Dithianes, cyclic

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

Propenyl)-1,3-dithiane, after lithiation and addition of zinc chloride, reacts with ethyl 2-oxopropanoate to give preferentially the. vvn-adduct37, which is an intermediate in the synthesis of racemic /ra .s-tetrahydro-2,3-dimethyl-5-oxo-2-furancarboxylic acid. It is assumed, that the ethoxycarbonyl group is brought to a pseudoaxial position in the cyclic transition state by the chelating zinc cation. 

Similarly, only selected cyclic systems containing more than one sulfoxide or sulfone groups have been included and discussed here, primarily in the thietane (i.e. 1,2- and 1,3-dithietanes) and thiane (i.e. 1,2-, 1,3- and 1,4-dithianes) series. The criterion for the inclusion of these multifunctional heterocycles was their contribution to the understanding of the physical properties and chemical reactivity of cyclic sulfones and sulfoxides, and the effects of these groups on either their immediate vicinity or on the behavior of the whole molecule. 

Cyclic dithioketals and acetals represent another important class of sulfur containing chiral auxiliaries, which are available in chiral form by biooxidation. Biotransformations were performed on a preparative scale using whole-cells (wild type and recombinant) and isolated enzyme. Again, enantiocomplementary oxidation of unsubstituted dithianes (linear and cyclic, R = H) was observed when using and CPMOcomo (Scheme 9.28) [211,212]. Oxygenation of functionalized substrates (R = substituted alkyl) with gave preferably trans... 

In 1863 Husemann prepared an intermediate, to which he assigned the formula C2H4S, by the action of sodium sulfide on ethylene bromide. From it he obtained the cyclic dimer, dithiane, by distillation. Mans-feld (1886) reinvestigated the intermediate and concluded that it is a polymer. As a reminder of the significance of the term polymer at that time it is to be noted, however, that Mansfeld suggested the cyclic trimeric formula for the intermediate, which is now known to be a linear polymer. Other polymers prepared similarly by Husemann (1863) include methylene sulfide (—CH2—S—)a and methylene tri-thiocarbonate (—CH2—S—CS—S—) . Neither was recognized as a polymer, and neither has since been investigated from this standpoint. 

The ligand properties of a cyclic dithioether, 1,4-dithiane monosulphoxide (DTMSO), have been studied by physical measurements . The infrared spectra indicate that the metal cation coordinates to the oxygen lone pair electrons of DTMSO. Both infrared and ligand-field spectra show the presence of octahedral ions MfDTMSO) in the compounds M(DTMS0)g(C104) and M(DTMSO)g(BF4) . In the case of M = Cu these ions are distorted from the regular octahedral structure. 

Some cyclic thioacetals have an A-SE2 hydrolysis mechanism,206 as do some 2-aryl-2-methyl-l,3-dithianes, except for the 4-NO2 derivative, which looks more A2-like.207 In 10 vol% dioxane/aqueous HC104 mixtures, reactive 2-aryl-2-phenyl-l,3-dithianes are believed to have an A-SE2 hydrolysis mechanism, whereas the least reactive ones have an A2 mechanism.130 Isothiocyanates are believed to hydrolyze by a mechanism that involves simultaneous proton transfer to nitrogen and attack of water at carbon in a cyclic transition... 

Further evidence for the above-mentioned mechanism of HOMO elevation by group 14 elements is provided by studies of thioethers. The decrease in oxidation potential of silyl ethers as compared to ethers is not realized in the case of a-silylthioethers whereas a-stannyl substituents in thioethers cause a considerable cathodic shift in oxidation potential. Moreover, the effect is geometry-dependent. Values for substituted cyclic dithianes 15 are summarized in Table 21. The difference between Si and Sn in this case is illustrative. The lone nonbonding pair in the 3p orbital of sulfur is much too low in energy compared to... 

Hexafluorophosphate derivatives, such as [M(Me2SO)n][PF6]3 (M = La, Lu, Y), have been synthesized (365), and infrared data show O-bonding of the sulfoxide with ionic hexafluorophosphate groups. Analytical data are incomplete for this series, as decomposition, postulated to be to lanthanide fluorides, occurs. The (CH2)4SO complexes [M( CH2 4SO)t.5][PF6]3 are reported (145), which are 3 1 electrolytes with uncoordinated anions, implying a possible semibridged structure, as previously mentioned. Complexes of other cyclic sulfoxides, including thioxane oxide (146) and trans-1,4-dithiane-1,4-dioxide (147) derivatives of hexafluorophosphate salts have also been prepared. 

Secondary alkyl selenides are reduced by (TMS)3SiH, as expected in view of the affinity of silyl radicals for selenium-containing substrates (Table 4.3) [40]. Reaction (4.23) shows the phenylseleno group removal from the 2 position of nucleoside [50]. Similarly to 1,3-dithiolanes and 1,3-dithianes, five- and six-membered cyclic selenoacetals can be monoreduced to the corresponding selenides in the presence of (TMS)3SiH [51]. The silicon hydride preferentially approached from the less hindered equatorial position to give transicis ratios of 30/70 and 25/75 for the five-membered (Reaction 4.24) and six-membered cyclic selenoacetals, respectively. 

Carbolithium compounds of moderate reactivity open the O—C bond of cyclic sulfate esters or cyclic sulfamidates to produce new C—C links, as shown, for example, in equation 112 for a chiral cyclic sulfamidinate and 2-lithio-l,3-dithiane (380a). More reactive organolithium species, such as n-BuLi and PhLi, yield mixtures of products, probably due to attack on the S atom of the sulfonamido group too °. 

Dithioketals, especially the cyclic dithiolanes and dithianes, are also useful carbonyl-protecting groups. These can be formed from the corresponding dithiols by Lewis acid-catalyzed reactions. The catalysts that are used include BF3, Mg(03SCF3)2, Zn(03SCF3)2, and LaCl3.100 S-Trimethylsilyl ethers of thiols and dithiols also react with ketones to form dithioketals.101... 

Krabbe SW, Spafford MJ, Mohan RS (2010) Bismuth(in) triflate catalyzed allylation of cyclic acetals and dithianes followed by in situ derivatization to generate highly functionalized esters. Org Prep Proced Int 42 363-371... 

Also the preparation of methyltriphenylphosphonium dichromate ((Ph3PMe)2Cr207) and the application of this compound as efficient, inexpensive, stable and mild reagent for coupling a variety of aliphatic and aromatic thiols and aliphatic dithiols to their corresponding acyclic and cyclic disulfides has been published <2006JSF441> 1,2-dithiane was obtained by this procedure in 90% yield. 

The chemistry of chiral 1,3-dithiane 1-oxides, in particular their use as chiral auxiliaries, has been reviewed <19980PP145>. Some further developments in this field are the stereoselective a-alkylation with alkyl halides <1997T13149> or a-hydrazination with di-fert-butyl azodicarboxylate (DBAD) <2000T9683>. The carbonyl group of 2-acyl-l,3-dithiane 1-oxides was also used as an electrophile (Scheme 82). Interestingly, acyclic enolates react with these substrates to give a 95 5 mixture of anti- and ry -adduct, whereas cyclic enolates produce a mixture of anti- and ry -adduct in 8 92 ratio <2000JOC6027>. 

Methylenedithiol was used to construct cyclic meat flavor compounds, such as l,3-dithian-5-one 217 <1998FFJ177>. The reaction of the geminal dithiol with a 1,3-dibromide proceeds with pyridine as base in 44% yield (Equation 81). 

The reactions of perfluorinated cyclic sulfides, e.g. perfluorotetrahydrothiophene, per-fluoro-l,3-dithietane, and perfluoro-l,4-dithiane. proceed in a similar way.98... 

Double silylation is also observed in the reaction of a,/3-unsaturated ketones with a bis(disilanyl)dithiane, resulting in high yields of cyclic silyl enol ethers [Eq. (65)].58 The catalyst for this reaction is a cyclic bis(silyl)pal-ladium(II) bis(ferf-butyl isocyanide) complex. Analogous reactions of ester... 

---