Oxidative deprotection, benzene

A second example from the same group is the synthesis of an elaborate diethynyltriphenylene derivative (Scheme 7 Table 8,entries 12,13) [58].Zn/Pd-promoted homocoupling of a 4-iodo-l,2-dialkoxybenzene furnishes the desired tetraalkoxybiphenyl, an electron-rich aromatic system. Iron trichloride-catalyzed Friedel-Crafts arylation of the biphenyl derivative with dimethoxy-benzene furnishes an unsymmetrical triphenylene derivative. Deprotection, oxidation, and subsequent Diels-Alder reaction with cyclohexadiene is followed by catalytic hydrogenation and reoxidation. TMS-CC-Li attack on the quinone delivers the alkyne modules, treatment with SnCl2 aromatizes the six-mem-bered ring, while KOH in MeOH removes the TMS groups cleanly to give the elaborate monomer. 

Although THP ethers243 resist the action of PCC under the relatively mild conditions used for the oxidation of alcohols, PCC in boiling benzene is able to deprotect THP ethers and perform an in situ oxidation of the resulting alcohol to ketone.260... 

Methylcyclohexenone 281 upon oxidation with Mn(OAc)3 in benzene under reflux gave 282, which reacted with phenylmagnesium chloride and CuBr-Me2S to form two isomeric ketones 283 and 284. Further, 283 has been transformed to vinylsilane 285 followed by its hydrolysis to form the free alcohol 286, which in turn was alkylated with methoxyallyl bromide to give 287. Oxalic acid-mediated deprotection of 287 led to the formation of the ketone 288. Ozonolysis of 288 in methanol afforded the fused 1,2,5-trioxepine 289 in low yields (Scheme 66) <1997BML2357>. 

As regards the protecting effect, the complex is stable to Lewis acids. Also, no addition of BH3 occurs. As Co2(CO)6 can not coordinate to alkene bonds, selective protection of the triple bond in enyne 137 is possible, and hydroboration or diimide reduction of the double bond can be carried out without attacking the protected alkyne bond to give 138 and 139 [32], Although diphenylacetylene cannot be subjected to smooth Friedcl Crafts reaction on benzene rings, facile /7-acylation of the protected diphenylacetylene 140 can be carried out to give 141 [33], The deprotection can be effected easily by oxidation of coordinated low-valent Co to Co(III), which has no ability to coordinate to alkynes, with CAN, Fe(III) salts, amine /V-oxidc or iodine. 

Cyclohexanone (202) was converted to compound (203) whose transformation to cyclohexanone (204) was accomplished in three steps. It underwent cyclialkylation with boron trifluoride etherate affording the cyclized product (205) (R=R,=OMe) in 64% yield along with naphthalene (206) (R=Ri= H,H). Compound (205) on heating under reflux with DDQ in benzene produced ketone (207) whose tosylhydrazone on treatment with sodium cyanoborohydride afforded reduced product (208). Deprotection of the aryl methyl ethers and oxidation with ceric ammonium nitrate led to the formation of miltirone (197). 

This current-control feature can be illustrated in the electrochemical decarboxylation of vicinal diacids. These reagents have long been used as protecting groups for double bonds, since electrochemical deprotection by two-electron oxidation causes bis-decarboxylation and production of a C=C double bond [60]. In contrast, when platinized titanium dioxide is irradiated in the presence of one such vicinal diacid (cyclohexenedicarboxylic acid), the major reaction pathway leads to monodecarboxylation, rather than to benzene formation (Eq. 12). 

Materials. Cyclohexane, benzene, tetrahydrofiiran, ethylene oxide, styrene, isoprene and 1,3-butadiene were purified as described previously (27-30). Protected hydroxyl-functionalized initiators (see Scheme 2) (FMC, Lithium Div.) were used without further purification after double titration analyses (31). All chemicals for deprotection were used as received. 

A similar oxidative cyclization initiated by the irradiation of a substrate in the presence of (diace-toxyiodo)benzene and iodine can be used for the deprotection of benzyl ethers (e.g., 611) situated next to the hydroxyl in the a, p, or 7-position [642]. Depending on the substrate, the corresponding cyclic ethers of diols (such as 612) can be isolated (Scheme 3.239). 

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