Dihydro-1,4-dioxin

The vibrational spectrum of 1,4-dioxin was studied at the MP2 and B3-LYP levels in combination with the 6-3IG basis set [98JST265]. The DPT results tend to be more accurate than those obtained by the perturbational approach. The half-chair conformation of 4//-1,3-dioxin 164 was found to be more stable than the corresponding conformations of 3,4-dihydro-1,2-dioxin 165,3,6-dihydro-1,2-dioxin 166, and of 2,3-dihydro-1,4-dioxin 167 (Scheme 114) [98JCC1064, 00JST145]. The calculations indicate that hyperconjugative orbital interactions contribute to its stability. 

Most of the examples previously reported involve the cycloaddition reaction or photooxidation of 2,3-dihydro-1,4-dioxin <1996CHEC-II(6)447>. A more recent Diels-Alder cycloaddition was done between the dimethylidenedi-hydrobenzodioxin 109 and the reactive 1,2-dibromocyclopropene. Reaction of the cyclopropene (generated in situ... 

Dimethylthexylsilyl trifluorometh-anesulfonate, 74 using other methods Allyl chloroformate, 9 Benzyl trichloroacetimidate, 32 Bromodimethylborane, 47 Chloromethyl ethyl ether, 75 2,3-Dihydro-1,4-dioxin, 112 p-Methoxyphenol, 181 of aldehydes and ketones as acetals or dithioacetals... 

The kinetics and mechanism of the acid-catalysed hydration of dihydro-1,4-dioxin have been reinvestigated. The solvent isotope effect (A h+Ad+ = 2.2) indicates that the reaction proceeds by a rate-determining proton transfer from the catalyst to the substrate68 rather than by a pre-equilibrium mechanism. 

Dioxins, 1,4-oxathiins, and 1,4-dithiins are commonly prepared by elimination reactions from saturated analogues (see Section 4.3.4.1.4). A convenient synthesis of 2,3-dihydro-1,4-dioxins (e.g., 442) starts from propargyl chloride and 1,2-ethanediol (Scheme 206) . Another approach to substituted 2,3-dihydro-... 

In this regard, clear chemical and spectroscopic evidence for the disproportionation of the intermediate radical cations, photochemically and/or thermally generated, were achieved on 2,3-diphenyl-5,6-dihydro-1,4-dioxin, and derivatives 22a - c [90, 111, 57-159]. In fact, the 2,4,4,6-tetrabromo-2,5-cyclohexadien-l-one (TBCHD)-sensitized photooxygenation of 22a affords the corresponding 1,2-ethanedioldibenzoate, 67, the cleavage product of the intermediate 1,2-dioxetane, 24a, together with minor amounts of 4a,8a-diphenyl-2,3,4a,6,7,8a-hexahydro-p-dioxino[2,3-b]-p-dioxin 68 [158] ... 

Interestingly, as reported in the previous section, two unconnected mechanisms seem to operate in the TBCHD-sensitized photooxidation of several substituted 2,3-diaryl-5,6-dihydro-1,4-dioxins 22a-c, and substituted 2,3,5,6-tetraphenyl-1,4-dioxin, 64a-d, yielding, as major products, 1,2-ethanedioldibenzoates 24a-c, and (Z)-stilbenediol dibenzoates 66a-d respectively, together with variable amounts of the corresponding a-diketones [90, 111] (Eqs. (27)]. 

In contrast to dihydropyran rearrangements, the rearrangement of 1,4-dioxins is believed to proceed via a chairlike transition state with the substituents in axial positions. Dihydro-1,4-dioxins 10 and 11, obtained by acid-catalyzed double-bond isomerization of 9. on heating in a sealed tube, undergo a sigmatropic shift to give dihydropyrans 12 and 13322 323. 

Most partially saturated ring systems are well investigated, with 2,3-dihydro-1,4-dioxin (10),... 

In 2,3-dihydro-1,4-dioxin, -1,4-oxathiin, and -1,4-dithiin derivatives and their benzo analogues, the values of the vicinal proton coupling constant ratios and ring dihedral angles were determined and found to be consistent with half -chair conformation <820MR(18)92>. 

Dioxin is considered to have a greater stability towards dilute acid than vinyl ethers or 2,3-dihydro-1,4-dioxin but otherwise it behaves as a typical unsaturated ether. The 2,3,4,5-tetraphenyl derivative (19) reacts with two equivalents of tris(p-bromophenyl)aminium tetrafluoroborate in a dichloromethane/ethylene glycol mixture to give the tra 5-diaxial bisketal addition product (35) (Equation (4)) <87S947>. 

The cyclic vinyl ether 2,3-dihydro-1,4-dioxin is converted into its cyclic hemiacetal hydration product, tetrahydro-2-hydroxy-1,4-dioxin, in aqueous solution by an acid-catalyzed reaction <870K2746, 89JP043). Treatment of an alcohol with excess of 2,3-dihydro-1,4-dioxin at room temperature in the presence of copper(II) bromide in tetrahydrofuran leads to the corresponding acetal. This new protective group for alcohols, which is stable towards lithium aluminum hydride and organolithium reagents, can be removed by treatment with acidified aqueous methanol <85S806>. 

Cyano-5,6-dihydro-1,4-dioxin reacts with bromine to afford the corresponding dibromide deriva-... 

The treatment of one equivalent of 5-lithio-2,3-dihydro-1,4-dioxin with 0.5 equivalent of copper(I) cyanide solubilized as its lithium chloride (0.5 equiv.) complex at — 15°C affords the corresponding cyanocuprate. The reactivity of this cuprate was assessed by its conjugate addition to cyclic enones, and by nucleophilic epoxide opening the presence of boron trifluoride etherate led to enhancement... 

Dioxacyclohexyne (5,6-didehydro-2,3-dihydro-1,4-dioxin) has been generated at low temperature in tetrahydrofuran solution from 5,6-dibromo-2,3-dihydro-1,4-dioxin, via 5-bromo-6-lithio-2,3-dihydro-1,4-dioxin, and isolated as the corresponding cyclotrimer (97) <88CC942>. 

Metal-assisted decompositions of a-diazocarbonyl compounds in 1,2-dialkoxy-l-alkenes give dialkoxycyclopropylcarbonyl systems. Thus, reaction of 2,3-dihydro-1,4-dioxin with ethyl diazoacetate at 80°C over copper bronze yielded the cyclopropane (111) acid catalyzed solvolysis of this with water, and with ethanol, gave hemiacetal and acetal esters (112) and (113), respectively <85JOC4681>. 

Reaction of 2,3-dihydro-1,4-dioxin with ethyl diazopyruvate over (trimethylphosphino)copper(I) chloride produces dihydrofuroic ester (114) and a similar reaction with dimethyl diazomalonate, followed by acid hydrolysis, afforded the lactone ester (115) <85JOC4681>. 

Paterno-Biichi cycloaddition of 2,3-dihydro-1,4-dioxin to carbonyl compounds yields the expected compounds but these adducts have not yet been examined further <85JA3819>. 

Cycloaddition of diphenylketene to 2,3-dihydro-1,4-dioxin (10) has been described. The resulting cyclobutanone (119) is transformed almost quantitatively to a ketone (120) on heating with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (Scheme 3) <89H(28)52l>. 

The preparation of cyclobutanone (123) via the [2 + 2] cycloaddition of ketene (121), and the photolytic reaction of chromium carbene complex (122) with 2,3-dihydro-1,4-dioxin (10 has also been described <92TL927>. 

Cycloaddition of 2,3-dihydro-1,4-dioxin (10) to 1,1-dichloroketene, generated from zinc and trichloroacetyl chloride by sonication, leads to 7,7-dichloro-2,5-dioxabicyclo[4.2.0]octan-8-one (124), which upon acid hydrolysis affords semisquaric acid (125) <90S583>. 

---