Control of Chemoselectivity

Chebanov VA, Muravyova EA, Sakhno YI, Saraev VE, Kappe CO, Desenko SM (2009) Microwaves vs. ultrasonication towards control of chemoselectivity. Microwave and flow chemistry conference, Jolli Beach, Antigua, 52... 

Carbenoids derived from the metal catalysed decomposition of diazo compounds undergo various chemical transformations. Control of chemoselectivity by choice of the appropriate catalyst has significantly increased the synthetic viability of catalytic cyclopropanation reactions. Intermolecular reaction of unsaturated alcohols with carbenoids derived from catalytic decomposition of alkyl diazoesters has been reported by Noels and... 

Solventless reactions of 5(4//)-oxazolones 139 with hydroxycoumarins 140 exhibited excellent control of chemoselectivity leading to O- and C-acylation products 141 and 142 <03SL1710>. 

Control of Chemoselectivity. The reaction of allylic alcohols with r-butyl hydroperoxide and vanadium acetylacetonate or titanium tetraisopropoxide provides a highly chemoselective method for the preparation of epoxides, as exemplified below. Catalysis by vanadium is envisioned to involve a complex of r-BuOOH, the OH group of the allylic alcohol, and the metal. 

In addition to catalyst control of chemoselectivity, catalyst control of stereochemistry was also demonstrated in this series. Treating substrate 16 with [Rh(CH2=CH2)Cl]2 and the novel ligand P[OCH(CF3)(o-CH30Ph)]3 in toluene at 60 °C provides a 91 9 (trans to cis) ratio of 17a and b in 87% yield (Eq. 56). In contrast, reaction of the same substrate with Rh(CH2=CH2)Cl]2, AgOTf, and the novel ligand P[OCH(CF3)(2,6-(CH3)2CgH3)]3 provides a complementary... 

An interesting example of the control of chemoselectivity by counterions has been observed for the reaction of 0-(2-butynoyl)salicylaldehyde (X) with phenyl... 

In a similar way, enetetraynes can undergo a pentacyclization as observed for compound 6/1-272, which gave 6/1-273 in 66% yield (Scheme 6/1.72). In this case the initial alkenyl-Pd-species is formed by an oxidative addition of Pd° to the alkenyliodide moiety in 6/1-272 [130]. In comparison to the reaction of6/1-270, this procedure has the advantage of complete control of the chemoselective formation of the alkenyl-Pd-species, but the disadvantage that the starting material is less easily accessible. 

Positional control of conjugation is essential at two levels in the assembly of nanobiological devices [6-8], Recently developed chemoselective conjugation... 

Elastin-mimetic protein polymers have been fabricated into elastic networks primarily via y-radiation-induced, radical crosslinking of the material in the coacervate state [10]. Although effective, this method cannot produce polymers gels of defined molecular architecture, i.e., specific crosslink position and density, due to the lack of chemoselectivity in radical reactions. In addition, the ionizing radiation employed in this technique can cause material damage, and the reproducibility of specimen preparations may vary between different batches of material. In contrast, the e-amino groups of the lysine residues in polymers based on Lys-25 can be chemically crosslinked under controllable conditions into synthetic protein networks (vide infra). Elastic networks based on Lys-25 should contain crosslinks at well-defined position and density, determined by the sequence of the repeat, in the limit of complete substitution of the amino groups. 

The detailed study of the MCRs involving barbituric acids and 5-aminopyrazoles was published by Muravyova et al. [58]. The article describes the development of chemoselective cyclocondensations with help of microwave and ultrasonic irradiation. It was established that the temperature was the main factor in controlling the direction of the MCRs studied. 

Key to these applications has been the control of selectivity-chemoselectivity, regioselectivity, diastereoselectivity, and, especially, enantioselectivity. Here the design of chiral catalysts, begun already in the 1960s, has allowed access to one product when multiple products would, in the past, have been expected. Both electronic and steric control are important, and different metal ions with their associated chiral ligands can have unexpected effects. 

The controlled photoelectrochemical oxidation of imides presents an interesting example of chemoselectivity. The photoinduced oxidation of lactams to imides, Eq. (22) and of N-acylamides to imides, Eq. (23) introduces oxygen into the organic substrate, but if the reaction is conducted in the presence of Cu(I), unsaturation is induced instead, Eq. (24)... 

In addition, a high degree of chemoselectivity can be achieved with the proper selection of catalyst, as shown in Eq. 5.23, which describes exceptional control of product selectivity based... 

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