1.3- Oxathian-2-ones

The crystal structures of [Ag(l,4-dioxane)]AsF69 9 and [Ag2(l,4-oxathiane)](N03)295° have been reported. Two adducts of the 4-nitropyridine A-oxide with silver nitrate have been characterized, one is a mononuclear tetracoordinated compound [Ag(N03)(0NC5H4N02)2] and the other is a dinuclear pentacoordinated species [Ag2(N03)2(/u-0NC5H4N02)2(0NC5H4N02)2] (130).951,952 In the mixed compound /nmv-[RhCl2(py)4]N03-AgN03 exists the dinitroargentate(I) ion in a distorted tetrahedral environment 953... 

In the reactions which we have just discussed, the initial reaction involves only the carbonyl group. We therefore looked next at reactions of tetrachlorobenzyne with simpler aldehydes. It is noteworthy that o-carboxybenzenediazonium salts are known to yield 1,3-benzodioxan-4-ones, for example, (129) with carbonyl compounds 166>, and 2,2-diphenyl-3,l-benzo-oxathian-4-one (130) with thiobenzophenone 167>. These products are presumably formed via reactions with the intermediate (128). 

Benzyltriethylammonium tetrathiomolybdate, [BnNEt3]2MoS4, is a useful sulfur transfer reagent which has now been shown to effect a one-pot, three-step sequence of S-transfer— reduction—Michael addition. The intramolecular version successfully produces 1,4-oxathianes from the bromoenone 108 (Scheme 65). In like manner, the bicyclo[3.3.1]nonane skeleton incorporating a thiopyran ring can be constructed . 

S)-Ethyl-6-methyl-1,4-oxathian-2-one 2,4,6-trinitrobenzenesulfonate S J. Chem. Soc., Chem. Commun. 1975,629... 

Similarly, using either sulfuric acid, the SOs/dioxane complex, or a solution of SO3 in chloroform/dioxane, 4,6-diphenyl-l,2-oxathiin 2,2-dioxide was obtained from phenyl acetylene <1999RJ0415>, 3,6-disubstituted-l,2-oxathiane 2,2-dioxides were obtained from allylphenol <2002RJ01210>, and 3,4-dihydro-6-phenyl-l,2-oxathiin-4-one 2,2-oxide was obtained from Ph-CO-CH2-COMe <1991CIL253>. 

Reports on the new syntheses of six-membered ring systems with two oxygen and/or sulfur atoms in 1,2-positions are rather limited and are covered already in Section 8.10.9. Often, only one really successful synthetic path has been described or the derivatives obtained were simply by-products. Thus, a comparison of various synthetic strategies for obtaining certain dioxane/oxathiane/dithiane derivatives is not meaningful. 

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. 

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. 

The solid-state structure of one benzo derivative of 1,3-oxathiane 96 has been studied the heterocyclic moiety was found to adopt a half-chair conformation with the aryl substituent in an equatorial position <1998JFA4002>. 

One of the rare examples of an intramolecular ring expansion leading to a 1,3-oxathiane is depicted in Equation (97). The sulfurane precursor 235 thus upon heating rearranged to 1,3-oxathiane 236 <1996JA697>. 

The cyclodextrin-supported cleavage of 1,3-oxathianes with IBX (cf Scheme 76, Section 8.11.6.4.6) <2006SC3771>, as well as the copper-catalyzed aminolysis of 1,3-dithianes <2006OL2547> has been published. l,3-Dioxane-2-ones readily undergo a Grob fragmentation (Equation 100) <2006CC4303>. This reaction is catalyzed by Ni- (24-99% yield) or by Pd-complexes (42-93% yield). 

One of the main research interests in this area is to find new oxidizing agents having the highest chemoselectivity to obtain the sulfoxide derivatives without overoxidation to sulfones. 1,4-Oxathiane 18 was often used as a model sulfur-containing compound to afford 82 (Table 2) and 17 similarly gave 83. m-2,6-Dimethyl-l,4-dithiane was oxidized to the... 

In a similar way, 1,4-oxathian-2-one and l,4-dioxan-2-one were obtained from the condensation of thioglycolic acid and glycolic acid, respectively <1979J(P1)1893, 1998T11445>, and from palladium-catalyzed <1999JOC6750> or oxoammonium salt <2004JOC5116> oxidations of 2,2 -thiodiethanol or diethylene glycol. Asymmetric syntheses... 

Synthesis of 2,3-dihydro-l,4-dithiin 11 was accomplished from l,3-dithiol-2-one 247 in the presence of dibro-moethane and potassium hydroxide <1998JOG3952>, while reaction of 2,3-dichloro-l,4-dioxane with powdered Zn in hexamethylphosphoramide (HMPA) was used for the synthesis of 1,4-dioxene 10 <1998JPP10067773>. To obtain substituted 1,4-oxathianes, the hydrogenation of the corresponding partially saturated compounds has been employed <2001J(P1)2604>. 

Six-membered non-aromatic heterocycles containing more than one heteroatom Morpholines, 1,4-dioxanes, 1,4-oxathianes Six-membered aromatic heterocycles containing one heteroatom Pyridine... 

Chemical shift trends for the remaining protons in the 1,3-series and related compounds can be deduced from the representative data given in Table 6. The rules outlined above also satisfactorily accommodate the data for methylene groups adjacent to just one heteroatom in the 1,3-dithiane the opposing effects of the j3 C—C and C—S bonds cancel whereas in the 1,3-dioxane the effect of the C—C and C—O bonds reinforce. At the C-5 methylene group, the axial proton is to lower field of the equatorial proton in the 1,3-dioxane and 1,3-oxathiane but to higher field in the 1,3-dithiane. +... 

In addition, the X-ray crystal structures of two 1,4-oxathiane derivatives were published (00AX1510, 01AX560) the di-axial conformation of the hydroxymethyl group in position 3 and of the l,9-dihydro-6//-purin-6-one group in position 6 in compound 82 is surprising (01AX560) (cf. Scheme 30). The X-ray structures of the sulfones of the isomeric 2,6-di-OEt-3-OMe-l,4-oxathianes were published (00CEJ1858) the 4,4-dioxa-... 

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>. 

Efficient syntheses of tetrahydrothiopyran-4-one and 3,6-dihydro-4-trimethylsilyloxy-277-thiopyran 58 from dimethyl 3,3 -thiobispropanoate have been described <07S1584>. Carbene insertion into a C-H bond of diazosulfones leads to tetrahydrothiopyran sulfones 59 with stereoselectivity for the trans disposition of the 2- and 3-substituents. When applied to diazosulfonates, the corresponding 1,2-oxathianes are produced <07OL61>. 

The reaction between arylpropiolates and 1,2-mercaptoalcohols is catalysed by BU3P which initiates a-S-addition to the triple bond and the umpolung adduct then cyclises to give 3-arylidene-l,4-oxathian-2-ones 70 <07OL3925>. 

---