Bridgehead carboxylic acids

NMR spectra of the compound 13a showed a broad singlet at 4.41 for the clibyl protons, whereas compound 13b showed two multiplets. Conversion of bridgehead carboxylic acids to the corresponding halides using Pb(OAc)4 and iodine in refluxing benzene under illumination is reported. This is considered to be an alternative to Barton s method, because of its simplicity and ease of preparation, but it involves toxic lead compounds. 

Bridgehead carboxylic acids, e.g. adamantane-1-carboxylic acid, are transformed into acyl hypoiodites, followed by photochemical decarboxylation to yield the corresponding bridgehead iodides1091. 

Decarboxylation. Della and Patney recommend a two-step method for decarboxylation of bridgehead carboxylic acids. The first step is the Cristol-Firth modification of the Hunsdiecker reaction (I, 657, improved by use of methylene bromide as solvent). The second is photochemical reduction of the bromides by tri- -butyltin hydride (I, 1192-1193) with azobisisobutyronitrile as initiator. Yields are 80-95% in the first step and 80-90% in the second step. 

Decarboxylation.—Direct decarboxylation of hindered geminal diesters has been achieved using amine bases (DBN Dabco). This attractive one step process generally gives good yields and avoids the use of both acidic and aqueous conditions. An improved method for the oxidative decarboxylation of vicinal diacids to olefins employs copper(i) oxide and 2,2 -dipyridyl in quinoline temperatures of 180 °C are however required. Excellent yields have been achieved in the decarboxylation of bridgehead carboxylic acids by sequential bromination and photochemical reduction in the presence of tri-n-butyltin hydride. 

In view of continuous efforts to develop an alternative procedure for the preparation of bridgehead halides from the corresponding carboxylic acids using... 

Adamantanecarboxylic acids with a carboxylic group at a bridgehead position, e.g. adaman-tane-1-carboxylic acid (8),111 react with sulfur tetrafluoride in the conventional way giving high yields of the corresponding trifluoromethyl-substituted adamantanes. l-(Trifluoromethyl)-adamantane (9) can also be formed in one step from adamantane by treatment with a mixture of sulfur tetrafluoride, hydrogen fluoride and formic acid the latter serves as a source of carbon monoxide. The reaction is believed to proceed via carbonylation of an intermediate carbocat-... 

Adamantane-2-carboxylic acid, in which the carboxy group is at a secondary carbon atom, on treatment with sulfur tetrafluoride gives a product with fluorine atoms at bridgehead positions, 5,7-difluoro-2-(trifluoromethyl)adamantane (10), in over 90% yield (see Section 8.2.10.).87... 

Adamantanecarboxylic acids, on treatment with sulfur tetrafluoride, react in a conventional manner to give (triiluoromethyl)adamantanes (see Section 8.2.4.1.). However, in the presence of anhydrous hydrogen fluoride in large excess, substitution of bridgehead hydrogen atoms by fluorine occurs in addition to fluorination of carboxylic acid groups di- or trifluoro(tri-fluoromethyl)adamantanes 7 and 8 arc obtained in one step.8 119186... 

Functional group manipulations are equally successful for the preparation of a wide variety of 2-substituted adamantanes. Starting with the methyl ester of 2-adamantane carboxylic acid S7> 2061, 2-t-butyladamantane (71) may be prepared in a manner anlogous to that described above for the synthesis of the bridgehead isomer (Scheme 17) 2M These reactions illustrate the... 

Bridgehead carbons of adamantane [86], pinane [87], and fused norbornanes [85a, 88] undergo selective hydroxylation under similar reaction conditions. Alkyl-substituted cyclopropane is oxidized selectively at the a-position to cyclopropane ring (Eq. 3.54) [89]. The methyl group of toluene can be converted into the corresponding carboxylic acids (Eq. 3.55) [91]. 

Gust, Moore, Moore and coworkers covalent cartenoid-porphyrin-quinone molecular triads 55-60 contain a cyclized hydrogen bond within the quinone acceptor framework [143], The naphthaquinone moiety of 55 is fused to a norbomene system whose bridgehead position bears a carboxylic acid, which can hydrogen bond to an adjacent quinone. Photoinduced electron transfer from the porphyrin to the quinone leads to a marked p/fg increase of the latter, resulting in a fast proton transfer ( pt 10 s ) to form the semiquinone. Back electron transfer from the semiquinone is attenuated as a consequence of the proton-stabilized charge-separated species. This leads to a two-fold increase in the quantum yield of the charge-separated state of 55, as compared to those of the reference triads 56 and 57 (see Volume III, Part 2, Chapter 2). 

Quasi-Favorskii rearrangement Skeletal rearrangement of bicyclic a-halo ketones in which the halogen is located at the bridgehead position to afford carboxylic acids or carboxylic acid derivatives. 370... 

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