Vinylogous eliminations

This method is also useful for vinylogous eliminations, in which case hetero substituted dienones may be produced ... 

The opening of epoxides with neighboring keto groups, accompanied by vinylogous jS-elimination, has already been mentioned (section V). This is an extension of the jS-elimination reactions which occur when a,p- or jS,y-epoxy ketones are opened with acid or base. a,jS-Epoxy ketones give a-substituted a,jS-unsaturated ketones [(162), for example], and j5,y-epoxy ketones give y-substituted a,jS-unsaturated ketones [(163), for example] ... 

The stereospecific generation of enamines by -elimination reactions (187) and a vinylogous elimination, which leads to a dienamine (188), have been reported. The loss of an a substituent from a tertiary amine is seen in the generation of enamines by elimination of hydrogen cyanide from benzylic a-aminonitriles (189,190). 

Several enamines also participate in these cycloaddition reactions. For example, the addition of methyl lithium to benzaldehyde 5 and the sequential introduction of the vinylogous amide and magnesium bromide results in the cycloaddition elimination product chromene 63 (method G, Fig. 4.33).27 The introduction of methyl magnesium bromide to a solution of the benzaldehyde 5 and two equivalents of the morpholine enamine produces the cycloadduct 64 in 70% yield with better than 50 1 diastereoselectivity (method F). Less reactive enamides, such as that used by Ohwada in Fig. 4.4, however, fail to participate in these conditions. 

The solid-state interaction of enamines (428, 333a) with trans-l,2-diben-zoylethene (87) provides quantitative yields of the pyrrole derivatives 445 or 446 [140]. These remarkable 5-cascades consist of initial vinylogous Michael addition, enol/keto tautomerism, imine/enamine tautomerism, cyclization, and elimination, all within the crystal without melting. A waste-free extraordinary atom economy is achieved that cannot nearly be obtained in solution. The milling times are unusually long here (3 h) but it s certainly worth the effort... 

When a,/3-unsaturated esters are the target molecules, the use of a thioaldehyde or thioketone is advisable for the easy elimination of H2S from the intermediate thiol adduct. This strategy has been applied to thiolactams 94a and to thionolactones 94b, opening a route to vinylogous carbamates 95a143 and carbonates 95b144(equation 57). 

Stereochemical investigation on the vinylogous 1,5-elimination reactions of 16 to give cyclopropylalkenes has revealed highswi stereoselectively of the intramolecular SN process (equation 18)30. The stereoselectivity of the cyclization reaction of 17 to give the ( )-isomer is improved by the increase in steric bulkiness of the sulfonyl substituent (equation 19)31. 

The vinylogous elimination to give alkenylcyclopropanes may also be effected via rt-allyl palladium complexes34,35. The palladium(0)-catalyzed substitution of allylic esters with stabilized carbon nucleophiles via 7i-allyl palladium derivatives stereospecifically proceeds with net retention (double inversion) of configuration. Thus, the chirality of an allylic substrate is transferred to resultant alkenylcyclopropanes in the intramolecular S J/ reaction via 7i-allyl palladiums (equation 21)36,3. ... 

A vinylogous Peterson reaction has also been described, and the elimination has been found to be stereospecifically syn, as shown in equations 135 and 136208. 

At 140° in methanol, a much more extensive decomposition of akuammicine occurs, and the products isolated are 3-ethylpyridine and 2-hydroxycarbazole (XXXV). The formation of 3-ethylpyridine is presumably the result of a normal Hofmann degradation of the pyri-dinium ester corresponding to XXXIII the other product should consequently have been methyl 3-vinyl-2-indolylacetate (XXXIV), but this was not obtained. It was accordingly suggested that the 2-hydroxycarbazole obtained was formed from XXXIV by intramolecular nucleophilic attack of the vinylogous enamine methylene group on the carbomethoxy group (arrows in XXXIV), followed by elimination of methanol and aromatization (39). 

The E2 Reaction. The stereochemistry of the E2 process is even less well understood. It is exemplified by the decarboxylative eliminations of the vinylogous /3-hydroxy acids 5.16 and 5.18, which are both largely, although not exclusively, syn. The corresponding E2 reaction with /3-hydroxy acids is highly anti selective, in the usual way for /3 eliminations. 

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