1,3 -benzoxathioles

Hydro ythiophenols react with dibromomethane to produce 1,3-benzoxathioIes [27] (Table 4.6). 1,3-Benzoxathioles have also been obtained by the one-pot base-catalysed ring opening of 1,3-benzoxathiol-2-ones and subsequent reaction with dibromomethane in the presence of Aliquat [28]. 

Method B from 1,3-benzoxathiol-2-ones NaHCO, (12.6 g, 0.15 mol) and Aliquat (1.5 g, 3.7 mmol) in H20 (35 ml) are added to the l,3-benzoxathio -2-one (0.05 mol) in CH2Br2(10 ml) and the mixture is refluxed for 3 h. Excess CH,Br2 (ca. 8 ml) is then allowed to distil and the reaction temperature is raised to ca. 100°C and maintained at this temperature until TLC analysis shows complete disappearance of the benzoxathi-olone. The reaction mixture is extracted with Et20 (2 x 25 ml) and the ethereal extracts... 

An interesting rearrangement was observed when 2,3,6-trisubstituted-l,4-benzoxathiin 37 was reacted with HI or TMSI/H2O (TMSI = trimethylsilyl iodide). As shown in Equation (2), a selective 1,2-migration of the sulfur atom affords 1,3-benzoxathiole 38 <2004TL3729>. 

Cabiddu et al47 have prepared 1,3-benzoxathiole (lib) from 2-hydroxy-benzenethiol (10). The yields are higher under biphasic conditions than those obtained using DMSO. 

To a solution of 30 g 85% KOH in 75 mL wann H20, there was added an equal volume of warm MeOH followed by 16 g 5-methoxy-1,3-benzoxathiol-2-one, and the mixture was held under reflux conditions for 2 h. After cooling to room temperature, the mix was acidified with HC1 and extracted with 2x 100 mL CH2CL Removal of the solvent from the pooled extracts gave a yellow oil that crystallized on standing. The product, 2-mercapto-4-methoxyphenol, weighed 14 g and had a mp of 56-57 °C. 

A solution of 10 g 2-mercapto-4-methoxyphenol in 100 mL MEK was added over the course of 1 h to a vigorously stirred suspension of 25 g finely powdered anhydrous K.CO, in 200 mL MEK that contained 14 g methylene bromide. The reflux was maintained for 48 h. After cooling, the mixture was freed of solids by filtration and the filter cake washed with 50 mL additional MEK. The combined washes and filtrate were stripped of solvent under vacuum, and the product distilled to give 3.3 g of 5-methoxy-1,3-benzoxathiol as a yellowing oil that had a bp of 110-120 °C at 1.7 mm/Hg. There was considerable residue in the pot, which was discarded. The NMR spectrum was excellent, with the methylene protons a two-hydrogen singlet at 5.6 ppm. 

To a mixture of 3.2 g POC1, and 2.8 g N-methylformanilide that had been heated briefly on the steam bath (to the formation of a deep claret color) there was added 2.3 g 5-methoxy-l, 3-benzoxathiol, and steam hath heating was continued for an additional 5 min. The reaction mixture was poured into 100 mL H.O, and after a few minutes stirring, the insolubles changed to a loose solid. This was collected by filtration, H,0 washed and, after sucking as dry as possible, recrystallized from 30 mL boiling MeOH. This provided 1.9 g of 6-formy 1-5-methoxy-1,3-benzoxathiol as brownish needles that melted at 119-120 °C. 

The second special case is formed by the photoreactions of 2,4-dinitro-6-(phenyliodonio)phenolate (266) with several nucleophiles759. Upon irradiation of this fairly stable zwitterion in methanol, 6-methoxy-2,4-dinitrophenol is formed in 65% yield. Photoreaction with pyridine affords 2,4-dinitro-6-pyridiniophenolate (85%) and irradiation in the presence of phenyl isothiocyanate in acetonitrile affords a mixture of two stereoisomeric 2-phenylimino-5,7-dinitro-1,3-benzoxathioles (269) (71%) which could not be separated. The reaction starts with attack of the nucleophile on the positively charged iodine atom, leading to an iodinane (267) and proceeds by expulsion of iodobenzene. The mechanism is illustrated for phenylisothiocyanate, a case in which the substitution product (268) can undergo further photocyclization to a benzoxathiole derivative (269) (equation 196). 

The 1,3-dioxolane ring is usually inert to catalytic hydrogenation unless forcing conditions are employed. Hydrogenolysis over rhodium or palladium catalysts in the presence of an acid catalyst gives mainly hydroxy ethers as shown in equation (8). Raney nickel cleaves the oxathiolane ring (Scheme 9) and converts 1,3-benzoxathioles into phenol derivatives. 

Treatment of 1,3-benzoxathiole derivatives 87 with benzaldehydes in the presence of piperidine/acetic acid gives thioaurone derivatives 88. This reaction probably involves an oxidation step. These are also synthesized by the reaction of disulfide 89 with aldehydes (Equation 4) <2005T8648>. 

Benzoxathiol 5-(l-Ethoxy-ethoxy)-2-oxo- E14a/1, 327 (R—OH + Enol-ether)... 

Benzoxathiol 2-Phenylimino-4,5,6,7-tetrachlor- E19b, 1349 (Carben-l,3-dipolare Cycloaddition)... 

Deprotonation of 1,3-benzoxathiole A-oxide with LDA followed by stereoselective alkylation with a variety of electrophiles has been studied <2001T10365, 2003JHC979>, and this is the way in which compounds 52-55 were obtained. 

Gross, H., and CostiseUa, B., a-Substituted phosphonates. Part 28. Synthesis of 2-phosphorylated 1,3-benzodioxoles, 1,3-benzoxathioles and 1,3-benzodithioles, Synthesis, 622, 1977. 

As mentioned in CHEC-I, electrophihc substitution on the heterocyclic ring of dioxoles or oxathioles is essentially unknown. Reaction of 1,3-benzoxathioles bearing bulky substituents at... 

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