Oxane derivatives

Frozen conformational equilibria. Although the rate of the interconversion of the various conformers (chair, boat, twisted) of the oxane derivatives at ambient temperature is fast on the NMR timescale, this ring-inversion process is sufficiently slow at low temperature (T < -60°C) to permit the observation of individual signals for each conformer. Direct integration delivers K and -AG°, respectively [see Eqs. (1) and (2)]. 

Barriers to Ring Interconversion [AG (Real mol )] of Oxane Derivatives... 

AG values of oxane derivatives not covered by the mentioned references are given in Table XII and are briefly discussed. 

Examples of variation of the anomeric effect AG(AEl) with the substituents are shown in Tables II and III for oxane derivatives. The application of Eq. 1 to the results of the measurements of anomeric and conformational equilibria have established that the anomeric effect decreases in approximately the following order halogen > PhC02 > AcO > AcS > RO > RS > HO > NH2 > Me02C0 > imidazolium > pyridinium. 

Influence of the Solvent on the Axial Preferences of the Hydroxyl, Methoxyl, Methylthio, Aziridinyl, and Carbamoyl Groups in 2-SubstituM Oxane Derivatives"... 

The extent to which this kind of calculation is able to predict the effect of the solvent on conformational properties of carbohydrates has been thoroughly tested on 2-substituted oxane derivatives, D-glucopyranose, and methyl a- and y -D-glucopyranoside. In the model applied, the cavity term in Eq. 7 is based on an expression taken from the Scaled Particle Theory, and the electrostatic term is calculated according to the reaction held theory. The dispersion term takes into account both attractive and repulsive nonbonding interactions by using a combination of Lx)ndon dispersion energy and Bom-type repulsion. ... 

PCILO Calculated Bond Lengths (pm) and Bond Angles (degrees) for 2-Substituted Oxane Derivatives, Compared with... 

Fio. 25.—Orientation of Two Non-equivalent Lone-pairs of the (rand it Type in the a and e Forms of 2-Substituted Oxane Derivatives. 

Ayyanger, N. R.,S.Madan Kumar, and K. V. Srinivasan, Facile One-Pot Synthesis of 2,1,3-Benzoxadiazole A-Oxide(Benzofur-oxan) Derivatives under Phase-Transfer Catalysis, Synthesis, 616 (1987). 

Electrochemical fluorination of tetrahydrofuran carboxylic acids derivatives proceeds with preferential formation of F-tetrahydrofurane (5), however, in case of furans containing carbonyl group in the side chain, an interesting formation of spiro-ethers 7 is observed. F-Oxanes 8 and 9 are prepared in low yield by ECF of the corresponding oxane derivatives (Fig. 9.2). 

In parallel with these experiments, studies on the mechanism of formation of TXBj were carried out [19]. It appeared conceivable that more than one reaction would be needed for the formation of the non-prostanoic acid structure of TXB2 from PGG2/PGH2. An attractive sequence of reactions consisted of enzymatic formation of a bicyclic oxetane-oxane derivative followed by non-enzymatic hydrolysis of the oxetane ring to yield TXB2. In order to test this possibility, short time incubations (30-60 s) of arachidonic acid with platelets were carried out. The... 

Reaction of pyroc techol with epichlorohydrin in the presence of base affords the benzodioxan derivative, 136, (The reaction may well involve initial displacement of halogen by phenoxide followed by opening of the oxirane by the anion from the second phenolic group.) Treatment of the alcohol with thio-nyl chloride gives the corresponding chloro compound (137). Displacement of halogen by means of diethylamine affords piper-oxan (138), a compound with a-sympathetic blocking activity. 

Reaction of 3,4-bis(phenylsulfonyl)-l,2,5-oxadiazole oxide isomers with ethanol and ethanethiol in basic medium gave the expected alkoxy- and alkylthio-substituted (benzenesulfonyl)furoxans, respectively <1996JHC327, 1997FES405>. Nucleophilic substitution of the sulfonyl group of 3,4-bis-(benzenesulfonyl)furoxan 222 in the presence of aqueous NaOH in tetrahydrofuran (THF) furnished the corresponding 3 -0-(3-benzenesulfonylfur-oxan-4-yl) derivative 223 in 79-92% yield (Equation 44) <2004JME1840>. 

This article deals with the conformational analysis of substituted oxanes (tetrahydropyranes) and derivatives in which ring methylenes are replaced by further oxygen atoms (di-, tri-, tetroxanes, pentoxanes, and O ) or by carbonyl group(s) (oxanones, Meidrum s acid derivatives) and, if conforma-tionally of interest, systems incorporating these rings in polycyclic structures... 

The chair conformation is the preferred conformer for the oxane ring and substituted derivatives. In the case of poly-substitution (e.g., 1,3-diaxial groups), twist conformers can also participate in the equilibrium. Substituents can adopt the axial and equatorial positions ring interconversion between the chair conformers is fast on the NMR timescale at ambient temperature but becomes slow at low temperature (AG = 10.3 kcal mol- ) (73JA4634). 

Barby et al. [82JCS(P2)249] investigated the conformational equilibria of 2-NR2-substituted oxanes. The cis/trans isomeric 2-NR2-4-Me-derivatives proved to adopt preferred conformations (cis isomers 2-eq-A-eq trans isomers 2-ax-A-eq, except for /ram-2-NHMe-4-Me-oxane preferring the 2-ax-A-eq conformer by only 0.4 kcal mol ) the 2-NR2 monosubstituted oxanes (see Table I) prefer the equatorial position and do not show notable anom-eric interaction with the ring oxygen atom [82JCS(P2)249]. The same is true for 2-NHCOMe-oxane, but not for N3 and NCO substituents, respectively, in the 2-position (70ZOR863) (see Table I). 

The conformations of spiro-compounds containing the oxane ring have been reported (Scheme 5) (81CJC1132). They exist only in conformation 9a at room temperature at lower temperature, no indication of a second conformer was found. The methyl-substituted derivatives 10a and similar tricyclic analogs occur in the same conformation 11a. 

As In the case of the examples given in Tables I and III, the N-thiocarbamate derivatives were generally equal to the parent methylcarbamate in toxicity to house flies, particularly after taking into consideration Increase in molecular weights. Except for the methomyl and oxan l derivatives, substantial improvement in mosquito larvlcldal activity again appeare d to be a function of the lipophilic properties of the derivative. With the exception of the oxanyl derivatives, the data clearly indicate that the N-thiocarbamate derivatives are markedly less toxic to the white mouse than the parent methylcarbamate. Since... 

Pyrans (Oxanes) The six-membered cyclic ethers are commonly named as derivatives of pyran, an unsaturated ether. The saturated compound has four more hydrogen atoms, so it is called tetrahydropyran (THP). The systematic term oxane is also used for a six-membered ring containing an oxygen atom. 

Flavins (8), 8-azaflavin (9) and their analogs are characteristic oxidants toward thiols under anaerobic and other specific conditions (equation 12), 5-Arylidene-l,3-dimethylbarbituric acid derivatives (10) also worii as the oxidant in oxane at 120-150 C, and the method was applied to synthesis of unsymmetrical disulfides. Electrochemical oxidation s and photolysis of thiols to disulfidqs are also known. 

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