Thiocarbonyl 5-oxides , cycloadditions

Thiocarbonyl oxides are a subject of active investigation. The natural occurrence of sulfines and related compounds in plants of the genus Allium (onion, garlic, etc.) is included in a superb and extensive review by Block [91]. Two detailed papers [92, 93] report the isolation of zwiebelanes from onions and their chemical synthesis involving intermediate sulfines produced by oxidation of di-l-propenyl disulfide, subsequent sulfoxide accelerated [3.3] sigmatropic shift and the [2+2] cycloaddition of the C=S and C=S=0 moieties. A further article [94] provides a great deal of information on the mechanism of formation of (Z)-propanethial S-oxide, the lachrymatory factor of the onion, as well as its chemical synthesis and reactions. Techniques of analysis of the volatiles of onions have been further improved [95]. 

I.3.4.2.5. Carbonyl and Thiocarbonyl Compounds a-(Hydroxyimino)phenyl-acetonitrile oxide (generated in situ at room temperature from PhC( NOH)C ( NOH)Cl in the presence of NaHC03 or Et3N) reacts with simple aldehydes and ketones R1R2CO to give 1,4,2-dioxazoles 180 (347). Related dioxazoles, formed by cycloaddition of benzonitrile oxide to aromatic aldehydes, upon treatment with I-BuOK, undergo cyclo-reversion, allowing direct conversion to substituted benzoic acids or their esters (348). 

A-Sulfinylamines (R—N=S=0) are known to function as reactive dienophiles and dipolarophiles, and some examples of [3 + 2] cycloaddition with thiocarbonyl ylides have been reported (176). For example, the reaction of thiobenzophenone (5)-methylide (16) with both A-phenyl and N-tosylsulfinylamines occurs regiose-lectively to give 1,3,4-dithiazolidine 3-oxides (135). In the case of thiocarbonyl ylide 69, reaction with N-phenyl sulfinylamine selectively afforded the analogous product 136 (R = Ph). However, the corresponding reaction with Al-tosyl sulfinylamine resulted in a mixture of the N,S-adduct (136) (R =Tos) and the 0,S-adduct 137. Formation of a mixture of products is compatible with a stepwise reaction via a zwitterionic intermediate. 

A cycloreversion mechanism is suggested for the transformation of the nonisolable cycloadduct 90 to the aldehyde 91 and isothiocyanate 92 <1996BCJ719> and for the spiro-1,4,2-oxathiazole intermediates 94 to the dioxothiazoline 95 and the aryl isothiocyanate 92 <2001MOL510>. Both cycloadducts are obtained by cycloaddition reactions of nitrile oxides 88 to thiocarbonyl compounds (Scheme 12). 

A precursor of a thiocarbonyl ylid, a dihydrothiadiazole, was reacted [259] with a sulfine, thiobenzophenone S-oxide, to lead mainly to a dithiolane S-oxide, the formation of which was rationalised through a 1,3-dipolar cycloaddition of the ylid with the C=S bond of the sulfine. It is interesting to note that an opposite regioselectivity was observed for thiofluorenone S-oxide. 

Cycloaddition between thiocarbonyl derivatives as hetero dipolarophiles and various 1,3-dipoles provides easy entry to five-membered thiaheterocycles. The reaction of thiobenzoyltrimethylsilane lb and thiobenzoyltriphenylsilane la with benzonitrile oxides, diphenylnitrilimine, and benzonitrile-4-nitroben-zylide gave regiospecifically 5H-1,4,2-oxathiazoles 12a,b, 1,3,4-thiadiazoline 13, and 4,5-dihydrothiazole 1430 (Scheme 11). The regiochemistry of this reaction was assigned through the protiodesilylation of the adducts (vide infra). 

Reaction with thiocarbonyl compounds. The thiocarbonyl compounds obtained by photochemical oxidation of phenacyl sulfides can be trapped efficiently by a 1,3-dipolar cycloaddition with 1 to give 2. This heterocycle can be cleaved to carbonyl compounds by Bu4N F or (CjH5)3N HF. This process is more efficient and more general than photolysis of phenacyl sulfides in the presence of oxygen. 

Cycloaddition of Sulfur Monoxide or Thiocarbonyl S-Oxides to Unsaturated Compounds... 

Cycloaddition of thiocarbonyl 5-oxides was first observed in the reaction of dichloro-methanethione 5-oxide with diazo compounds77. This reaction initially gives an unstable 2,5-di-hydrothiadiazole 5-oxide which undergoes elimination of nitrogen to give three-membered cyclic sulfoxides 3. 

Thiocarbonyl 5-oxides behave as dienophiles and undergo cycloaddition with 1,3-dienes to form cyclic sulfoxides (cf. Houben-Weyl, 4th ed., Vol. Ell, p 940). Thus, dichloromethanethione 5-oxide and cyclopentadiene react exothermically at — 40 °C to form a mixture of unstable diastereomeric 3,3-dichloro-2-thiabicyclo[2.2.1]hept-5-ene 2-oxide (7)77. 

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