2.3- Dihydrothiophene dioxide, thermolysis

Dihydrothiophene dioxide (2-sulfolene, 76) is partly converted to ethyne, ethene, and sulfur dioxide (93PHC1). Tetraphenylthiophene dioxide (77) is split to diphenylacetylene and S02 under FVP conditions, whereas solution thermolysis leads to different products (77RTC282). 

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. 

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. 

The solvent-dependence of cheletropic reactions has also been investigated [158aj. The thermolysis of 3-methyl-2,5-dihydrothiophene-l,1-dioxide appears to involve a concerted fission of the two (T-bonds. This is in accordance with the very small solvent effect on rate observed in six solvents of different polarity [158a]. 

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). ... 

A modified method for the synthesis of (139) has been described. Birch reduction of 2-acylthiophens and of 2-acyl 5-alkylthiophens, followed by alkylation with alkyl halides, gave 2-acyl-, 2-alkyl-, or 2-acyl-2,5-dialkyl-thiophens in good yields. Oxidation of these compounds to the 1,1-dioxides by MCPBA, followed by thermolysis, offers a convenient route to 1,3-dienyl ketones. The Birch reduction of 2-t-butyl-5-pivaloylthiophen to the corresponding 2,5-dihydrothiophen was a key procedure in the synthesis of... 

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