2.7- Dihydrothiepine 1,1-dioxide

Dihydrothiophene-1,1-dioxides (42) and 2,17-dihydrothiepin-1,1-dioxides (43) undergo analogous 1,4 and 1,6 eliminations, respectively (see also 17-38). These are concerted reactions and, as predicted by the orbital-symmetry rules (p. 1067), the former is a suprafacial process and the latter an antarafacial process. The rules also predict that elimination of SO2 from episulfones cannot take place by a concerted mechanism (except antarafacially, which is unlikely for such a small ring), and the evidence shows that this reaction occurs by a non-concerted pathway.The eliminations of SO2 from 42 and 43 are examples of cheletropic reactions, which are defined as reactions in which two a bonds that terminate at a single atom (in this case the sulfur atom) are made or broken in concert. 

The first synthesis of thiepin 1,1-dioxide (131) was performed by Mock in 1967 75>. With exess bromine in chloroform 2,7-dihydrothiepin 1,1-dioxide (129), prepared by 1,6-addition of sulfur dioxide to m-hexatriene, gave a dibromide 130 which, on treatment with two equivalents of triethylamine, afforded 131. Upon catalytic reduction, 131 rapidly absorbed three molar equivalents of hydrogen to yield hexahydrothiepin 1,1-dioxide (132). The 1,1-dioxide 131 is a fairly stable compound ... 

An alternative synthesis of thiepin 1,1-dioxide (131) has also been reported (Paquette, L. A., Maiorana, S. J. Chem. Soc. Chem. Commun. 1971, 313). Thus, the reaction of vinyldiazomethane with sulfur dioxide yields 4,5-dihydrothiepin 1,1-dioxide which was converted to 131 by bromination (NBS) and dehydrobromina-tion (Et3N) sequence. 

A further example of a concerted thermal elimination reaction of a thiepane derivative was the formation of c/s-hexatriene and the extrusion of sulfur dioxide from heating 2,7-dihydrothiepin 1,1-dioxide (116) (67JA1281). That this reaction was under orbital symmetry control was deduced from the results obtained by heating cis- (117) and trans-... 

A double nucleophilic substitution reaction on 1,6-dibromohexane with sodium sulfide has been found to give an acceptable yield (59%) of the thiepane (35 equation 62) (81SC409). The reversible 1,6-addition of sulfur dioxide to ds-hexatriene (equation 63) provides a convenient synthetic route to the 2,7-dihydrothiepin 1,1-dioxide (116) (67JA1281). 

Ramberg-Backlund rearrangement of fused dihydrothiepine 1,1-dioxides obtained via [6n+4n] cycloaddition (Section 13.03.5.2) led to benzannulated polycycles <1996JOC7644, 1999JA8237>. The rearrangement was conducted by the sequential treatment of dihydrothiepin 1,1-dioxides 102-106 with Bu OK and A-chlorosuccinimide (NCS) or. V-iodosiiccinimidc (NIS) at —105 °C, followed by the addition of another equivalent of the base (Scheme 13). The benzannulated product 111 was transformed to (-l-)-estradiol 112. 

The reaction of trimethylsilylacetylenedicobalt hexacarbonyl complex and divinyl sulfone in refluxing toluene afforded a dihydrothiepine 1,1-dioxide 218 in 30% yield (Scheme 29) <1999JA8237>. Subsequent bromination-dehydrobromination gave 4-trimethylsilylthiepine 1,1-dioxide 219 in 70% yield. Metalation using [(MeCN )3Cr(CO)3] provided the complex 220 in 85% yield, which was used for [6n+2n] cycloaddition and subsequent Ramberg-Backlund rearrangement (Sections 13.03.5.2 and 13.03.6.2). 

Thiepin 1,1-dioxide (4). When sulfur dioxide is passed into a cold ethereal solution of vinyldiazomethanc (1), nitrogen is evolved, and the sulfone 4.5-dihydrothiepin 1.1 -dioxide (3) is obtained in 29 % yield. The t fj-divinyl episulfone (2), which can under-... 

CycloaddUion. Sulfur dioxide reacts with c -3-hexatriene (1) (20°, several weeks) to form 2,7-dihydrothiepin 1,1-dioxide (2). This cycloaddition appears to be general indeed substituted co-3-hexatrienes react more readily... 

Dihydrothiophene 1,1-dioxides from 1,3-dienes s. 6, 615 2,5-dienc-2,5-dihydro-thiophene 1,1-dioxides from conjugated diallenes s. K. Kleveland and L. Skatteb0l, Acta Chem. Scand. B 29, 827 (1975) 2,7-dihydrothiepin 1,1-dioxide ring s. W. L. Mock and J. H. McCausland, J. Org. Chem. 41, 242 (1976)... 

With trienes, the extrusion process dominates and the ring opens in a con-rotatOTy fashion. This indicates that in this reaction, one of the components is acting in an antarafacial manner. For example, cis- and [Pg.241]

Two preliminary communications describe the reaction of sulphur dioxide with vinyldiazomethane, which gives 4,5-dihydrothiepin 1,1-dioxide (16) (Scheme 4). Initial formation of the sulphone (14) is postulated. 

The stereochemical consequences of orbital symmetry control and the mechanism of the reversible addition of sulphur dioxide to hexatrienes, which yields 2,7-dihydrothiepin 1,1-dioxides, have been examined, and the scope of the reaction has been explored. "... 

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