Diels-Alder reactions CUMULATIVE

Allenes as versatile synthons including Diels-Alder reactions and especially intramolecular cycloadditions of this type were reviewed by Aso and Kanematsu [338], In some cases of intramolecular Diels-Alder reactions of open-chain starting materials such as 340 [339], 342 [339] and similar acceptor-substituted allenes [156], the formation of two new six-membered rings seems to be favorable if possible (Scheme 7.48). The non-activated cumulated C=C bond of 340 takes part in the [4+ 2]-cycloaddition and hence the necessary reaction temperature is high. On the other hand, the progressive truncation of the tether and the electron deficiency of the allenic C=C bond involved give rise to a remarkable Diels-Alder reactivity of the sulfone 346 generated in situ from sulfoxide 345 [339]. 

Diels-Alder reactions of dienes/dienophiles with cumulated... 

Ab initio and DFT calculations of the Diels-Alder reactions of vinylallene and diallene with dipolarophiles show that these reactions are concerted processes and that cumulated dienes are less reactive than non-cumulated dienes.153 Electron-rich... 

A sense of the growth in importance of the Diels-Alder reaction is gained by looking at the Cumulative Indexes of Chemical Abstract. Only two entries were listed under the name Diels-Alder reaction in the ten-year period from 1927 to 1936, whereas such entries had increased to 180 by the decade spanning 1947 to 1956, during which time Diels and Alder shared the Nobel Prize in Chemistry. In the five-year period from 1987 to 1991, a total of about 2400 entries appeared under this heading, and the number of citations presently exceeds 800 per year. 

Carbonyl diisocyanate (46) was shown to undergo a Diels-Alder type of cycloaddition with azomethines to give 2,3,6,7-tetrahydro-4/7,8/7-[l,3,5]triazino[2,l-Z>][l,3,5]oxadiazin-4,8-diones (16) (Scheme 5). A large variety of the triazino[2,l-ft][l,3,5]oxadiazines were realized by the suitable selection of dienophiles. Thus, l,3,5-triazino[2,l-Z>][l,3,5]oxadiazines (17), (18 R = NMe), and (19) and were synthesized by the reaction of carbonyl diisocyanate with alkyl or aryl isocyanates, dimethyl cyanamide, or aliphatic carbodiimides, respectively <86CB1133>. Due to the high reactivity of the cumulated double bonds, carbonyl diisocyanate (46) was also found to undergo [4 + 2] cycloadditions with cyclohexanone to yield cyclohexan-l-spiro-9 -[l,3,5]-oxadiazino-[3,4-e][l,3,5j-dioxazin-5 -spiro-l"-cyclohexane-2, 7 -dione (47) (Scheme 6) <76LA1634>. 

Interesting two-component cyclo-addition processes occur in the reactions of ketenes (and of other systems containing cumulated double bonds) with olefins.f A typical example is given by the reaction of ketene with cyclo-pentadiene, (Equation 6,68). It is notable that the 6-membered ring, the normal Diels-Alder product, is not formed. 

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