aldehyde to 2-propyl esters is also possible using only catalytic amounts of pivalic acid." ... 

Despite the lack of direct evidence, the most likely mechanism for the formation of botryococcoid ether (67) is the coupling of two monoepoxides (70) and (71) derived from the trienic hydrocarbon (4) (Fig. 15). 

Horn JS, Paul AG, Rapoport H (1978) Biosynthetic conversion of thebaine to codeinone. Mechanism of ketone formation from enol ether in vivo. J Am Chem Soc 100 1895-1898 Kametani T, Ihara M, Honda T (1970) The alkaloids of Corydalis pallida var. tenuis (Yatabe) and the structures of pallidine and kikemanine. J Chem Soc (C) 1060—1064 Kirby GW, Massey SR, Steinreich P (1972) Biosynthesis of unnatural morphine derivatives in Papaver somniferum. J Chem Soc Perkin Trans 1 1642-1647 Kleinschmidt G, Mothes K (1959) Physiology and biosynthesis of alkaloids in Papaver somniferum. Z Naturforsch 14b 52-56... 

Kita et al. further developed PIFA-induced CDC reactions between phenyl ether derivatives and cyclic 1,3-dicarbonyl compounds as nucleophiles (Scheme 8.2). The reactions with 1 equiv. of PIFA in hexafluoro-2-propyl alcohol attach nucleophiles onto the ort/jo-position of para-substituted phenyl ethers to afford the dehydrogenative coupling products 8 in moderate yields. UV and electron spin resonance (ESR) spectroscopic studies support a reaction mechanism involving the formation of the charge-transfer complex 9 followed by the generation of the cation radical intermediate 10. This is the first example of the reaction of aromatic compounds with PIFA that involves the formation not of diatyliodonium(m) salt 11 but the cation radical intermediate 10 as a key intermediate. 

Stabilised sulphur ylides react with alkenylcarbene complexes to form a mixture of different products depending on the reaction conditions. However, at -40 °C the reaction results in the formation of almost equimolecular amounts of vinyl ethers and diastereomeric cyclopropane derivatives. These cyclopropane products are derived from a formal [2C+1S] cycloaddition reaction and the mechanism that explains its formation implies an initial 1,4-addition to form a zwitterionic intermediate followed by cyclisation. Oxidation of the formed complex renders the final products [30] (Scheme 8). 

Misonidazole [27 l-methoxy-3-(2-nitroimidazol-l-yl)-2-propanol] and the model compound l-methyl-2-nitroimidazole have been used as radiosensitizers in the treatment of certain types of human tumors. One important property of these compounds is that they are more toxic to hypoxic cells than to aerobic cells, indicating that reductive metabolism of the drug is involved in the toxicity. Results of a number of studies suggest that intracellular thiols play a significant role in the hypoxic cell toxicity, and it was found that reduction products formed stable thio ethers with GSH (for literature see References 181-183). The reaction mechanism of thio ether formation has not been fully established. It has been suggested that the 4-electron reduction product was involved in thio ether formation181,184,185, and that the hydroxylamine rather than the nitroso derivative was the reactant. On the other hand, an intermediate nitroso derivative is expected to give a sulfenamide cation (see Scheme 1) which easily allows thio ether formation. 

If chiral catalysts are used to generate the intermediate oxonium ylides, non-racemic C-O bond insertion products can be obtained [1265,1266]. Reactions of electrophilic carbene complexes with ethers can also lead to the formation of radical-derived products [1135,1259], an observation consistent with a homolysis-recombination mechanism for 1,2-alkyl shifts. Carbene C-H insertion and hydride abstraction can efficiently compete with oxonium ylide formation. Unlike free car-benes [1267,1268] acceptor-substituted carbene complexes react intermolecularly with aliphatic ethers, mainly yielding products resulting from C-H insertion into the oxygen-bound methylene groups [1071,1093]. 

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