ABSTRACT
The cyclization reaction of an epoxyalcohol is catalyzed by a synthetic cavitand receptor with an inwardly directed carboxylic acid function. The receptor features a hydrophobic pocket in which the substrate is bound and positioned to react in a regioselective manner. The nature of this substrate-catalyst complex and its dynamic properties were investigated by NMR methods and with the aid of a model compound lacking the epoxide function. The kinetic parameters of the cyclization reaction were also studied. A catalytic cycle is proposed and diverse inhibition mechanisms are identified that parallel those encounterd in enzymology.
Subject(s)
Alcohols/chemistry , Ethers, Cyclic/chemistry , Resorcinols/chemistry , Catalysis , Cyclization , Epoxy Compounds , Kinetics , Magnetic Resonance Spectroscopy/methodsABSTRACT
A deep, self-folding cavitand responds to minor electronic differences between suitably sized adamantane guests. Binding constants range from <0.5 to 4000 M(-1) for guests as similar as 1-bromoadamantane and 1-cyanoadamantane. The barriers to guest exchange also vary up to 3 kcal mol(-1).
Subject(s)
Electrons , Ethers, Cyclic/chemistry , Resorcinols/chemistry , Adamantane/chemistry , Models, Molecular , Molecular ConformationABSTRACT
A cavitand functionalized with a Kemp's triacid derivative was used to catalyze the epoxide ring-opening cyclizations of 1,5-epoxyalcohols. A deep, cylindrical cavity containing electron-rich aromatic walls and an inwardly directed carboxylic acid displayed the necessary characteristics to bind different 1,5-epoxyalcohols and initiate their cyclization reactions. The reactions inside this synthetic receptor occurred in a catalytic and regioselective manner. These results highlight that the arrangement of functionality and unique solvation provided by the structured interiors of natural enzymes can be incorporated into synthetic systems having useful physical and chemical properties.
Subject(s)
Alcohols/chemistry , Carboxylic Acids/chemistry , Chemistry, Organic/methods , Epoxy Compounds/chemistry , Ethers, Cyclic/chemistry , Resorcinols/chemistry , Catalysis , Cyclization , Molecular StructureABSTRACT
The limitations of stereoelectronic models in assessing the stereoselective nucleophilic substitution reactions of cyclic oxocarbenium ions at high reaction rates are discussed. Evidence is provided suggesting that the diastereoselectivity of nucleophilic substitution reactions is attenuated at the limits of diffusion control. The low diastereoselectivities observed in the reactions of trimethylsilyl cyanide with five- and six-membered ring oxocarbenium ions are attributed to the high reactivity of the nucleophile and its reactions with these electrophiles at diffusion control rates.
Subject(s)
Acetals/chemistry , Acetals/chemical synthesis , Cyanides/chemistry , Trimethylsilyl Compounds/chemistry , Diffusion , Ions/chemistry , Models, Chemical , Molecular Conformation , Organosilicon Compounds/chemical synthesis , Organosilicon Compounds/chemistry , StereoisomerismABSTRACT
[reaction: see text] The O-to-C rearrangement of vinyl acetals is used to demonstrate that tight ion pairing is not involved in the stereoselective nucleophilic addition reactions of alkoxy-substituted cyclic oxocarbenium ions.
