Dioxanes disubstituted

Table 5 NMR assignment of stereochemistry and erythro/threo-conf gurat on of 4,5-disubstituted-1,3-dioxane derivatives 73...

The erythro/threo diastereomers of a larger variety of 4,5-disubstituted 1,3-dioxanes (chiral conformationally restricted arachidonic acid analogs 54-59) proved to be of enantiomerically pure stereochemistry (cf. Scheme 17) (99TA139) the epimers were clearly identified by the coupling patterns of the protons in positions 4, 5, and 6, reflecting the ax,equ (threo) and equ,equ (erythro) relationships of the two substituents. 

Calamitic compounds which exhibit a smectic and/or nematic phase usually consist of a relatively rigid central core containing co-linear six-membered rings, either aromatic rings, such as 1,4-disubstituted-phenylene, 2,5-disubstituted-pyridine, 2,5-disubstituted-pyrimidine, 3,6-disubstituted-pyridazine, and alicyc-lic rings, such as /ra j-l,4-disubstituted-cyclohexane, 1,4-disubstituted-bicy-clo[2.2.2]octane, 2,5-disubstituted-dioxane. Heteroaromatic rings tend to lead to the formation of smectic phases rather than the nematic phase unless combined with a polar terminal function, such as a cyano group. The dependence of the liquid crystalline transition temperatures on the nature of... 

Dioxene can be used to prepare trisubstituted annulated furans in a three-step sequence. By lithiation of 1,4-dioxene, followed by carbonyl addition, an allylic alcohol 13 is obtained, which can be reacted with silyl enol ethers in the presence of a Lewis acid to furnish disubstituted dioxanes of type 14. These compounds rearrange to furans under mild conditions upon treatment with camphorsulfonic acid (Scheme 16) <1999TL2521>. 

In a base-free medium (dry MeCN), Fe Ch activates HOOH to form a reactive intermediate that oxygenates alkanes, alkenes, and thioethers, and dehydrogenates alcohols and aldehydes. Table 11 summarizes the conversion efficiencies and product distributions for a series of alkene substrates subjected to the Fe Cfi/HOOH/MeCN system. The extent of the Fe Cb-induced monooxygenations is enhanced by higher reaction temperatures and increased concentrations of the reactants (substrate, Fe Cls, and HOOH). For 1-hexene (representative of all of the alkenes), a substantial fraction of the product is the dimer of 1-hexene oxide, a disubstituted dioxane. With other organic substrates (RH), Fe Cb activates HOOH for their monooxygenation the reaction efficiencies and product distributions are summarized in Tables 11(b). In the case of alcohols, ethers, and cyclohexane, a snbstantial fraction of the product is the alkyl chloride, and with aldehydes, for example, PhCHO, the acid chloride represents one-half of the product. In the absence of snbstrate the Fe Cls/MeCN system catalyzes the rapid disproportionation of HOOH to O2 and H2O. 

The mechanism for the homopolymerization of epoxides may proceed via a bimolecular reaction yielding a symmetrical 2,5-disubstituted dioxane as shown in Reaction Scheme 6. This reaction should be described by the following kinetic expression for the disappearance of epoxides ... 

The synthesis can be conducted both in solution and without solvents. The reaction in solvent (e.g., methanol, ethanol, dioxane, dimethylformamide) is recommended for volatile 1,3-diynes and amines in this case the pyrroles are purer and the yield is higher. With disubstituted diacetylenes, ammonia and primary alkyl- and arylamines produce 1,2,3-trisubstituted pyrroles under the same conditions (65CB98 71MI1). Since disubstituted diacetylenes are readily obtained by oxidative coupling of acetylenes (98MI2), this reaction provides a preparative route to a wide range of pyrroles. 

Unsaturated substituents of dioxolanes 36-38 and dioxanes 39-41 are prone to prototropic isomerization under the reaction conditions. According to IR spectroscopy, the isomer ratio in the reaction mixture depends on the temperature and duration of the experiment. However, in all cases, isomers with terminal acetylenic (36, 39) or allenic (37, 40) groups prevail. An attempt to displace the equilibrium toward the formation of disubstituted acetylene 41 by carrying out the reaction at a higher temperature (140°C) was unsuccessful From the reaction mixture, the diacetal of acetoacetaldehyde 42, formed via addition of propane-1,3-diol to unsaturated substituents of 1,3-dioxanes 39-41, was isolated (74ZOR953). 

Only c/s-disubstituted and trisubstituted alkenes yield l,4-dioxan-2-ones by way of a cycloaddition reaction when oxidised by dimethyl a-peroxy lactone. An open 1,6-dipolar intermediate is postulated, involving steieoelectronic control <96JA4778>. 

Cyclopenta[fc]dioxanes (44) are accessible from the reaction of the dioxenylmolybdenum carbene complex (43) with enynes <96JOC159>, whilst an intramolecular and stereoselective cyclisation of (Ti5-dienyl)tricarbonyliron(l+) cations affords chiral frans-2,3-disubstituted 1,4-dioxanes <96JOC1914>. 2,3-Dimethylidene-2,3-dihydro-1,4-benzodioxin is a precursor of the 3,8-dioxa-lff-cyclopropa[i]anthracene, which readily dimerises to dihydrotetraoxaheptacene (45) and the analogous heptaphene <96AJC533>. 

Electron-donating groups in the para position of the perbenzoic acid tend to decrease the rate of reaction . The reverse seems to be true when these groups are present in a 4,4 -disubstituted diaryl sulphoxide. The effect of ring size on the oxidation of cyclic sulphoxides is apparently very small in dioxane-water solution under either acidic or basic conditions . This suggests that no major hybridisation change occurs for the sulphur atom in going to the transition state. 

Under some circumstances, acid-catalyzed ring opening of 2,2-disubstituted epoxides by sulfuric acid in dioxane goes with high inversion at the tertiary center.116... 

This latter compound, 36, and the isomeric 9-oxabicyclo[4.2.1]nonane, 37, were obtained as the sole products, in ca 13 87 ratio, by reaction of 3 with A-chlorosuccinimide (NCS) in protic solvents (methanol, dioxane-water mixtures)72. It is noteworthy that similar ratios of the two disubstituted bicyclononane derivatives were obtained, independently of the solvent, also by using A-bromosuccinimide (NBS) as electrophile, whereas a strongly solvent-dependent ratio was observed when A-iodosuccinimide (NIS) was used. Since these reactions should proceed through hydroxy- or alkoxyhalogenation of one of the double bonds, followed by transannular attack of the oxygen function on the cationic center which is formed on the other side of the ring by the reaction of another electrophile with the second double bond, the isomer ratio has been rationalized in terms of a different nature of the intermediates. 

Dioxo-l,3-dioxanes ring-open under basic conditions. Cleavage of the 5,5-disubstituted derivatives in the presence of quininium or quinidinium alkoxides produces chiral malonic hemi-esters (ee 30-40%) in high yield [11]. The addition of cetyltrimethylammonium bromide promotes the base-catalysed cleavage of p-keto esters to form ketones under sonication [12]. 

Disubstituted 2-phenyl-277-1,2,3-triazole-1-oxides (150) can be easily obtained from the corresponding bis(hydroxyimino)butanes 148 in three steps. Thus, treatment of dioximes 148 with diluted HCl in dioxane with subsequent interaction with PhNHNH2/EtOH/AcOH afforded a-hydrazinooximes 149 in excellent yields. Reaction of 149 with A-iodosuccinimide (NIS) in CCI4 or with CUSO4 in aqueous pyridine afforded triazoles 150 (equation 65) . Similar cyclization in the presence of SOCI2 also leads to... 

Similarly, using either sulfuric acid, the SOs/dioxane complex, or a solution of SO3 in chloroform/dioxane, 4,6-diphenyl-l,2-oxathiin 2,2-dioxide was obtained from phenyl acetylene <1999RJ0415>, 3,6-disubstituted-l,2-oxathiane 2,2-dioxides were obtained from allylphenol <2002RJ01210>, and 3,4-dihydro-6-phenyl-l,2-oxathiin-4-one 2,2-oxide was obtained from Ph-CO-CH2-COMe <1991CIL253>. 

The conformations of three 2,2 -disubstituted-l,3-dioxane derivatives 65-67 have been elucidated by NMR spectroscopy <1998CHE141, 1999PAC385, 2001ARK(xii)7> only the conformers with the more polar substituent in an axial position have been assessed as being in agreement with the anomeric effect. 

The erythrolthreo-KoracK of three 4,5-disubstituted-l,3-dioxanes 73 (cf. Table 5), synthesized as chiral building blocks based on the 1,3-dioxane core, were assigned by H NMR spectroscopy (Table 5) <1999TA139> additionally, they are enantiomerically pure. 

Harwood and co-workers (105) utihzed a phenyloxazine-3-one as a chiral derived template for cycloaddition (Scheme 4.50). An oxazinone template can be formed from phenylglycinol as the template precursor. The diazoamide needed for cycloaddition was generated by addition of diazomalonyl chloride, trimethyl-dioxane-4-one, or succinimidyl diazoacetate, providing the ester, acetyl, or hydrogen R group of the diazoamide 198. After addition of rhodium acetate, A-methylmaleimide was used as the dipolarophile to provide a product that predominantly adds from the less hindered a-face of the template in an endo fashion. The cycloaddition also provided some of the adduct that approaches from the p-face as well. p-Face addition also occurred with complete exo-selectivity. Mono- and disubstituted acetylenic compounds were added as well, providing similar cycloadducts. 

Functionalised 2,3-dihydro-l,4-dioxins can be synthesised in a three step-sequence from P-keto esters. The key step is the insertion of a Rh-carbenoid derived from an a-diazo-p-keto ester into an 0-H bond of a 13-diol <99H(51)1073>. The reaction of 2-(l,4-dioxenyl)alkanols with silyl enol ethers yields 23-disubstituted 1,4-dioxanes. When 13-bis(trimethylsilyloxy)-cyclobut-l-ene is used, the expected cyclobutanone products are accompanied by a spirocyclopropane derivative <99TL863>. 1,4-Dioxane-monochloroborane 57 is a highly reactive hydroborating reagent <990L315>. 

In addition to these examples, the late transition metals such as ruthenium, rhodium, and iridium have shown their effectiveness in catalyzing the PKR. In 1997, two groups independently showed that [Ru3(CO)i2] can catalyze the PKR. The group led by Murai reported the conditions that employ dioxane as a solvent " another group led by Mitsudo employed DM AC as a solvent." Both conditions required high pressure of CO (10-15 atm) and the scope is limited to the disubstituted alkynes. 

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