1.4- Oxathiin-2-ones

A theoretical study by DFT and MP2 of the dimerization of thioformylketene 50 has been reported <20050L5817>, wherein both [4-1-2] and [4-1-4] pathways were considered. Interestingly, the barriers for [44-2] cycloadditions, for example, in formation of the 1,3-oxathiin-one derivative 51 (Equation 2), are low and the dimerizations are sufficiently exothermic that they are not expected to be in equilibrium with 50 at room temperature. 

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>. 

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 fully unsaturated compounds containing two oxygens (e.g., 1,4-dioxin 61), two sulfur atoms (e.g., 1,4-dithiin 62), and one each of oxygen and sulfur atoms (e.g., 1,4-oxathiin) are electron-rich and highly reactive toward electrophiles. Benzo derivatives are usually most reactive in the fused heteroring (84M113). 

Obviously, one can not precisely quantify the elements of Figure 8. It is nevertheless highly indicative. Resistance of a practical nature is rarely, if ever, encountered in normal field practice by the sulfenimides (or for example, the dithio-carbamates, Figure 9) - insoluble, multisite protectives. The water soluble single site and variably translocatable antibiotics, benzimidazoles, oxathiins, etc., etc., all exhibit degrees of field resistance. As far as this author knows, this analysis can be extended over the whole class of plant protection fungicides. 

Adamantane-2-thione undergoes a concerted regioselective cycloaddition reaction with alkynoic acids to give 2-spiro-linked l,3-oxathiin-6-ones (Scheme 32). When applied to thiofenchone, the reaction also appears to be stereospecific <03EJO3727>. 

Indenoll,2-e]-l,4-diazepin-6-one, 838, 976 Indeno[2,l-[Pg.772]

Dithiins can also be generated by cyclization of a five-atom fragment and a one-atom unit. 1-Alkynylethynyl sulfides react with sodium sulfide in methanol to afford monosubstituted 1,4-dithiins <86SR123>. Likewise, dipropynyl sulfone is transformed conveniently into 3,5-dimethyl-1,4-oxathiin... 

The monobenzo-fused derivatives of 1,4-dioxin, 1,4-oxathiin, and 1,4-dithiin can all be prepared by routes in which the first step is a base-catalyzed reaction between the appropriate 1,2-disubstituted benzene and an a-haloketal. The product is a 2-alkoxy-2,3-dihydro derivative from which the unsaturated heterocycle is obtained in one or more steps <84CHEC-I(3)943>. 

A synthesis of annulated 1,4-oxathiin-2-ones involves an anionic rearrangement under directed metalation conditions as the key step. A,A-Diethyl o-methylsulfanylaryl carbamates, readily available from phenols, are deprotonated at the SMe group which initiates rearrangement to a thioacetamide from which the oxathiinone can be obtained (Scheme 31) <0415215>. 

Dihydro-l,4-benzodithiines and -oxathiines. A remarkable one-pot synthesis of these heterocycles from cyclohexanones and ethanedithiol or 2-mercapto-ethanol is mediated by NBS at 0°C. 

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