O-thioquinones

Various 1,4-oxathiins have been converted to the sulfoxides in a highly stereoselective reaction. The sulfoxides undergo a thermal retro-Diels-Alder reaction to ot,a -dioxosulrines which can behave both as electron deficient dienes and as dienophiles, affording different oxathiins and dihydrothiopyran 1-oxides, respectively <96T12233>. In like manner, o-thioquinone 5-oxides have been generated and they too exhibit diene and dienophile behaviour <96T12247>. 

The material exploded when the temperature of evaporation of the slurry was increased by adding a higher-boiling solvent. This may have been caused by presence of some of the tautomeric ac/-nitro-o-thioquinone. Such salts are thermally unstable. 

Hetero-Diels-Alder reactions in which o-thioquinones behave as the 4ji component towards pentafulvenes lead to 1,4-benzoxathianes (Scheme 67) <00TL6919>. 

A few examples of hetero-Diels-Alder adducts have been reported [75-81]. A thio-chroman-fused fuUerene adduct was synthesized by the reaction of o-thioquinone with CgQ in o-dichlorobenzene at 180 °C [77]. The obtained cyclic sulfide 84 (Figure 4.4) can be oxidized to the corresponding sulfoxide and sulfone with m-chloroperoxybenzoic acid. Reaction of azadienes with Cgg leads to hetero Diels-Alder adducts such as, for example, 85 (Figure 4.4) [79]. The tetrahydropyrido[60]-fullerene 85 is formed in refluxing o-dichlorobenzene. 

The lower reactivity of the o-thioquinone methide 81 (7.0 x 104 M-1 s 1) compared with the o-l (8.4 x 105 M-1 s 1) in acidic solution contrasts with the much higher reactivity of 81 at neutral pH (Table 1). This may represent the balance between the smaller concentration of the protonated thioquinone methide compared with protoned o-l due to the weaker basicity of o-S (pATa < —3 Table 1) compared with o-S (pKa < —1.7) and the presumably greater intrinsic reactivity of H-81 +. 58 However, the observed effects of sulfur for oxygen substitution on carbocation reactivity have in the past proven very difficult to rationize,130,160 and in the present case are probably not fully understood. 

Benzo[3]thiete is a source of o-thioquinone methides, heterodienes which react with 1,4-naphthoquinones or 1,4-epoxynaphthalenes to give the benzo[ ]thioxanthene system. With the former reagents, the initial products undergo an autooxidation and 6,ll-dihydro-12//-benzo[7]thioxanthen-6,ll-diones result. The 6,11-epoxy adducts which are formed from the epoxynaphthalenes can be dehydrated to 12//-benzo[7]thioxanthenes or converted into the dihydrobenzothioxanthene-6-ol (Scheme 131) <1995JPR379>. 

The photolysis of 4-phenyl-l,2-dithiole-3-thione (71) gives a dimer (72) (73TL1561), but benzo-l,2-dithiolane 2,2-dioxide (73a) gives an o-thioquinone methide (74) (78JOC3374). 

Benzo-l,2-dithiole-3-thione (77b) also reacts, but the monoadducts corresponding to (95) have o-thioquinone methide structures (cf. structure 104) and form diadducts rapidly. The use of dehydrobenzene (benzyne) gives 2-thioacylmethylenebenzo-l,3-dithioles (95 R3, R4 = (CH=CH)2). 

Dithiins are formed regioselectively and in high yield when fused pentathiepins 48 are treated with an electron deficient alkyne and PhjP at room temperature the stepwise loss of S atoms leading to an o-thioquinone is proposed <06OL4529>. 

The cycloaddition reaction of o-thioquinones with acyclic 1,3-dienes is a finely tuned process that can involve either reagent behaving as the diene component. It has now been established that formation of a 2-spiro-linked thiopyran by a [2-1-4] reaction is kinetically controlled and that the [4-1-2] alternative leads to the thermodynamic product, a 1,4-benzoxathiin (Scheme 31). Cyclic dienes yield only the oxathiin <03T5523>. Incorporation of the thionocarbonyl diene unit into fiiran and pyran rings and reaction with carbohydrate glycols leads to tricyclic oxathianes, desulfurisation of which yields 2-deoxydisaccharides <03T4249>. 

A series of stabilized and isolable o-thiobenzoquinone methides and related o-thioquinone methides, initially prepared by the photochemical addition of olefins to 1,2-benzodithiole-3-thione [Eq. (15)],33 34... 

Aromatic phthalimidosulfenylationr The reaction usually takes place at room temperature. No catalyst is required for activated arenes. From the products, a synthesis of o-thioquinones is developed. 

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