1.3- Dithiolane-4-thione

This method involving bond formation between a sulfur atom and a carbon atom (Figure 3) has been little used, but the structural requirements are present in the disulfides, formed by reaction of 2-mercaptoacrylic esters, or 2-mercaptobenzoic esters with chloroacetyl chloride. These cyclize in acids to l,2-dithiole-3-thiones (Scheme 15) . The 3-acyldithiopropanoyl chlorides cyclize to 1,2-dithiolane-thiones (Equation (9)) <74IZV700>. 

An original method has recently been developed starting either from 1.3-dithiolane (10) or from 1.3.4-thiadiazole-2-thione derivatives (11) (Scheme 4) (3). This method does not work when Ri is an aryl group. 

C3S2 s s — — — l,2-Dithiolane-4-carboxylic acid 3-phenyl-l,2-dithiolylium iodide 4-methyl-1,2-dithiole-3-thione ... 

Ethynylquinoxalme (144) and bis(isopropoxythiocarbonyl) disulfide (145) gave 2-(2-oxo-l,3-dithiol-3-yl)quinoxalme (146) [l,l -azo(cyclohexanecar-bonitrile), PhMe, reflux, N2, 7 h 77%] somewhat similarly, 2-(acetylethy-nyl)quinoxaline and 4-phenyl-l,3-dithiolane-2-thione gave 2-(5-acetyl-2-... 

Dithiolane isocyanate iminium methylides (55), are a new type of azomethine methylide derived 1,3-dipole, and undergo efficient and regioselective cycloaddition to thiocarbonyls to yield predominandy thiazolidine-2-thiones (56) <96TL711>. 

In the synthesis of l,3-dithiolan-2-ones from spirocyclic intermediates, via episulfides, substituted tetrathiacyclododecane and the related pentathiacyclopentadecane were isolated in good yields <96JCS(P1)289>. Preparation and molecular dynamics studies of 2,5,8,17,20,23-hexathia[9.9]-p-cyclophane have been reported <96P4203>. The syntheses and properties of thiocrowned l,3-dithiole-2-thiones and their conversion to tetrathiafiilvenes via treatment with triethylphosphine have been described <96LA551>. 

Gotthard, however, showed that by lowering the thione concentration the thietane-dithiolane yield ratio could be systematically directed to favor the four-membered heterocycle. A low thione concentration inhibits addition of the intermediate diradical to another thiophenone molecule to produce the dithiane (Scheme 5). 

Other C=S compounds that have proven to be useful for the preparation of 1,3-dithiolanes are dithioesters, (161-163) trithiocarbonates, (161) and 1,3-thiazole-5-(4//)-thiones (164). 

Ueno and Okawara (184) were the first to explicitly formulate a conjugated thiocarbonyl ylide as an intermediate in the reaction of l,3-dithiolane-2-thione with 4-bromophenacyl bromide. The initially formed thiocarbonyl ylide undergoes deprotonation with sodium hydride to give 2-(4-bromophenyl)-l-oxa-4,6,9-trithias-piro[4.4]non-2-ene. 1,3-Diacylated thiocarbonyl ylides of type 149 (Scheme 5.45) have also been proposed as intermediates in the reaction of 1,3-diphenylpropane-1,3-dione with thionyl chloride. This reaction leads to 2,2,4-tribenzoyl-5-phenyl-... 

Secondary thiols.8 A new preparation involves dithiolane cleavage with n-butyllilhium to thiones, which are known to be reduced by the same reagent to sec-thiols by /8-hydride transfer. Yields are 78 90% from saturated thioketals. The mtthod is less useful for preparation of aryl substrates owing to competitive ItlflUltttion of the aromatic ring. 

Diazomethane attacks the thione group of the 3,l-benzothiazine-4-thione (32) to give the bis-(l,3-dithiolane) (33), whereas diazoketones afford 4-methylene derivatives (34) (71BSF187). 

A thioaldehyde complex (93) was formed, although in very poor yield (<1%), when the l,3-dithiolane-2-thione complex 92 was deprotonated with LDA at low temperature, treated with CS2, and finally alkylated with [Et30]BF4 [Eq. (19)]. No reaction took place in the absence of CS2. Studies using 14C-labeled CS2 confirmed that CS2 was incorporated into the complex.62... 

For R = Me the yield was 96% after 40h. l,3-Dithiolane-3-thione (R,R = CH2CH2) was obtained in a 70% yield together with some polymeric material. The yield dropped to 30% for l,3-dithiane-3-thione (R,R=CH2CH2CH2). 

Flash vacuum pyrolysis (FVP) of 1,3-dithiolane 1-oxides (88-90), as a possible route to thiocarbonyl compounds, has been studied.57 Thiobenzophenone and thiofenchone are obtained from (88) and (89), respectively, but there was no evidence of heptane-4-thione formation from (90). A stepwise homolytic cleavage has been proposed. 

The 8 + 2-cycloaddition of 2//-cy c I o h cpta[h ] furan -2 - one (155) with acyclic 1,3-dienes provides a facile route to bicyclo[5.3.0] ring systems (156) (Scheme 61).291 2H-Benzo[Z>]thiete in the o-quinoid form undergoes 8 + 2-cycloaddition with 1,3-dithiolane-2-thione, l,3-dithiole-2-thiones, and adamantanethione to produce 4//-1,3-benzoditliianes.292... 

Benzo[7]thiete 41 reacts in the ortho-qumoiA isomer 41 with cyclic trithiocarbonates such as l,3-dithiolane-2-thione, (ethylene trithiocarbonate), l,3-dithiole-2-thiones, and adamantanethione <1997LA1603>. In boiling toluene, 1,3 dithiolane and dithiole derivatives formed the spiro compound 42 and 43 in good yield. The related l,3-dithiole-2-thiones reacted chemoselectively at the C=S double bond. The use of adamantanethione yielded the polycyclic adduct 44 in a smooth and quantitative reaction (Scheme 3). 

It was shown (78JFC(12)193) that the product of the reaction of perfhioro-2-methylpent-2-ene with potassium ethyldithiocarbamate in dimethylformamide is 5,5-bis(trifhioromethyl)-5-(tetrafluoroethylidene)-2-fhioro-2-ethoxy-l,3-dithiolane. Analogously, the reaction of 1,1,1-trifluoro-methyl-2-bromoprop-2-ene with thiourea and its N-substituted derivatives yields 5-trifhioromethyl-2-thione-thiazolidines 57 (92JFC(58)365). 

The 1,2-dithiole system and related compounds possess two adjacent sulfur atoms in a five-membered ring. The types are known respectively as 1,2-dithiolanes (1), 1,2-dithioles (2), l,2-dithiol-3-ones (3a), l,2-dithiole-3-thiones (3b), 3-iminodithioles (3c), l,2-dithiol-3-ylidenes (3d) and 1,2-dithiolylium (4) ions. 

Dithiolanes (1), in contrast to other dithioles, are not planar. In the carboxylic acid (28a) the CSSC dihedral angle is 26.6° (B-61MI43100), leading to a strain of 67 kJ mol-1. These compounds are unstable and the parent may be obtained only in dilute solution. Lipoic acid (29) is more stable, but does polymerize. The instability may account for its particular reactivity (B-61MI43101). l,2-Dithiol-3-ones (3a) and -3-thiones (3b) are very stable, as is evident from their modes of formation. Simple l,2-dithiol-3-imines (3c) are also stable, but some readily interconvert to isothiazoline-3-thiones (30) (Section 4.31.3.2). 

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). 

---