1.2- Oxathianes

2-Phenyl-l,3-oxathiolanes undergo a ring expansion on treatment with diazoacetates and this reaction becomes of synthetic value when a diazo(triethylsilyl)acetate is used in the presence of a Cu(ll) complex 1,4-oxathianes are produced as a mixture of diastereoisomers 

The thermolysis of P-allyloxy and P-propynyloxy /-butyl sulfoxides, accessible from /-butyl methyl sulfoxide, generates a sulfenic acid which forms 1,4-oxathiane iS-oxides as two diastereomers by intramolecular attack of the S atom at the unsaturated bond 050BC404 . 

Both enantiomers of the chiral sulfoxide 41 have been polymerised and then treated with triflic acid which protonates the sulfoxide. Electrophilic attack on one side of the adjacent phenylene ring forms a 1,4-oxathiane ring and a helical ladder structure based on fused phenoxathiine rings results 05CL164 . 

05ACR813 M. Isaka, P. Kittakoop, K. Kirtikara, N. L. Hywel-Jones, Y. Thebtaranonth, Icc. Chem. Res. 

05AJC864 A.J. Liepa, O. Nguyen, S. Saubem, Aust. J. Chem. 2005,58, 864. 

Attack of the aryl radical derived from the benzylic thioester 49 at the S atom results in formation of a 1,2-benzoxathiane 2-oxide accompanied by expulsion of an alkyl radical 06AG(E)633 . 

Thiosalicylic acid affords l,3-benzoxathian-4-ones 50 on treatment with imidoyl chlorides 06S3195 . 

Flash vacuum pyrolysis of 2,2-dimethyl-47/-3,l-benzoxathian-4-one 51 results in decomposition to 2-methyl-4Ef-l-benzothiopyran-4-thione in high yield. Homolytic cleavage of the C-2 - O bond followed by ring closure involving one of the methyl groups is postulated 06HCA991 . 

Ethyl 3-arylpropynoates undergo a Bu3P-catalysed addition of mercaptoalcohols which is followed by an intramolecular cyclisation that leads to 2-arylidene-l,4-oxathian-3-ones 52 06OL3925 . 

The directing effect of the sulfoxide function in substituted 1,4-benzoxathiane 4-oxides 53 ensures that their borane reduction to the 2,3-dihydro derivative proceeds in an asymmetric fashion 06S3389 . 

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Oxathiane 101 is readily deprotonated using s-BuLi, and the resulting anion reacts with alkyl halides, ketones, and benzonitrile (85JOC657). The majority of work in this area, however, is due to Eliel and coworkers and has involved chiral 1,3-oxathianes as asymmetric acyl anion equivalents. In the earliest work it was demonstrated that the oxathianes 102 and 103, obtained in enantiomeri-cally pure form by a sequence involving resolution, could be deprotonated with butyllithium and added to benzaldehyde. The products were formed with poor selectivity at the new stereocenter, however, and oxidation followed by addition... 

Besides 1,3-oxathianes, the 1,3-dithiane 1-oxide moiety can be used for directing the nucleophilic addition of an organometallic reagent to a carbonyl group in a diastereoselective manner. The addition of methylmagnesium iodide to the 2-acyl-l,3-dithiane 1-oxide 23A leads exclusively to the diastereomer which is formed by Re-side attack. On the other hand, addition... 

Another chiral auxiliary used in diastereoselective addition reactions is the 1,3-oxazine derivative 4a which shows a close structural resemblance to the 1,3-oxathiane 16 (vide supra). However, in contrast to the oxathiane, 4a cannot be readily acylatcd in the 2-position. Therefore, the benzoyl derivative 4b was prepared by condensing amino alcohol 3 with phenylglyoxal. 

The 1,3-oxathiane 8, derived from (5)-l,2,4-butanetriol, is lithiated to form the equatorial anion 9, which adds benzaldehyde with high induced but moderate simple diastereoselectivity (4 1) to form the alcohols 10 and 1117. The selectivity is enhanced to 7 1 by metal exchange by means of magnesium bromide. Deprotection affords (5)-2-hydroxy-l,2-diphenylethanone with 75% ee. It is expected that the method could be extended to aliphatic aldehydes. 

The lithio-derivative derived from cyclohexyl phenyl sulfone underwent 1,2-addition to cyclohexenylideneacetaldehyde or cinnamaldehyde to give the corresponding / -hydroxysulfones387. Reactions of 2,2-dimethyl-4-lithio-1,3-oxathiane 3,3-dioxide 308... 

The radicals derived by hydrogen abstraction from 1,3-dithianes and 1,3-oxathianes undergo intramolecular addition to ot,P-unsaturated esters and hence facilitate the synthesis of cycloalkanones <96T9713>. 

In a quite different approach, the 1,3-oxathianes 79 undergo a tandem [4++2 ] -eye 1 oaddi ti on -elimination with alkenes yielding 3,4-dihydrothiopyrans a preference for the tis-isomer was noted (Scheme 55), The oxathianes, which are readily synthesised, thus serve as a,P-unsaturated thioaldehydes <00TL371>. 

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. 

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. 

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. 

Ab initio calculations at the MP2 level of theory of a collection of substituted 1,3-dioxanes, 1,3-oxathianes, and 1,3-dithianes have been employed to study both the position of the conformational equilibria and the validity of the Perlin effect <2005T7349>. The 7h,c coupling constant proved to be a valuable tool in conformational analysis both twist conformers, in addition to the chair and alternative chair forms, could be readily identified simply by comparing experimental Vh,c coupling constants to the corresponding calculated values in the particular forms. In addition, the Perlin and reversed-Perlin effects of the C2-H fragments, c in 1,3-dioxanes,... 

Also, the reaction pathways of the Corey-Chaykovsky epoxidation reaction have been compared quantum-chemically <1999JOC4596>. As models for one transition state, 1,3-oxathiane compounds such as 52, suitably substituted to allow comparison with experiment (Equation 3), were calculated and these predicted both the absolute stereochemistry of the main product 53 and the distribution of the other stereoisomers, as supported by experimental results. Thus, this theoretical study was able to identify the transition state which proved to be responsible for the stereoselectivity of the catalytic Corey-Chaykovsky epoxidation reaction. 

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