1.3- Oxathiane function

A-Aryl ketenimines bearing a 1,3-oxathiane function at the ort/j< -position of the keteniminic nitrogen atom convert into spiro[l,3-oxathiane-2,4 (3 H)quinolines] under thermal treatment in a two-step sequence involving a [1,5]-// migration followed a 6 r-electrocyclic ring closure (Scheme 35)." ... 

Another type of heterocycle containing two heteroatoms susceptible of being cleaved reductively are 1,3-dioxanes or 1,3-oxathianes 422. They were treated with lithium and a catalytic amount of DTBB (4.5%) in THF at room temperature (Y = O) or at —78 °C (Y = S) to yield, after hydrolysis with water, functionalized homobenzylic alcohols 425 (Scheme 119) . The participation of intermediates 423 and 424 has been postulated in order to explain the obtained results. 

Density functional theory (DFT) calculations employing sum-over-states DF perturbation theory were applied to calculate both the H and chemical shifts of 1,3-dioxane, 1,3-oxathiane, 1,3-dithiane, and the parent cyclohexane <1997JMT(418)231>. Both normal and anomalous trends in the H chemical shifts could be reproduced well and. 

Various 2-functionalized-l,3-oxathianes have been prepared from 1,3-thioalcohols by a combined SNV/Michael addition sequence using (Z)-l,2-bis-phenylsulfonylethylene (BPSE) as Michael acceptor. The yields were in the range of 72-90% for aliphatic 1,3-thioalcohols and somewhat lower for 2-hydroxymethyl-substituted aromatic thiols (33%) (Equation 90) <2003TL5723>. 

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 products obtained by the action of peracids upon 3-alkoxy-3,5-dienes are dependent upon the reaction conditions. Aqueous organic solvents and the gradual addition of peracid favour the formation of 6 -hydroxy-4-en-3-one, but when an excess of peracid is added in one portion the product, obtainable in high yield, is the unsaturated aldehyde-ester (393) which has also been obtained (as ethyl ester) by photochemical oxidation of the oxathian (394) followed by a reductive desulphuration. In non-polar solvents a 1 1 adduct (395) is formed between m-chloroperbenzoic acid and 3-acetoxy-3,5-dienes which is sufficiently stable (in the presence of a 17-ketone function) to be isolated and acetylated to the 6 -acetate or oxidised by chromium trioxide in pyridine to the b-ketone. ... 

Chiral thiols 59 were derived from 10-camphorsulfonic add by reduction of the sulfony] chloride with lithium aluminum hydride, yielding a 1 4 mixture of enrfo/exo-diastereomers, which can be separated by chromatography60. Containing a hydroxy and a thiol functionality, 59 can be used for the formation of sulfur-containing heterocycles such as oxathianes which can react with carbanions (Section D.l.3.4.4.) or form enolates (Section D. 1.5.3.4.2.1.) and can direct osmium tetroxide catalyzed dihydroxylations (Section D.4.4.). 

We found that PhSCH2OCH3 can be lithiated completely within 45 min at temperatures between —40 and — 45 °C using n-BuLi in THF and hexane. Subsequent functionalizations gave high yields. When during the lithiation or derivatization reaction the temperature was allowed to rise above — 20 °C, however, reduced or low yields of impure products were obtained. Since reactions with most electrophiles can be completed within 1 to 2 hours at temperatures below — 40 °C, the lower stability of PhSCH(Li)OCH3 is not a serious drawback in syntheses performed on a small or moderate scale. Problems may arise if first an alkyl chain is introduced and the methyne proton in the product PhSCH(Alkyl)OCH3 is to be replaced by lithium. 1,3-Oxathiane has been incidentally used as a substrate for lithiation-functionalization reactions [4]. 

Whereas thioanisole, CH3SPh, is reported to give a mixture of ring-metallated and side-chain-metallated products upon interaction with butyllithium, the 0,S-acetal CH3OCH2SPh is lithiated only on the methylene carbon atom by BuLi in THF or sec-BuLi-TMEDA in THF [1-3]. Analogous metallations have been realized with 1,3-oxathiane [4]. The obtained lithium compounds have a limited thermal stability [5], so functionalizations with alkyl halides and epoxides, which are usually less fast than reactions with other electrophiles , give reduced yields. The 0,S-acetal PhSCH2OCH3 can be lithiated in a reasonable time with butyllithium in a THF-hexane mixture but the temperature has to be kept below — 40 °C to prevent decomposition of the lithiated intermediate into PhSLi and other (unidentified) products [5],... 

Regioselective one-pot reactions of nitromethane with isothiocyanates and oxiranes provided a series of functionalized 1,4-oxathianes in good-to-exceUent yields (Scheme 86) (14SL1839).The synthesis of a single example of a 1,3,5-dioxathiane resulted from the adamantanethione with two... 

The unusual branched thiosugar derivative (42) was produced by Pummerer rearrangement of the functionalized sulfoxide (41), which was formed by condensation of cw-2,6-dihydroxy-1,4-oxathiane with L-menthyl cyanoacetate, followed by acetylation and oxidation. The preparation of 5-thiolactose and 3-0-fucosyl-5-thiolactose by use of glycosyltransferases is covered in Chapter 3. 

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