Vinylogous carboxylic esters

If vinylogous imidazole-A-carboxylates (route A) are treated with nucleophiles such as alkoxides or amines, the corresponding vinylogous carbonic esters or amides are obtained. While reaction of the vinylogous imidazole-TV-carboxylate with a thiol (route A) yields the addition product only, that of the corresponding imidazolium compound (route B) leads to the carbonic thioester in a substitution reaction [3]... 

Interesting rearrangements proceed upon refluxing the azido diene 105 in benzene solution and form 61% of the vinylaziridine 106 as a mixture of diastereoisomers and the vinylogous urethane 108 (28%) (equation 37)53. It was shown that the process 106 - 108 occurs entirely at elevated temperature (refluxing xylene, ca 140 °C). However, treatment of the aziridine 106 with p-toluenesulfonic acid in THF at room temperature gives rise to trans,trans-, 3-butadiene carboxylic ester 107 in 98%53. 

The reduced electrophUicity of indole-3-carboxylic esters (they are vinylogous carbamates) means that they are much more versatile directors of lateral lithiation than the comparable benzoates, as illustrated by the synthesis of 564 (Scheme 221... 

Vogel and coworkers have described the synthesis of anti-1,6 7,12-/ wmethano[ 14]annulene 44 for studies of its -electron structure.32 Incorporation of AIBN into decarboxylative bromination of vinylogous carboxylic acid 45 via Barton esters increased the efficiency of this reaction, Scheme 16b. Harvey... 

By analogy with cyclopropanes and oxiranes, 2-acylaziridines, aziridine-2-carboxylic esters and amides, are also suitable substrates for one-electron reductive ring cleavage. This is an efficient and highly regioselective method for the synthesis of yS-amino carbonyl compounds. Vinylogous substrates are, furthermore, transformed into <5-amino y8,y-unsaturated carbonyl derivatives azetidines can also be used to achieve y-amino functionalization (Scheme 19) [72]. 

Case 11. (See also Case 5 above.) The formation of Thiele s ester (223)184 is a remarkable example of several kinds of selectivity, all of which can be explained by frontier orbital theory. The particular pair of cyclopentadienes which do actually react together (221 and 222) are not the only ones present. As a result of very rapid 1,5-sigmatropic hydrogen shifts (see p. 99), all three isomeric cyclopentadiene carboxylic esters are present, and any combination of these is in principle possible. As each pair can combine in several different ways there are, in fact, 72 possible products. We shall take up other aspects of this selectivity later, in their proper place (p. 167), but for now we may note that the regio-selectivity shown is a vinylogous version of case 11. 

Nothing remarkable has been reported, but a few useful results should be noted. Carboxylic esters at C-6 of the [1,2-a] fused system may be viewed as part of a vinylogous urethane, and are significantly less reactive than are those at C-5 <94TL2157>. 8-Hydroxyimidazo[l,2-a]pyridine showed useful oxine-like metal-chelating properties <91MI 810-02>. 

Enamines from thioimino-esters. - Vinylogous carboxylic acid amides. Startg. thioimino-ester dissolved in triethyl phosphite, and warmed 20 hrs. at 60° under Ng -> product. 

Similar BINOL-derived phosphoric acids were also found to effectively catalyze the asymmetric vinylogous Mannich reaction. Sickert and Schneider reported that various /V-PMP-protected imines react with acyclic silyl dieno-lates in the presence of the Brpnsted acid catalyst 137 to afford the desired 5-amino a,p-unsaturated carboxylic esters (139) in good yields (83-94%) with complete regi-oselectivity and high enantioselectivities (80-92% ee) (Scheme 11.29) [96], Remarkably, the one-pot three-component process (i.e., imine is formed in situ) is comparable... 

Aminocrotonate ester or aminocrotonitriles are cyclocondensed with vinylogous amidinium salts or enaminone ketals to give pyridine-3-carboxylic acid derivatives via electrocyclic ring closure of an intermediate adduct68-70 (equation 50). 

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