1.3- Dioxane structure

Two major groups of polycyclic compounds including the 1,3-dioxane structure are interesting conformationally. The first, the spiranes, conserve... 

An enormous number of different 1,3-dioxane structures have been reported since 1996 in Figure 3, mono-, bicyclic and spiro variants are presented, while Figure 4 contains examples of tricyclic structures with the 1,3-dioxane moiety. The conformations, bond lengths, bond and dihedral angles of the 1,3-dioxane rings are determined by the ring fusion, the attached substituents, and the presence of exocyclic double bonds. Thus, published structures are classified as either monocyclic (mono), spiro-substituted (spiro), bicyclic (bi), or tricyclic (tri). The well-known Meldrum s acid derivatives (M) have been most intensively studied. 

It is worth of note that the formation of a dimer, with different dioxane structure, in the treatment of 3,4-dihydroxyprecocene I with p-toluensulfonic acid has been recently reported (13). Further work, to study the reactivity of precocene epoxides with selected nucleophiles, which can shed light on the mode of action of these compounds, is in progress. 

Some known odour and taint problems derived from polyurethane adhesives include the formation of cyclic 1,3-dioxolane and 1,3-dioxane structures from the reaction of 1,2-diols and 1,3-diols with aldehydes and ketones, and... 

Studies, by means of nuclear magnetic resonance measurements with pertrimethylsilyl ethers, of the behavior in solution of glycol-aldehyde and related substances formed by periodate oxidation gave striking results. Products formed under mild conditions showed that the original material was composed of dimers having p-dioxane structures products separated after complete mutarotation showed eight dimeric forms. ... 

The NMR spectnim of the white crystalUne solid obtained when the crude product was dissolved in ethanol revealed a single P signal at 8.30 ppm that split into a triplet when coupled to protons. In the C H NMR spectrum, this soUd exhibited a signal at 95.45 ppm that is characteristic of the C-2 of the 1,3-dioxane structure. 

All characteristic last steps in the synthesis of EDOT, i.e. the ring closure to the dioxane structure, are also sufficient for the formation of the analogous seven-membered rings (1,3-dioxepanes), the 3,4-propylenedioxythiophenes (ProDOTs) Williamson ether synthesis [13], transetherification [30] and Mitsunobu reaction [25]. The analogous basic five-membered ring compound 3,4-methylenedioxythiophene (MDOT, a 1,3-dioxolane derivative) is also accessible by Williamson ether synthesis using bro-mochloromethane [31]. 

It is shown that this reaction proceeds in two stages without an intermediate o-cation formation. The 7t-cation (lb-5b) is directly transformed into the 1,3-dioxane structure (ld-5d) though the corresponding transition states (lc-5c). The calculated values of the thermodynamic parameters are presented in Table 9.1. 

The structure of the 3c transition state for butane-1 and FD reaction somehow differs from the structure of other transition states. It ean be considered as an intermolecular cyclization of o-cation. In fact the distance and bond order C2-C4 values are eharaeteristic for a single bond C-C and amount to 1.539 and 0.877 A aceordingly. The 03-Cl bond has not been formed (R = 2.688 A) and the distanee and bond order values C3=C4 amount to 1.459 A and 1.017. And this bond is quite similar to the single bond C-C at C+ in carbocation. These parameters eonsiderably differ from the similar values of other transition states where the C3-C4 bond can be treated as a sesquialteral one. However, a stable o-eation in the butane-1 and FD reaction was not discovered. The forming re-eation 3b as in other alkenes is direetly transformed into a dioxane structure. 

Due to the analysis of the calculated data of the transition states structure of 4-alkyl-l,3-dioxane formation from formaldehyde oligomers and al-kenes, it is found that 1,3-dioxane structures are formed in the result of direct isomerization of re-cation on the first stage. A free o-cation formation is not observed here. 

Al-Phenyl-iV-allylmethacrylamide (23) gives a polymer with a five-membered ring structure with true asymmetric centers in the main chain by free-radical cyclopolymerization [63]. When the polymerization is carried out in the presence of SnCl4/(-)-menthol, the resulting polymer was optically active ([a]] -5.6°). Chiral induction was also observed in the copolymerization of 23 with MMA. Cationic cyclopolymerization of 24 using a ZnClJ 10-camphorsulfonic acid (25) initiator system gives an optically active polymer having a 1,3-dioxane structure in the main chain([a]435-17°)[64]. 

Schwarz G and Savko P 1982 Structural and dipolar properties of the voltage-dependent pore former alamthicin in octanol/dioxane Biophys. J. 39 211-19... 

Another method for the hydroxylation of the etliylenic linkage consists in treatment of the alkene with osmium tetroxide in an inert solvent (ether or dioxan) at room temperature for several days an osmic ester is formed which either precipitates from the reaction mixture or may be isolated by evaporation of the solvent. Hydrolysis of the osmic ester in a reducing medium (in the presence of alkaline formaldehyde or of aqueous-alcoholic sodium sulphite) gives the 1 2-glycol and osmium. The glycol has the cis structure it is probably derived from the cyclic osmic ester ... 

Thio-2-oxoselenazolidine can be obtained in 89% yield by treating the 2,4-dione with P4SJ0 in boiling dioxane (81). This structure is found in heterocyclic indigo-type dyes (Scheme 78). 

When two heteroatoms are present in a saturated six-membered ring their effects are approximately additive. Apart from the case of two a oxygen atoms—in 1,3-dioxane (48) the shift of C-2 is S 95.4 instead of 5 108 which a double shift of 40 p.p.m. would require—predictions of shift made on this basis are usually accurate to within 5 p.p.m. and are generally much closer than this. Observed shifts for a few representative examples are shown in structures (48)-(52). 

In aqueous dioxane, the endo-anti isomer gave a product mixture consistent of alcohol N and the corresponding ester (derived from capture of the leaving group p-nitrobenzoate). The other isomers gave much more complex product mixtures which were not completely characterized. Explain the trend in rates and discuss the structural reason for the stereochemical course of the reaction in the case of the endo-anti isomer. 

The alkynylation of estrone methyl ether with the lithium, sodium and potassium derivatives of propargyl alcohol, 3-butyn-l-ol, and propargyl aldehyde diethyl acetal in pyridine and dioxane has been studied by Miller. Every combination of alkali metal and alkyne tried, but one, gives the 17a-alkylated products (65a), (65c) and (65d). The exception is alkynylation with the potassium derivative of propargyl aldehyde diethyl acetal in pyridine at room temperature, which produces a mixture of epimeric 17-(3, 3 -diethoxy-T-propynyl) derivatives. The rate of alkynylation of estrone methyl ether depends on the structure of the alkyne and proceeds in the order propar-gylaldehyde diethyl acetal > 3-butyn-l-ol > propargyl alcohol. The reactivity of the alkali metal salts is in the order potassium > sodium > lithium. 

While the mechanistic scheme as outlined so far accounts for the majority of structural changes in ring A-dienone isomerizations, a few cases require modifications of this general pathway. The B-nor dienone (215) is transformed exclusively to the linear dienone (217) in dioxane solution. The preferential fission of the 5,10-bond in the hypothetical precursor (216) has... 

The foregoing conclusions are further supported by a refined X-ray analysis of pyrid-2-one, which indicated that the mobile hydrogen atom is attached to the nitrogen atom in the solid state and that individual molecules are bound into helices by N—H- -0 hydrogen bonds. An oxo structure is also indicated by the molar refractivity of pyrid-2-one. The dipole moment of 4-methoxypyridine is ca. 3.0 debyes in dioxane, whereas the values for pyrid-4-one and its 1-methyl derivative are much higher, ca. 6.0 debyes indicating the... 

The ultraviolet spectra of the pyridinecarboxylic acids (334) were initially interpreted assuming that the proportion of the zwitterion structure 335 was not appreciable,and the early pK work was inconclusive. However, Jaffe s calculations based on the Hammett equation indicated that about 95% of nicotinic and isonicotinic acids existed in the zwitterion form, and ultraviolet spectral data showed that the actual percentages of picolinic, nicotinic, and isonieotinie acids existing in the zwitterion form in aqueous solution are 94, 91, and 96%, respectively.This was later confirmed by Stephenson and Sponer, who further demonstrated that the proportion of the zwitterion form decreases in solvents of low dielectric constant, becoming very low in ethanol. Dipole moment data indicate that isonicotinic acid exists as such in dioxane, and 6-hydroxypyridine-3-carboxyiic acid has been shown to exist in form 336 u.sing pK data. ... 

Intramolecular cycloadditions of substrates with a cleavable tether have also been realized. Thus esters (37a-37d) provided the structurally interesting tricyclic lactones (38-43). It is interesting to note that the cyclododecenyl system (w = 7) proceeded at room temperature whereas all others required refluxing dioxane. In each case, the stereoselectivity with respect to the tether was excellent. As expected, the cyclohexenyl (n=l) and cycloheptenyl (n = 2) gave the syn adducts (38) and (39) almost exclusively. On the other hand, the cyclooctenyl (n = 3) and cyclododecenyl (n = 7) systems favored the anti adducts (41) and (42) instead. The formation of the endocyclic isomer (39, n=l) in the cyclohexenyl case can be explained by the isomerization of the initial adduct (44), which can not cyclize due to ring-strain, to the other 7t-allyl-Pd intermediate (45) which then ring-closes to (39) (Scheme 2.13) [20]. While the yields may not be spectacular, it is still remarkable that these reactions proceeded as well as they did since the substrates do contain another allylic ester moiety which is known to undergo ionization in the presence of the same palladium catalyst. 

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