Sulfur extrusion from

Thus, sulfur extrusion from the twisted naphtho[3,4-c/]thiepin is rapid, because in this case less energy is required to form the thianorcaradiene isomer than for a total loss of aromaticity. 

In some sulfur extrusions from 2,7-di-/ Tt-butylthiepins a byproduct containing two sulfur atoms is obtained, along with the aromatic compound. The byproduct has been characterized as a thienothiophene by single crystal X-ray analysis.92... 

The many efforts of synthesizing the thiepin system described in Section 2 have revealed that extremely mild conditions have to be employed for the construction of the thiepin skeleton in order to avoid thermal sulfur extrusion from the resulting thiepins. This is an especially important prerequisite for the synthetic designs aimed at obtaining simple (thermolabile) thiepin derivatives. In our own study in this field we have previously developed new versatile routes for the synthesis of the thiepin skeleton. In this section we summarize our synthetic approaches to the relatively simple thiepin derivatives. 

Treatment of 5,6-dichloro-2,3-dimethyl- -benzoquinone with sodium sulfide followed by oxidation with nitric acid yields the thianthrene derivative (60). Sulfur extrusion from 60 with peracetic acid leads to 2,3,7,8-tetramethyl-l,4,6,9-dibenzothiophene tetrone (61) (overall yield 57%). ... 

When the reaction of Hiinig s base with sulfur monochloride was performed in boiling chlorobenzene, the corresponding bis[l,2]dithiolopyrroles 77-79 were formed by sulfur extrusion from intermediates 74-76 (Scheme 39). 

Avalos and co-workers (220-228) extensively investigated the 1,3-dipolar cycloaddition chemistry of 2-aminothioisomiinchnones with both acetylenic and olefinic dipolarophiles. For example, sugar derivatives of the mesoionic imi-dazo[2,l-Z7]thiazolium-3-olate system react regioselectively with a variety of acetylenic dipolarophiles [DMAD, diethyl azodicarboxylate (DEAD), methyl propiolate, ethyl phenylpropiolate] to give the corresponding imidazo[l,2-a]pyr-idin-4-ones (e.g., 323) following sulfur extrusion from the not isolable cycloadducts (220). Similarly, these thioisomtinchnones react with diethyl azodicarboxylate and arylisocyanates in the expected fashion (221), and also with aryl aldehydes to form episulfides (222). 

While the Diels-Alder reactions of thiophenes with various dienophiles are relatively rare, it is interesting to note that 2,5-dimethoxythiophene (297) prepared from 2,5-diiodothiophene does react with maleic anhydride to form the bis-adduct (299) (75JCS(P1)2483>. Sulfur extrusion from the primary cycloadduct delivers a new diene system (298) which then undergoes the second Diels-Alder reaction (Scheme 63). 

There are several examples of sulfur extrusion from 1,3,4-thiadiazines (144) either under basic conditions via the 877-anion (31) <75AG(E)581, 77S196), or by heating in acetic acid (70LA(741)45, 76JPR971). Both sets of conditions give pyrazoles in good yield. 

An interesting photochemical synthesis of 2,5-disubstituted-thiophenes 38 stems from the first report of the photoinduced sulfur extrusion from variously 3,6-disub-stituted 1,2-dithiins 35a by exposure to daylight in ethyl acetate or in DCM, or by irradiation at 365 nm in methanol (Scheme 12.11) [29]. Other examples involve naturally occurring thiarubrines 35b [30], as well as variously substituted synthetic derivatives [ 31]. This photochemical reactivity has been generalized with an application in the synthesis of selenophene 40 obtained in almost quantitative yield from 39 [32]. 

The present review deals with sulfur extrusions from heterocyclic compounds 5, giving rise to ring-contracted cyclic products 6 (Scheme 2). In the course of such ring contractions, the sulfur atom sometimes remains connected to the final ring system (formation of 7). This type of reaction is also considered to some extent in the following sections. 

Benzo[ ]-l,4-thiazepin-4-ones 85 (X = O) extrude sulfur to give quino-linones 86 (X = O). The same products, together with benzothiazoles, are formed by thermal H2S elimination from the corresponding 2,3-dihydro compounds (86CB3109). The sulfur extrusion from benzo[b]-l,4-thiazepine-4-thiones 85 (X = S) to quinolinethiones 86 (X = S) requires catalysis by sodium alkoxides (70HCA1697) (Scheme 23 ... 

Sulfur extrusion from the cyclic six-mem bered disulfide 135 with formation of the dione 136 can be achieved by reaction with carbenes (85TL5187) (Scheme 39). 

Imidazo[2,l-h]thiazolium-3-olate 160 (Section VI,C Scheme 47) underwent rapid cycloaddition with acetylene mono- and dicarboxylic esters to give the imidazo[l,2-a]pyridin-3-yl acyclo C-nucleoside 208 through sulfur extrusion from the intermediate 207 (91MI7) (Scheme 62). 

However, both a- and yS-methoxy-substituted thiophenes react with dimethyl acetylenedicarboxylate in xylene to give modest yields of phthalates resulting from sulfur extrusion from initial adducts in acetic acid as solvent only substitution products are obtained." ... 

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