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Multiple sequential reaction steps

The reaction mechanism depends on the chemistry of the active oxidant and chemical contaminants. Multiple sequential and parallel reaction steps occur frequently. Partial oxidation produces noxious byproducts. [Pg.147]

Under certain condition, however, reactions are still preferably conducted in solution. This is the case e.g., for heterogeneous reactions and for conversions, which deliver complex product mixtures. In the latter case, further conversion of this mixture on the solid support is not desirable. In these instances, the combination of solution chemistry with polymer-assisted conversions can be an advantageous solution. Polymer-assisted synthesis in solution employs the polymer matrix either as a scavenger or for polymeric reagents. In both cases the virtues of solution phase and solid supported chemistry are ideally combined allowing for the preparation of pure products by filtration of the reactive resin. If several reactive polymers are used sequentially, multi-step syntheses can be conducted in a polymer-supported manner in solution as well. As a further advantage, many reactive polymers can be recycled for multiple use. [Pg.382]

According to Ivar Ugi [5], the leading pioneer of modern MCR chemistry, MCRs can be classified in three major types, according to the number of irreversible steps. MCRs of Type I are characterized by multiple reaction equilibria among all reactants and intermediates, while MCRs of Type II have one irreversible step leading to the product. Type-Ill MCRs consist of sequential irreversible steps and are related to cascade or domino reactions having the various reaction components embedded in the structure of the starting materials. [Pg.199]

As many metabolic pathways are branched, feedback inhibition must allow the synthesis of one product of a branched pathway to proceed even when another is present in excess. Here a process of sequential feedback inhibition may operate where the end-product of one branch of a pathway will inhibit the first enzyme after the branchpoint (the conversion of C to D or C to E in Fig. lb). When this branchpoint intermediate builds up, it in turn inhibits the first committed step of the whole pathway (conversion of A to B in Fig. lb). Since the end-product of a metabolic pathway involving multiple enzyme reactions is unlikely to resemble the starting compound structurally, the end-product will bind to the enzyme at the control point at a site other than the active site. Such enzymes are always allosteric enzymes. [Pg.91]

One advantage of using free energy is that it is easier to evaluate the overall equilibrium/energy for a series of sequential reactions (its additive instead of multiplicative) ACtot = Sum[AG], Often use to predict feasibility of pathways, possible energy yields, and to determine when individual reactions are not at equilibrium (important for determining potential control steps etc.). [Pg.261]

Fig. (4). Proposed lignan biosynthetic pathway in Podophyllum (steps 1-3 have been characterized in Forsythia spp.). 1, Oxidase and dirigent protein create an 8-8 bond between two molecules of coniferyl alcohol 2, NADPH reductase catalyses two sequential reactions 3, NADP+ dehydrogenase. Dotted arrows indicate multiple uncharacterized steps. Fig. (4). Proposed lignan biosynthetic pathway in Podophyllum (steps 1-3 have been characterized in Forsythia spp.). 1, Oxidase and dirigent protein create an 8-8 bond between two molecules of coniferyl alcohol 2, NADPH reductase catalyses two sequential reactions 3, NADP+ dehydrogenase. Dotted arrows indicate multiple uncharacterized steps.
An alternate route to assembly of nanoparficles as hollow spheres that does not require a polymerization reaction step is sequential electrostatic assembly. Electrostatic-mediated multilayer assembly of charged particles was first demonstrated by R. Her on planar surfaces, wherein he established the proof-of-principle to deposit particles sequentially onto soUd substrates [25]. Decher advanced this scheme by assembling alternately charged PEs (e.g., polycations and polyanions) onto solid supports. Ever since, this scheme has been used to form capsules by sequential electrostatic deposition of single or multiple coatings of materials on preformed colloidal templates and subsequent removal of the template by calcination or solvent dissolution. This constitutes the LBL method for assembly of hoUow spheres. [Pg.92]

Multistep, one-pot sequential reactions are considered to be an ideal synthetic methodology because they do not require isolation and purification procedures for intermediates between reaction steps. They also reduce the necessary time and reagents. Multiple active components are necessary to promote several reactions however, active components are often opposing and mutually destructive when contacting with other components such as Hquid add and base reagents. Such difficulties could be setded by the concept of site isolation [127]. Sol-gel entrapment [128] and polymeric reagents [129-131] have been utilized to achieve the site... [Pg.141]

We may apply these ideas further to a synthesis plan, which is a sequential series of reaction steps leading to a target molecule. If each reaction s outcome is independent from its predecessors, then the overall yield may be interpreted as the overall probability that the sequence of reactions will ultimately lead to the desired product. Recall that the overall yield is the multiplicative product of all reaction yields along the longest branch in a synthesis plan. [Pg.164]

Sequential controls Sequential control functions perform real-time control of equipment to carry on a batch process to move a process through a succession of distinct states. An example of sequential control is opening a valve and running a pump for material transfer, material at any step, then after completing the step (may time controlled or quantity controlled) moves on to other step. Sequential controls in a batch process could he for major processes such as mixing, heating, and reaction. A phase may consist of a number of sequential control steps to manipulate equipment within a unit boundary. At time, it may be necessary that multiple units work in a synchronized fashion, for example, the transfer of material from one unit to another, with each unit having its own phase. Here, suitable communication between units is essential for synchronized operation. [Pg.417]

The first type of cascade reaction is sequential synthesis which describes the usage of multiple consecutive catalytic steps for building up complex stractures. Each step may be performed under different reaction conditions however, no purification of the intermediate products is necessary. Herein chemical catalytic steps may be combined with enzymatically catalyzed ones or the cascade may be operated with enzymes solely. [Pg.137]

Scheme 6.3 Cascade reactions in glyco-conjugate syntheses. Either one-pot, sequential, or convergent cascade reactions can be applied. Sequential syntheses reactions employ multiple consecutive catalytic steps, whereas in one-pot syntheses multiple catalytic steps are combined in one reaction vessel... Scheme 6.3 Cascade reactions in glyco-conjugate syntheses. Either one-pot, sequential, or convergent cascade reactions can be applied. Sequential syntheses reactions employ multiple consecutive catalytic steps, whereas in one-pot syntheses multiple catalytic steps are combined in one reaction vessel...
One-pot synthesis focuses on the combination of multiple catalytic steps in one reaction vessel under the same reaction conditions. Optimization of the reaction conditions to suit all involved catalysts is crucial to reach shorter reactions times than for the sequential reaction type. Again, here the combination of chemical steps and enzymatic steps in one-pot is possible. [Pg.138]

Sequential reactions also termed tandem reactions, involve the formation of multiple carbon-carbon or carbon-heteroatom bonds in a specific sequence without purification or separation of any intermediate products. Thus, sequential reactions can potentially provide complex, multifunctional products in a single step. However, for sequential reaction to be useful, they should be extremely efficient and specific to minimize formation of unwanted products. The reactions should also proceed with high yields. [Pg.405]

These types of tandem reactions involve multiple radical reactions or cyclizations. Curran and coworkers reported an excellent example of sequential radical reaction mediated by Sml2 as a critical step in the synthesis of (ib)-hypnophilin (scheme 4) (Fevig et al., 1988). [Pg.405]


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