Acid-catalyzcd reactions

Chloroalkenes are used as 7i-nucleophiIes in the acid-catalyzcd reaction of phthalimide-derived hydroxylactams. These cyclizations lead to sole formation of the thermodynamically less stable e/ r/(t-l-acetylpyrrolo[2,1-(v isoindole derivatives92. 

The iron(II)-catalyzcd reaction of phenazine (9) with an aliphatic substrate (RTl) and At,At-dialkyl-At-chloroamines in concentrated sulfuric acid yields the corresponding 2-alkyl derivatives 10. Under these conditions the reactive species is the diprotonated form of the phenazine. 

Acid-Catalyzed Elimination Reactions. The simplest kind of elimination reaction is catalyzed by acids and proceeds through a transitory carbonium ion (p. 44). Consider tert-butyl alcohol. In the presence of acid, an oxonium ion is formed (I) which can dissociate into water and a carbonium ion (II). As with all carbonium ions, there are then four courses of reaction open. (1) It can react with another water molecule or anion. (2) It can rearrange. (3) It can abstract a hydrogen atom with a pair of electrons from another molecule. (4) It can attract an electron pair from the carbon-hydrogen bond of an adjacent carbon atom so as to liberate a proton and to form an olefin (III to IV). The fourth possibility is the process by which many acid-catalyzcd elimination reactions occur. 

In studies of the asymmetric epoxidation of olefins, chiral peroxycarboxylic acid induced epoxidation seldom gives enantiomeric excess over 20%.1 Presumably, this is due to the fact that the controlling stereocenters in peroxycarboxylic acids are too remote from the reaction site. An enantiomeric excess of over 90% has been reported for the poly-(Y)-alanine-catalyzcd epoxidation of chalcone.2 The most successful nonmetallic reagents for asymmetric epoxidation have been the chiral TV-sulfonyloxaziridincs3 until asymmetric epoxidation reactions mediated by chiral ketones were reported. Today, the... 

Carboxylic acids react with butadiene as alkali metal carboxylates. A mixture of isomeric 1- and 3-acetoxyoctadienes (39 and 40) is formed by the reaction of acetic acid[13]. The reaction is very slow in acetic acid alone. It is accelerated by forming acetate by the addition of a base[40]. Addition of an equal amount of triethylamine achieved complete conversion at 80 °C after 2 h. AcONa or AcOK also can be used as a base. Trimethylolpropane phosphite (TMPP) completely eliminates the formation of 1,3,7-octatriene. and the acetoxyocta-dienes 39 and 40 are obtained in 81% and 9% yields by using N.N.N. N -tetramethyl-l,3-diaminobutane at 50 T in a 2 h reaction. These two isomers undergo Pd-catalyzcd allylic rearrangement with each other. 

In contrast to laboratory reactions, enzyme-catalyzed reactions often give a single enantiomer of a chiral product, even wiien the substrate is achiral. One step in the citric acid cycle of food metabolism, for instance, is the aconitase-catalyzcd addition of water to (Z)-aconitate (usually called m-aconitate) to give isocitrate. 

Stereoselectivity effectsThere are several recent reports that molecular sieves can improve the stereoselectivity of various reactions such as the Sharplcss epoxidation (c.g., 13,51). The beneficial effect has been attributed to water-scavenging by the sieves. Molecular sieves also show a marked effect on the diastereosclectivity of the Pd-catalyzcd cyclization of ci.s-1,2-divinylcydohexanc with chiral acids (equation I). In these reactions, additions of molecular sieves were found in some cases to increase the diastcrco-... 

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