Dioxane, 1,4-Dithiane, and 1,4-Oxathiane

In this chapter, the structures and chemistries of 1,3-dioxins, 1,3-oxathiins, and 1,3-dithiins are described, including both their fully saturated forms (1, 7, and 13) as well as their benzo analogs (6, 11, 12, and 17). The formally fully unsaturated monocyclic structures (4, 9, 10, and 16) contain only one endocyclic double bond with further unsaturation being accomodated by exocyclic double bonds (2, 3, 5, 8, 14, and 15), for example, by the introduction of a carbonyl group. Well known and intensively studied are the Meldrum s acid derivatives 18 and 19. In addition, 1,3-dioxane, 1,3-oxathiane, and 1,3-dithiane moieties can be part of spiro structures as well as hi- and tricyclic analogs. And finally, both the structures and chemistries of the corresponding sulfoxides and sulfones are also reported. 

The fluorination of oxathiane is described in Section 25.1.1.3.84 1,4-Dithiane reacts quite differently107 from dioxane and oxathiane over potassium tetrafluorocobaltate(III). The major products (ca. 60 % of the product mixture, itself in ca. 50 % yield) are the result of rearrangement to the 2-methyl-l, 3-dithiolane skeleton, e.g. 1-3. Fluorinated 1,4-dithiancs comprised about 30% of the product the major product is 4. The extent of fluorination, that is the number of fluorine atoms introduced, is much greater than that of dioxane under comparable conditions (tetrahydrothiophene and tetrahydrofuran show a similar contrast). 

Saturated rings like 1,4-dioxane 16, frequently used as a solvent, 1,4-dithiane 17, and 1,4-oxathiane 18 were first prepared at the turn of the twentieth century. 

Routes to benzo-fused derivatives of 1,4-dioxanes, 1,4-oxathianes and 1,4-dithianes make use of anions or dianions of the appropriate 1,2-disubstituted benzene. An alternative approach to the synthesis of 1,4-benzodioxanes involves Diels-Alder addition reactions of alkenes across the quinone function of 1,2-benzoquinones, e.g. (352) — (353). 

The chemistry of almost all of the numerous syntheses of 1,4-dioxanes, 1,4-oxathianes and 1,4-dithianes utilizes the nucleophilicity of negatively charged or neutral oxygen and... 

Competing dissociation and isomerization pathways in energy-selected ions of butene, pentene, 1,4-oxathiane, 1,4-dithiane, and dioxane A —> X Emission spectra of jet-cooled penta-1,3-diyne cations and deuteriated analogues. Vibrational frequencies and spin-orbit splittings A — X Emission spectra of jet-cooled CHjC C—C CCHJ and its [ Hj]- and analogues... 

The reactivity of these heterocycles towards hydrogen atom abstraction at C-2 has been shown to increase in the expected order, from 1,3-dioxane, through 1,3-oxathiane, to 1,3-dithiane. Hydrogen atom abstraction from C-2 in 4-methyl-1,3-dioxane, 2,4-dimethyl-1,3-dioxane, and 2-ethyl-4-methyl-1,3-dioxane has also been investigated <77MI 608-01 >. The radical-mediated alkylation of 2-stannyl-l,3-dithianes (67) with alkenes and enol ethers has been observed (Equation (30)) <92CL1229>. 

Dioxane absorbs at 180 nm and is thus transparent in the part of the UV region accessible to normal spectrometer systems. Replacement of the ring oxygen atoms by sulfur causes a batho-chromic shift, and both 1,4-oxathiane and 1,4-dithiane show absorption above 200 nm. 

Alkyl-l,3-dioxane derivatives are acetals and are generated in an equilibrium mixture from 2-alkylpropan-1,3-diols, 4.1, and aldehydes (Scheme 4) and the yield of product is optimized by azeotropic removal of water from the reaction mixture. A cis-/trans-mixture of products, with the tranj-isomer predominating, is obtained and repeated recrystallization gives the pure traus-products 4.2 in 50-60% yield [55-57]. The 1,3-oxathiane, 4.3, [58, 59] and 1,3-dithiane, 4.4, [57, 59-61] analogues of 1.3, dioxanes... 

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