Oxidative decarboxylation 1,4-dihydrobenzoic acids

A clever application of this reaction has recently been carried out to achieve a high yield synthesis of arene oxides and other dihydroaromatic, as well as aromatic, compounds. Fused-ring /3-lactones, such as 1-substituted 5-bromo-7-oxabicyclo[4.2.0]oct-2-en-8-ones (32) can be readily prepared by bromolactonization of 1,4-dihydrobenzoic acids (obtainable by Birch reduction of benzoic acids) (75JOC2843). After suitable transformation of substituents, mild heating of the lactone results in decarboxylation and formation of aromatic derivatives which would often be difficult to make otherwise. An example is the synthesis of the arene oxide (33) shown (78JA352, 78JA353). 

As mentioned earlier in the discussion of cyclizations leading to (3-lactones, the (3-lactones formed from halolactonization of 1,4-dihydrobenzoic acids readily rearrange to produce bridged ring y-lac-tones.19 In some cases, the substitution pattern favors formation of the y-lactone even under conditions of kinetic control (equation 23).20 Synthesis of a variety of y-lactones by iodolactonization of dihydroben-zoic acid derivatives has been reported recently by Hart (equation 24).91 Attempted iodolactonization of the acid in the case where R = H resulted primarily in an oxidative decarboxylation however, iodolactonization was effected using the amide derivative. 

The 1,4-dihydrobenzoic acids derived from reductive alkylation may undergo facile rearomatization with either loss of the carboxylic acid group or the alkyl group. The gibberellin synthesis intermediate (82), for example, was found to be especially labile, forming (83) simply on exposure to air." Oxidative decarboxylation may be deliberately achieved with lead tetraacetate or electrochemically." Loss of the 1-alkyl group is likely to be a problem when the alkyl moiety can form a reasonably stable free radical, since a chain reaction may then be sustained." ... 

Oxidative decarboxyiation of dihydroaromatic acids. Some years ago Birch mentioned that 1-substituted 1,4-dihydrobenzoic acids are decarboxylated to arenes by LTA. The starting materials are readily available by a one-step Birch reduction (Li-NHo) and alkylation from benzoic acids. Birch and Slobbe have now extended the early work and shown that this process is useful synthetically. One example is the synthesis of olivetol dimethyl ether (equation I), liquation II formulates the synthesis of a useful intermediate to dihydrojasmone. 

Yields are high (80—90%) the alumina probably assists in the hydrolysis of the ester. In a preliminary report, Birch has described a method for oxidative decarboxylation of the 1-substituted 1,4-dihydrobenzoic acid (25) and the dihydrofuran (26) using lead tetra-acetate. 

Benzo[c]thiophene may be prepared by low-pressure (20 mm) vapor-phase catalytic dehydrogenation of l,3-dihydrobenzo[c]thio-phene (Section III,A) at 330° under nitrogen,5,8 by decarboxylation of benzo[c]thiophene-1 -carboxylic acid (Section III,C) with copper in quinoline16,38 or by dehydration of l,3-dihydrobenzo[c]thiophene 2-oxide (Section VI,A) in acetic anhydride or over aluminum oxide at 20 mm Hg and 100°-125° in a sublimation tube.52 A trace of water appears to be beneficial to the first reaction, and it has been suggested53... 

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