1.3- Dihydrobenzo c thiophene 2,2-dioxide

During investigation of the synthesis of benzocyclobutane 193, Cava and coworkers118 found that o-quinodimethane 192, generated in the pyrolysis of 1,3-dihydrobenzo[c]thiophene 2,2-dioxide (191), can be trapped by Af-phenylmaleimide to give N-phenyl-1,2,3,4-tetrahydronaphthalene 2,3-dicarboimide (194) (equation 125). 

The synthetic utility of o-quinodimethane generated by cheletropic elimination of S02 has been amply demonstrated by Oppolzer and Nicolaou, who have conducted an intramolecular cycloaddition coupled with the alkylation of 1,3-dihydrobenzo[c]thiophene 2,2-dioxide122. When 1,3-dihydro-l-(4-pentenyl)benzo[c]thiophene 2,2-dioxide (201) prepared from 1,3-dihydrobenzo[c]thiophene 2,2-dioxide and 4-pentenyl bromide is heated in di-n-butyl... 

Charlton has recently reported the asymmetric induction in the reaction of dimethyl fumarate and 1,3-dihydrobenzo[c]thiophene 2,2-dioxide (198) containing a chiral a-alkoxy group at the 2-position (equation 128). A diastereomeric excess of 2.8 1 of 199 to 200 is achieved by using 198 derived from optically active a-methylbenzyl alcohol. 

Cyclizations can occur with heteroatoms present in the tether as long as the groups are not strongly nucleophilic. Decomposition of a-diazo- 3-arylmethanesulfonyl esters (173) resulted in the formation of 1,3-dihydrobenzo[c]thiophene 2,2-dioxides (174 equation 38),143 which are valuable precursors to o-quinodimethanes. Reaction with /V-aryldiazoamides (175) has been shown to be a useful method for preparing 2(3//)-indolinones (176 equation 39),22a while reaction of a-diazo-(3-keto esters (177) has been developed as a process to synthesize 3-acetylbenzofiiran-2(3//)-ones (178 equation 40).144... 

A solution of 200 mg (0.89 mmol) of (1 /J,S )-l-(1-phenylethoxy)-1,3-dihydrobenzo[c]thiophene-2,2-dioxide (I5a), 349 mg (3.56 mmol) of maleic anhydride (22) and 30 mg of powdered ZnO in 10 mL of benzene is refluxed for 20 h. The solution is filtered to remove ZnO, evaporated, and then heated to 100 CC under high vacuum to remove the maleic anhydride. The residue is recrystallized from benzene/hexane to give the major diastereomer as colorless crystals yield 286 mg (100%). 

A synthesis of (+)-estradiol from an alkyl 1,3-dihydrobenzo[c]thiophene-2,2-dioxide involving o-quinodimethane formation by thermal extrusion of SO2 and subsequent cycloaddition has been achieved in an overall yield of 50% (ref. 143). Thus a chiral cyclopentanone component (ref. 144) was used to alkylate the appropriate benzothiophene dioxide and the required tetracyclic stucture obtained directly with avoidance of the customary hydrogenation. It was found desirable at the alkylation stage to enhance deprotonation at Cl by having a cyano group in the benzenoid ring. The ( anotetracycle (R CN) was reacted... 

Light-catalyzed bromination of l,3-diphenyl-l,3-dihydrobenzo[c]-thiophene 2,2-dioxide gives either the 1-monobromo or the 1,3-dibromo derivative depending on the amount of bromine used.83... 

We have already mentioned that l,3-diphenyl-l,3-dihydrobenzo-[c]thiophene 2,2-dioxide gives 9-phenyl-9,10-dihydroanthracene as the major product [Eq. (6)] on being heated. Unlike 1,3-dihydro-... 

Thus, the ease of desulfonylation of a l,3-dihydrobenzo[c]thiophene 2,2-dioxide and therefore the products obtained is dependent on the stability of the intermediate o-quinodimethane. The photochemical decomposition of l,3-dihydrobenzo[c]thiophene 2,2-dioxide probably involves an excitation energy of >74 kcal/mole and is not possible under the conditions already mentioned, whereas its 1,3-diphenyl derivative may be photochemically desulfonylated under these conditions because the intermediate o-quinodimethane derivative (96) is stabilized relative to o-quinodimethane itself (see also Cava and Shirley96).101... 

Oxidation of freshly liberated 1,2,3,4-tetrahydrophthalazine (42) by mercury(II) oxide in the presence of sulfur dioxide gave an (unseparated) mixture of 1,4-dihydro-2,3-benzoxathiin 3-oxide (43) and l,3-dihydrobenzo[c]thiophene 2,2-dioxide (44) (substrate, CH2CI2, -20X, S02i, then HgOj, 20°C, 12h 45% of a 9 1 mixture in which the compounds were clearly identified). ... 

Dihydrobenzo[c]thiophene may be oxidized to its sulfone (Table VII) with peracetic acid,28,69,87,88 aqueous hydrogen peroxide,9 or potassium permanganate,21 and when benzocyclobutene and 1,2-diphenylbenzocyclobutene are heated with sulfur dioxide, they give l,3-dihydrobenzo[c]thiophene 2,2-dioxide89 and its cis-1,3-diphenyl derivative (83),69,90 respectively [Eq. (5)]. 1,3-Dihydro-benzo[c]thiophene 2,2-dioxide also arises when the thiadiazepine (17b) is heated just above its melting point (175°-180°).34a When sulfur dioxide is passed into a solution of diphenyldiazo-... 

For benzo[Z ]thiophene the heterocycle is rather more resistant to ring opening and oxidation with hydrogen peroxide in acetic acid at 95 C, for example, gives the 1,1-dioxide (Scheme 7.22) reduction with either sodium and ethanol or triethylsilane in trifluoroacetic acid affords 2,3-dihydrobenzo[Z)]thiophene. Electrophiles give mainly 3-substituted benzo[Z ]thiophenes, although these products are often accompanied by smaller amounts of the 2-isomers. 

Dimethyl-l,3-dihydrobenzo[c]thiophene 2,2-dioxide also fails to give a benzocyclobutene on being heated. In this case the intermediate o-quinodimethane derivative (97) undergoes a rapid 1,5-hydride shift to give o-ethylstyrene [Eq. (7)]. 88,89... 

Ab initio MO calculations on 3-formyl-(5 2-isoxazoline-BF3-complex suggests that s-trans conformation is preferred over s-cis conformation. Nucleophiles attack the stable s-trans conformer from the opposite side of the C-4 substituent, giving the respective anti adducts <93T7637>. The regioselectivity of the 1,3-dipolar cycloadditions of mesitonitrile oxide with benzo[b]thiophene S-oxide and S,S-dioxide yielding only 2,3-dihydrobenzo[b]thieno[2,3-d]isoxazolines, is discussed in terms of frontier MO interactions on the basis of photoelectron spectra and CNDO/S calculations <82JOC2461>. 

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