Enaminoketones cycloaddition

Dimethyl acetylenedicarboxylate (80) undergoes initial 1,2 cycloaddition with acyclic enamines to form cyclobutene intermediates which immediately decompose into acyclic dienaminodiesters (94,95). When an acyclic n/c-enediamine is used instead of a simple acyclic enamine, a dienediamino-diester is produced via a cyclobutene intermediate (95a). A cyclization reaction of dimethyl acetylenedicarboxylate with an acyclic enaminoketone... 

Enaminoketones undergo 1,4 cycloadditions with sulfene (162a). This is illustrated by the reaction of enamine 120 with sulfene to form sulfone 121 in an 80 % yield (162,163). 

The thionation-cycloaddition sequence is accompanied by the elimination of dimethylamine from the cycloadduct to afford thiopyrans. Similarly, when the thionation-cycloaddition methodology was applied to enaminoketones 98 and 99, obtained from thiochromanones, tricyclic compounds 100 and 101 were obtained (Figure 2.12). 

The use of 3 also allows for [3 + 2] cycloaddition with enaminoketones (38) (Scheme 8.3). When the morpholine-derived enaminoketon 6 was used, a mixture of A -pyrazoline 7 and pyrazole 8 was obtained. Complete transformation of 7 into 8 was achieved by treatment with water. In the case of the pyrrohdine-derived enaminoketone 9, pyrazole 11 and diazabicycloheptadiene 10 are formed competitively. In the formation of the latter compound, reaction of a second equivalent of 3 with the carbonyl group of 9 is involved. Reaction of uracil as... 

A clear correlation between the stabilisation of the endo-transition structure and the size of substituent at the 2-position of an 1-oxa-1,3-butadiene is again seen in the cycloaddition of the N-acetyl-enaminoketone 8-20 to 8-12. As expected, the reaction of 8-20 a to give 8-21 a and 8-22 a shows only a very small AAV, whereas with growing bulkiness of R as in 8-20 b and 8-20 c an increase of AAV is observed with the formation of the trans-cycloadduct 8-22 as the major product under high pressure. Because of the pressure effect it can clearly be deduced that 8-22 is formed via an endo-Z-anti-transition structure, presumably due to a strong hydrogen bond in the (Z)-diastereomer and a steric discrimination of the ( )-diastereomer of 8-20. However, an exo-E-anti-transition structure would give the same product (Fig. 8-7) [548]. 

An interesting approach to the canthin-4-one alkaloid family has recently been reported by Tremmer and Bracher and involves a 1,3-dipolar cycloaddition of the readily available 1-ethynyl-p-carboline 301 with various nitrile oxides (Scheme 54) (2015T4640). The initially formed isoxazoles 302 were then cleaved reductively, and this was followed by heating the resulting enaminoketones 303 in DMF at 150 °C to give 6-substituted canthin-4-ones 304 in high yields. 

In contrast to these results, the reaction of ketenes with j8-amino and (d-aIkoxy-a ,/ -unsaturated ketones (enaminoketones, vinylogous amides, and esters) results in satisfactory chemical yields of lactones (2,3-dihydro-2-pyranones) via a [4 + 2] cycloaddition. To date, more than 100 examples of the process, represented by the reaction of the enaminoketone 83 with dichloroketene to afford lactone 84 (92%) have been documented (1977FA794). A wide variety of substrates can be used under mild conditions to provide the corresponding lactone. However, dichloroketene, ketene, and diphenylketene are most commonly used (Scheme 28). 

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