Improvements or modifications

The long history associated with the Hantzsch pyridine synthesis has produced numerous approaches and methods to the reaction protocol in order to control the various factors directing the course of the reaction. 

It has been shown that TMSI is capable of mediating the reaction at room temperature. The classical three component coupling was carried out using aldehyde 82 and ketoester 83 with ammonium acetate in acetonitrile at room temperature with in situ generated TMSI. This gave a 73-80% yield of 1,4-dihydropyridines 84 in 6-8 h. The best results were obtained with 1 equivalent of TMSCl and 1 equivalent of Nal. 

The modified two component Hantzsch (Baeyer-Knoevenagel modification) was also examined. Shorter reactions times (2-3 h) were noted in this variation using 82 and 85 with slightly better yields (78-85%) being observed for the formation of 84. 

Indium trichloride has been found to catalyze the synthesis of tetrasubstituted 

Combinatorial approaches have been applied to this chemistry. In a method amenable to split and pool, PAL, or Rink resin, 89 is modified with an acetoacetate to generate the solid supported aminocrotonate 90. Either a two- or three-component Hantzsch protocol is followed to produce 91. Treatment with TFA carries out the cleavage from the resin and the cyclization to dihydropyridine 92. 

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The procedures described in this symposium by Brooks and by Dailey represent new synthetic paths with several advantages for scaled-up synthesis. The procedures described by Pepperman outline improvements or modifications in a path suggested by Sih (5). 

The varieties of copolymers that can be prepared with styrene have greatly expanded the use of the monomer. Dramatic improvements or modifications of physical properties can be achieved by choosing the right comonomer. The dynamic mechanical properties of these copolymers are strongly influenced by the characteristics of the comonomer, the copolymer composition and the miscibility parameter of the constituents parts to function as separable identities. 

Many improvements or modifications to the UCM model can be found in the literature. These csm lead to various classes of constitutive equations keeping the differential nature of the equation [2, 3, 35]. As pointed out by Larson [43], a systematic classification of these can be performed by rewritting the UCM model as ... 

The canonical Favorskii rearrangement has seen few improvements or modifications, which is not surprising given the simplicity of the reagents used to effect the transformation. However, a number of variations have been developed to expand its scope, though none has received extensive investigation. 

Further refinement is desirable concerning the preparation of grease type samples. Otherwise, the state of the art" appears satisfactory. No improvements or modifications of the test apparatus are anticipated presently. Results of this test are used as a basis for selection of materials for use in missile LOX systems. 

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