Lewis acid catalyzed -cycIoaddition

The nature of the substituents on the allene can have an impact on the outcome of a [2 -b 2] cycloaddition reaction, as was illustrated by the Lewis acid catalyzed cycioadditions of 1-thioaryl-3,3-dimethylallene (24) and 1-methyl-1-trimethylsilylallene to various 2-alkoxy-p-benzoquinones 25 (e.g. equation 8). The reactions were considered to proceed via carbocation intermediates foimed by nucleophilic attack of the thioallene on the Lewis acid activated quinone. At lower temperatures, these carbocations closed to cyclobutanes 26, whereas at higher temperatures, the theimodynamically more stable benzofurans 27 were formed. 

Introduction of an alkylthio group on the allene system increased the reactivity of the allene moiety in [2 + 2] cycloaddition reactions. It proved possible to conduct reactions of this allene at much lower temperatures. By adding Lewis acids, the reaction temperature could be decreased even more, as was illustrated by the Lewis acid catalyzed [2-1-2] cycioadditions of l-trimethylsilyl-l-methylthio-l,2-propadiene with a variety of electron-poor alkenes, including cyclic and non-cyclic enones, acrylates, methyl fumarate and acrylonitrile. When a chiral diol 21 based titanium catalyst was employed, the [2-1-2] cycloaddition reactions of /-acryloyl-l,3-oxazolidin-2-ones 17a and 17b with allenyl sulfides 18 yielded methylenecyclobutanes 19 and 20 with high optical purities (equation The highest yields were obtained with electron-poor allenophile 17b. 

Additional heterocyclic ring systems, such as benzofurans [125], dihydropyrroles and dihydroazepines [41], piperidines and dihydropyrimidines 36 [126], and fused oxazole derivatives [127], have been described (Eq. 7). The formation of epoxides and aziri-dines, formally emanating from ylides, was recently reported by Doyle et al. [77]. Rho-dium(II)-catalyzed isomiinchnone cycioaddition followed by Lewis acid-mediated ring opening has been used as an entry into the protoberberine azapolycyclic ring structure [128]. 

Although C—C connecting reactions are discussed elsewhere in Comprehensive Organic Synthesis , it may be mentioned that epoxides react regioselectively with carbon monoxide under rhodium catalysis (equation 51) and benzyl bromides undergo carbonylation and subsequent lactonization under the influence of a Pd" catalyst (equation 52). The [2 -( 2] cycioaddition of ketenes to carbonyl compounds, catalyzed by Lewis acids, is a familiar route to 3-lactones (equation 53). 

DIELS - ALDER Cyciohexene synthesis 4+2 Thermal cycioaddition between a diene and an activated alkene or alkyne, sometimes catalyzed by Lewis acids. 

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See also in sourсe #XX -- [ ]

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