Reviews and Accounts ARKIVOC 2012 (i) 570-609
7. Prins Reaction The acid catalyzed condensation of olefins with carbonyl compounds known as Prins reaction is an important carbon-carbon bond forming reaction.8a,b The tetrahydropyran ring is a part of the backbone of various important carbohydrates and natural products.9a,b
Yadav et al. reported that Amberlyst-15 catalyses the synthesis of tetrahydropyranols through the Prins-type cyclization. Thus, when 1-phenyl-3-buten-1-ol 19 and benzaldehyde was stirred in refluxing 1,2-dichloroethane in presence of Amberlyst-15 for 2 hrs. 2,6-Diphenyl-4hydroxytetrahydropyran 21 was produced in 88% yield with high diastereoselectivity. The formation of 21 may be explained by Prins-type cyclization of the intermediate hemi-acetal 20 (Scheme 9).10
OH Ph H Ph O + H+ from Amberlyst-15 1,2-dichloroethane OPh Ph OH2 OPh Ph OPh Ph OH H2O 88% 19 20 21 Scheme 9
8. Friedel-Crafts Reaction Friedel-Crafts reaction of aromatic and heteroaromatic compounds is one of the fundamental reactions for forming carbon-carbon bond. Friedel-Crafts alkylation and acylation reactions have been studied by using Amberlyst-15 as acid catalyst.
8.1 Alkylation Kadam et al. demonstrated that Amberlyst-15 can act as a powerful catalyst for the alkylation of activated arenes or heteroarenes and -amido sulfones. Friedel-Crafts alkylation by various amido sulfones 22 was achieved on treatment with 1,2,4-trimethoxybenzene using Amberlyst-15 in refluxing CH2Cl2 to give the products 23 in very good yield (Scheme 10).11a The reaction of N-benzyloxycarbonylaminophenyl-p-tolylsulfones with indoles (heteroarenes) afforded C-3 alkylation of indoles in moderate yield.
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