Ethers, benzenoid

Thiol groups on central benzenoid rings are alkylated <87SUL135>, and hydroxy groups may be oxidized to quinones, converted to ethers, or esterified <83MAC627,91CB2613). 

Over Raney Ni at 200-250°C, however, the main reactions of the dihydrocoumarin formed were saturation of the benzenoid ring to give octahydrocoumarin (3) and its conversion to the cyclic ether, hexahydrochroman (4), although polymeric products were also formed in 6-27% yields. The hydrogenation pathways and the products of coumarin and related compounds over copper-chromium oxide and Raney Ni are summarized in Scheme 10.4. For the formation of the cyclic ether 4 over Raney Ni,... 

The Claisen rearrangement of allyl ethers in the quinoline series is thus seen to be unusually fast, affording high yields of products. In no case has migration of the allyl side-chain to the benzenoid ring been observed. Migrations to the nitrogen atom and to the alkyl side-chain proceed by concurrent rather than consecutive paths. 

Selective transport of Hg(II) over Cu(II) through chloroform from a solution of these metal ions to an aqueous HCl solution was demonstrated for the troponoid dithiocrown ether (19). The benzenoid dithiocrown ether (20) was a much less effective transporter in spite of similar cavity size, suggesting protonation of the tropone ring assisted release of the Hg(II) from the complex. 

C.d. data recorded for a series of 5a-cholestane-3/3,6a-diol di-(4-chloro-benzoates) with additional substituents at C-4 have indicated that the diterpenoids caryoptin, 3-epicaryoptin, and clerodin have the same absolute configuration. C.d. data are reported for four isomeric steroidal 3-spiro-isoxazolidine[2,3- i]oxadiazolines, together with X-ray data confirming the structure (26) for one of them. These compounds result from 1,3-dipolar cycloadditions between benzonitrile oxide and the 3-methylene-steroid. An interpretation of the c.d. data at ca. 250 nm attributes the Cotton effect to coupled benzenoid transitions. A new sector rule is proposed for the c.d. of 2,2-dialkyl-l,3-oxathiolans (ketone hemithioacetals), which are essentially of thio-ether type with c.d. bands in the regions of 240 and 220 nm. 

Formation of substituted products by electrophilic reactions is one of the characteristics of benzenoid compounds. It has been shown that a few dehydro-[4/j + 2]annulenes afford substituted annulenes under strictly limited reaction conditions. Monodehydro[14]annulene, on treatment at room temperature with copper(ii) nitrate-acetic anhydride, oleum-dioxane and subsequent methylation and acetic anhydride-borontrifluoride etherate, yielded monosubstituted products, 169a, 169b and 169c, respectively. The electrophilic reactions must have resulted in substitution of one of the outer protons, but the exact point of attack has not been determined . 

A 2-aminodiaryl ether (79.2, X = O) is cyclized to the furopyridine (79.3, X = O) by using the Pschorr reaction—thermolysis ofa mixture of the diazonium salt and copper powder. The use of heterocyclic diazonium salts in organic synthesis has been reviewed [2980]. Rhodium(II) acetate catalyses the room-temperature cyclization of a-diazo-jS-oxo esters (79.4) by carbenoid insertion into a benzenoid C—H bond. 

Methoxy- and 2-acetoxy-furans are available from 2,5-dimethoxy- and 2,5-diacetoxy-2,5-dihydro-furans (18.1.1.4) via acid-catalysed elimination. They undergo Diels-Alder cycloadditions the adducts can be further transformed into benzenoid compounds by acid-catalysed opening. 3,4-Dihydroxyfuran is undetectable in tautomeric equilibria between mono-enol and dicarbonyl forms the dimethyl ether behaves as a normal furan, undergoing easy a-electrophilic substitution, mono- or dilithiation at the a-position(s), and Diels-Alder cycloadditions. 2,5-Bis(trimethylsilyloxy)furan is synthesised from succinic anhydride it too undergoes Diels-Alder additions readily. Both furan-2- and -3-thiols can be obtained by reaction of lithiated furans with sulfur in each case the predominant tautomer is the thiol form. ... 

The hexasubstituted benzenoid compound (145) is synthesized in one step by the condensation of 2 mol of the dienol silyl ether (146) with 1 mol of methyl orthoacetate and TiCU. The reaction is believed to proceed through the intermediate (147). Compound (147) can be considered as a potential 1,3,5-trielectrophile. The more reactive sites in (147) are at C-5 and at C-3 respectively. Condensation of (147) with a second mole of (146) gives therefore regioselectively the aromatic compound (145). By this method, the plant growth substance sclerin (148) can easily be prepared. 

ACSA(B)62]. The stannanes are available from enol ethers by a-lithiation and quenching with trialkylstannyl chloride. The coupling reactions have been run on derivatives that had either a chlorine atom in an activated position or a bromine atom in the benzenoid position. Mild acid hydrolysis of the a-pyrimidinylalkenyl ethers yields ketones, the acyl-substituted pyrimidines. In the 4,5-dichloro derivative (130), the masked acyl group is introduced into the electrophilic 4-position (131). In the 2,5-disubstituted pyrimidine (133), having a methyl group in the 5-position and a chlorine atom in the 2-position results in the addition of a masked acyl group in the electrophilic 2-position (134). When the 5-substituent in the latter example is a bromine atom, the chemoselectivity leads to masked acylation in 5-position (135). This reaction sequence constitutes a convenient... 

Ultra-violet and visible spectrophotometry can be effectively used for the control of purification and specification of purity of compounds. If a compound is transparent in the near ultra-violet and the visible regions, the purification is continued until the absorbancy is reduced to a minimum (e < 1). Traces of impurities present in pure transparent organic compounds can be readily detected and estimated, provided the impurities themselves have fairly intense, absorption bands. Before a liquid is used as a spectroscopic solvent, it should be tested for spectrophotometric purity. For example, commercial absolute alcohol usually contains benzene as impurity. The absence of benzene in the Alcohol should be confirmed spectrophoto-metrically by using sufficiently large cells (4 or 10 cm cells), before using the alcohol as a solvent. The presence of carbon disulphide in carbon tetrachloride may be detected by the presence of the disulphide absorption tend at 318 mytt. The detection of the characteristic benzenoid absorption in the spectra of many organic compounds (e.g. diethyl ether, cyclohexene) showed that the bands attributed to these compounds earlier were only due to the contamination by benzene1. 

---