Dihydrothiophene- 1-1-dioxides dienes

Another cheletropic reaction is the addition of sulphur dioxide under pressure to the hexa-2,4-dienes (134 and 136) to give the dihydrothiophen dioxides (135 and 137 respectively).129 These reactions are easily reversible simply by heating... 

Cycloaddition reactions of 1,3-dienes with sulfur dioxide, unlike its mono and bis imino derivatives, usually produce five-membered ring 2,5-dihydrothiophene dioxides rather than Diels-Alder adducts [Eq. (36)]. 

Hogeveen and Heldeweg found a rare example of a Diels-Alder reaction with sulfur dioxide where the [4 + 2] adduct is reasonably stable. Addition of sulfur dioxide to diene 76 at low temperature yielded [4 -I- 2] adduct 77 as the kinetic product. At temperatures above 2(J C products resulting from the dihydrothiophene dioxide 78 were observed (Scheme 1-XVI). 

The methiodide of 2,5-dihydrothiophene (239) is transformed in high yield to Z)-l-(methylthio)buta-l,3-diene (240) on treatment with alkali (81AJC1017). The thermal cheletropic extrusion of sulfur dioxide from both cis and trans isomers of 2,5-dihy-drothiophene 1,1-dioxides is highly stereospecific. For example, c/5-2,5-dimethyl-2,5-dihydrothiophene 1,1-dioxide (241) yields ( , )-hexa-2,4-diene (242) and sulfur dioxide (75JA3666, 75JA3673). 

The most synthetically useful cheletropic elimination involves 2,5-dihydrothiophene-1,1-dioxides (sulfolene dioxides). At moderate temperatures they fragment to give dienes and sulfur dioxide.301 The reaction is stereospecific. For example, the dimethyl derivatives 22 and 23 give the E,E- and Z,E-isomers of 2,4-hexadiene, respectively, at temperatures of 100°-150°C.302 This stereospecificity corresponds to disrotatory elimination. 

The thiophene ring system can be utilized as a synthetic scaffold for the preparation of nonthiophene materials as the sulfur moiety can be removed by reduction (desulfurization) or extrusion (loss of SO2). The extrusion of sulfur dioxide from 3-sulfolenes (2,5-dihydrothiophene 1,1-dioxides) give dienes (butadienes or o-quinodimethanes) that can be utilized to prepare six-membered rings by cycloaddition chemistry. For example, thermolysis of 3-sulfolene 120 provided tricyclic pyrazole 122 via an intramolecular cycloaddition of the o-quinodimethane 121 that results by extrusion of sulfur dioxide <00JOC5760>. Syntheses of 3-sulfolenes 123 and 124 <00S507> have recently been reported. 

The reaction of o-halomethylene benzyl halides, l,4-dihalobut-2-enes, cyclo-2-hexenols or 2,5-dihydrothiophene-l,1-dioxides with Na2Fe(CO)4 or Fe2(CO)9 results in the formation of (diene)Fe(CO)3 complexes108. In each case, the precursor is transformed in situ into the free diene ligand, followed by complexation. 

Intramolecular [4 + 2] cycloaddition reactions of enamides have provided a route to hydroindole and hydroquinoline ring systems (80JA3294,5274). In this work, the diene portion was initially masked as a 2-substituted 2,5-dihydrothiophene 1,1-dioxide. Thus, reaction of the acid chloride (312) with 3,4,5,6-tetrahydropyridine (311) afforded the masked enamido diene (313), which was converted to the enamido diene (314) upon brief refluxing in xylene. Thermolysis of (314) afforded the hydrolulolidine (315) in 45-55% yield. Additionally, (313) could be transformed to (315) directly by passage of a 1% solution in toluene through a vertical tube (600 °C oven temperature) (Scheme 67). The method was used to prepare a known precursor to aspidospermine. 

Chemical shifts and coupling constants indicate that thiophene 1,1-dioxides possess reasonable diene character <1984ZNB915>. The H NMR spectrum in CDCI3 shows the a- and /3-protons at 6.64 and 6.38 ppm, respectively, which is about 0.7 ppm upfield from the corresponding resonances in thiophene. Similarly, the H-2 and H-3 protons in 4,5-dihydrothiophene resonate at 6.06 and 5.48 ppm while that of the corresponding dioxide are found at 6.66 and 6.81 ppm, respectively. 

Five membered ring sulfones (thiolene-1, 1-dioxides, sulfolenes or dihydrothiophene-1, 1-dioxides) can be obtained by peracid oxidation of tetrahydrothiophene. The extrusion of sulfure dioxide from dihydrothiophene-1,1 dioxides or sulfolenes like (178) has been much studied as it provides a synthetic route to dienes, e.g. butadiene (152) (Scheme 70). The reverse reaction provides a method of synthesis of sulfolene (178) from butadiene (152) and sulfur dioxide. Thiolene dioxides or sulfolenes can be generally prepared by addition of sulfur dioxide to conjugated dienes for example, 1,4-dimethylbutadiene (179) in the presence of sulfure dioxide, triethylamine and formic acid affords 2,5-dimethylsulfolene (180) (Scheme 71). 

The theimolysis of 2-substituted 2,5-dihydrothiophene 1,1-dioxides leads to ( ) conjugated dienes via cycloieversion followed by the concerted cheletropic extrusion of sulfur dioxide. The thermolysis of the a, -alkylated sulfone (96) gives the intennediate, which loses SO2 to give 1-dodecadien-l-yl acetate (98), a component of the sex pheromone of the red bollwoim moth. Tliis procediue has been extended to the thermolysis of a,3-dialkylated sulfones in order to obtain ( , )-1,4-disubstituted-1,3-dienes (equation 46). Similar processes have been used for the syntheses of alkaloids. The synthesis of an Elaeocarpus alkaloid, elaeokwine A (100), makes use of the retrodiene extrusion of sulfur dioxide to give the 1,3-diene intermediate (99) that is subsequently consumed by an intramolecular imino DA reaction (equation 47). o-Xylylene (102) has been generated by rDA expulsion of SO2 from benzo-fiised 3,6-dihy(ho-l,2-oxathiin 2-oxide (101). ... 

Dihydrothiophen 1,1-dioxides (3-sulpholenes) have been demonstrated to be very versatile masked diene synthones in organic synthesis. Thus, alkylation with different alkyl halides can be used to mono-alkylate to (210) and dialkylate to a mixture of (211) and (212). Desulphonylation can be achieved in various ways. Treatment of (211) with LiAlH in ether gave exclusively (213) in 90% yield, while treatment of (212) with potassium hydroxide or potassium carbonate in ethanol at 125°C gave exclusively (214) in... 

Sulfur dioxide reacts with dienes to form a cyclic sulfone in a reaction somewhat reminiscent of the Diels-Alder reaction. In the case of Myrcene (3.1), the adduct produced is (3.2) and its remote double bond is selectively hydrated by mineral acid. Pyrolysis of the product (3.3) under neutral conditions leads to cleavage of the dihydrothiophene ring to give the hydrated diene (3.4) and Diels-Alder reaction of this product then produces Lyral (3.5). 

Dihydrothiophenes are oxidized to 1,1-dioxides by m-chloroperoxybenzoic acid. These compounds are also accessible by [4+1] cycloaddition of 1,3-dienes and sulfur dioxide. For instance, butadiene reacts even at room temperature with liquid sulfur dioxide to give an adduct 2,5-dihydrothiophene 1,1-dioxide, commonly known by the trivial name 3-sulfolene ... 

The Diels-Alder cycloaddition of 2,3-dihydrothiophene-1,1-dioxide with open and cyclic 1,3-dienes proceeds with moderate yields <94JCS(Pi)927>. FVP of the resultant adducts has been studied. 

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)... 

In keeping with these arguments, under thermal conditions, (2 ,4 )-hexa-2,4-diene and (2Z,4 )-hexa-2,4-diene undergo highly stereospecific addition of SO2 to give cis- and trani-2,5-dimethyl-2,5-dihydrothiophene 1,1-dioxide (cis- and trani-2,5-dimethylsulfolene), respectively (Scheme 5.4). 

Finally, the last entry in Table 6.5 is, like the first, another example of a cheletropic reaction. This time, in contrast to the first, a diene (rather than a monoene) and sulfur (IV) dioxide (rather than the more reactive carbene) are undergoing an orbital symmetry allowed (A + t[2s) pericyclic process. This allowed reaction occurs with retention of symmetry since (although not shown in Table 6.5) the geometry of the substituted diene is retained in the product. Thus, as shown in Equation 6.57, the diene, (2E,4E)-hexadiene (c Figure 4.42), enters the reaction in a suprafacial fashion and, as expected for a disrotatory process (Chapter 4), produces the d5 -2,5-dihydrothiophene-l,l-dioxide ... 

Dienes-1,1,4,4-d4. 2,5-Dihydrothiophene-2,2,5,5-d4 1,1-dioxide heated in a slow Ng-stream whereupon decomposition occurs at 130-162° during ca. 1 hr. 

The cheletropic elimination reactions are widely used for generation of relatively unstable dienes in situ for Diels-Alder reactions. For example, extrusion of nitrogen from diazene 146, and sulfur dioxide from 2,5-dihydrothiophene-l,1-dioxide 147... 

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