Concurrent cascades

As a rule, sulfonation takes place continually in a cascade or a falling film reactor (Table 14) at about 50-70°C. The S03 is steadily diluted to a concentration of 5-10 vol % with air or an inert gas. The LAB conversion reaches a value between 92% and 98% [156,157]. Mixing of the already formed alkyl-benzenes with fresh S03 leads to undesired highly sulfonated byproducts. In order to prevent these side reactions, all processes operate concurrently. 

Two immunization procedures designed to enhance the immune response to multiple antigen mixtures have been reported recently. The cascade immunization technique (20) utilized in vitro depletion of E. coli proteins (ECPs) which had previously elicited an antibody response. The removal of these dominant immunogens from the mixture was accomplished by immunoabsorption with antibodies obtained from an earlier antiserum. The passive immunization procedure (21) relied on in vivo blocking of strong immunogens by the concurrent administration of early antiserum obtained previously. This latter report demonstrated the presence of an apparently poorly immunogenic ECP to which a humoral response could only be elicited by this passive procedure. 

A couple of prototypical examples of the cyclic version of the Heck reaction, defined as a process consisting of alkene carbopalladation followed by -elimination, were reported during the 1984-1985 period [9,10]. Almost concurrently, seminal examples of both the non-Heck cyclic carbopallation reactions [10,30] were reported during the 1983-1985 period. Thus, with due respect paid to earlier discoveries of alkyne cyclooligomerization via cascade carbopalladation [7,8] as well as copolymerization [24] and cocyclization [25,... 

Figure 19.15. Staged permeation cascade with rectification and stripping sections. The individual membrane modules may be operated concurrently or counterconcurrently, or perfect mixing may be assumed to occur.

Although ketoprofen is highly bound to plasma proteins, one recent study indicated that it did not alter the coagulation cascade parameters or prothrombin time in humans treated concurrently... 

Polystyrene supported versions of catalyst Id have been reported/ In addition, Frechet and coworkers demonstrated that catalyst Id can be encapsulated in the core of a soluble star polymer, which enables reagents to diffuse, allowing catalysis to take place/ Interestingly, the concurrent mixing of different chiral and achiral catalysts encapsulated in a similar manner onto different star polymers enabled one-pot multicomponent asymmetric cascade reactions of otherwise incompatible catalysts. 

The reactions were highly regioselective and only minor side products were formed (4-10% yields) under the established conditions. In addition, this reaction could also be carried out on a 500 mg scale under mild conditions. Recently, Kroutil and co-workers developed a novel chemo-en matic derace-mization reaction, which involves two enantioselective oxidation steps and one non-stereoselective reduction step. This concept combines stereoinversion of one substrate enantiomer with a kinetic resolution to transform a racemic substrate to an optically pure product. By using this method, dera-cemization of benzylisoquinolines rac-39 to berbines (S)-41 via the cascade transformation using monoamine oxidase (MAO), BBE, and morpholine-bo-rane concurrently gave optically pure product [S]-41 (>97% ee, HPLC) in up to 88% isolated yield (Scheme 2.14c). 

H (II), I (III), and J(IV) has been achieved through a flexible and expedient strategy that features a cascade sequence involving two concurrent [3,3] sigmat-opic rearrangements and an unusual intramolecular formal [2 + 2] thermal cycloaddition reaction between an electron-rich stilbene and a prenyl group [109]. 

The substrate requires a hydride acceptor proximal to a C—H bond serving as hydride donor, and the reaction is initiated by a hydride shift (or related H-atom-transfer step), which formally oxidizes the carbon donor and reduces the hydride acceptor. The new C-X bond will be formed at the hydride-donor atom, after the hydride shift takes place. The defining characteristic for these reactions is the functionalization of a C-H bond concurrent with a hydride shift. The names proposed by Sames ( HT-cyclization ) and Akiyama ( Internal Redox Cascade ) would seem more appropriate if focusing more on the unique hydride-shift mechanism that draws together a diverse group of substrates, at least for intramolecular examples. 

The four-stage low-pressure cascade impactor incorporates both the impactor and wire screen methods (Tokonami et al., 1997). This system can measure the activity size distribution of radon decay products in a low level environment within 90 min. Figure 6.9 shows a block diagram of the activity-weighted size distribution instrument. In the first air inlet, unattached radon decay products are collected on a metal wire screen (300 mesh openings 118.2 cm wire diameter 3.75 x 10 cm). A silicon semiconductor detector, SSD, is set opposite the metal wire screens where both collection and detection are concurrent. Output signals from the silicon semiconductor detector are sent through a preamplifier, PA, and the internal amplifier of a multichannel analyser, MCA, and then to the multichannel analyser. 

Siva Kumar, K., Mahesh Kumar, R, Sreenivasa Rao, V., Jafar, A. A., Meda, C. L. T, Kapavarapu, R., Parsa, K. V. L., Pal, M. (2012). A new cascade reaction concurrent construction of six and five membered rings leading to novel fused quinazolinones. Organic Biomolecular Chemistry, 10, 3098-3103. 

Schenie11.2 Concurrent oxidation and reduction in the biocatalytic production of hydromorphone by a three-enzyme cascade system. 

However, when biocatalysts showing a sufficient cofactor specificity are not naturally available, possible undesired interferences between the cofactor regeneration systems can be circumvented by different approaches still maintaining the one-pot fashion of the multienzymatic process. For example, in the biocatalyzed synthesis of 12-ketoursodeoxycholic acid, the performances of the investigated cascade system, in which five enzymes were involved in concurrent oxidation and reduction reactions at different sites of the starting substrate cholic acid, were significantly improved by simple compartmentalization of the oxidative and reductive enzymes in two membrane reactors (Scheme 11.4b) [11]. 

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