Thioquinone

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. 

Phenols79 react with phthalimidesulphenylchloride (PhthNSCl) to yield N-thiophthalimides. These when treated with tertiary amines afford unstable ortho-thioquinones. Base-catalysed CN-elimination from camphoryl thiocyanate yields the a n -l,3-dithietane as the sole stereoisomer via thioketone80 (Scheme 9). 

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. 

A related cycloaddition approach of t>/X t>-thioquinone methides promoted by Lewis acids has also been reported <2005SL469>. 

This monograph covers the period 1989-1995 and is partially an update of the work by Schaumann1. The scope is essentially the same, attention being focused primarily on thioaldehydes (thials) and thioketones (thiones). In keeping with the spirit of Reference 1, only some new developments in the field of thioketene, hetero cumulene and thioquinone chemistry are considered. Because of the fast evolution in the field, the spirit is somewhat different and physicochemical and theoretical topics are emphasized. Whenever possible, systematic comparisons with homologous carbonyl compounds, a natural yardstick, are carried out. 

In spite of many synthetic efforts, thioquinones 49 still remain very elusive and only the anthraquinone member 51 has been isolated as a stable compound232. In this case, the reason for this is the higher stability of the cyclic forms, benzothietes, 50 (see Section II). 

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>. 

Heating the benzobisthiete 486 in the presence of ( )-l,2-bis(4-hexyloxyphenyl)ethene affords the 2 1 adduct 487 <1994AGE465>. Similar treatment of 486 with an epoxynaphthalene produces the dithiaheptacene 488 (Scheme 175) <1997LA1173>. The stepwise opening of the four-membered rings to thioquinone methides operates. 

Ortho-thioquinones are also accessible by this route [238, 239]. They were treated with glycals as dienophiles with success (Table 5, entry 2). These reactions were designed [240] to offer an original glycosyl transfer, by achieving a Diels-Alder reaction with the appropriate a,adioxothione (Table 5, entry 4), and a simple Raney nickel reductive desulfurisation of the cycloadduct. 

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). 

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