Armed strategy

In 1980, a Merck group disclosed the results of a model study which amply demonstrated the efficiency with which the strained bicyclic ring system of thienamycin can be constructed by the carbene insertion cyclization strategy.12 Armed with this important precedent, Merck s process division developed and reported, in the same year, an alternative route to carbene precursor 4.13 Although this alternative approach suffers from the fact that it provides key intermediate 4, and ultimately thienamycin, in racemic form, it is very practical and is amenable to commercial scale production. The details of this interesting route are presented in Schemes 4-6. 

The generic features of these approaches are known from experience in anionic polymerization. However, radical polymerization brings some issues and some advantages. Combinations of strategies (a-d) are also known. Following star formation and with appropriate experimental design to ensure dormant chain end functionality is retained, the arms may be chain extended to give star block copolymers (321). In other cases the dormant functionality can be retained in the core in a manner that allows synthesis of mikto-arm stars (324). 

Hyperbranched and dendronized polymers such as 40, 41, and 42 have also been synthesized using the transition metal coupling strategies in recent years.32 These polymers are fundamentally different from those traditional linear polymers. They possess dendritic arms within die polymer or along the polymer backbone. It is believed that they possess interesting properties and have potential applications in many fields such as nanotechnology and catalysis ... 

Norr, L.C. 1991 Nutritional Consequences of Prehistoric Subsistence Strategies in Lower Central America. PhD dissertation. University of Illinois at Urbana-Champaign. Arm Arbor, MI, University Microfilms. 

Star-block copolymers are star polymers in which each arm is a block (diblock or triblock) copolymer. There are several methods used for the synthesis of star-block copolymers [142], and the most commonly used strategies are given in Scheme 67. 

With the multifunctional linking strategy either homo- or heterofunctional linking agents can be employed. The homo-approach was widely used in the past for the synthesis of well-defined //-stars with predetermined number and molecular characteristics of arms [163]. New applications of these approaches have been recently reported for the synthesis of A2B, A3B and A2B2 type //-stars. 

A few years later, the same group extended this strategy in order to access metabolically stable C-glycosyl clusters containing long-arm spacers via a sequence of transition metal-catalyzed transformations (Scheme 11).93 In this context, crossmetathesis reactions of various C-glycosyl compounds with alkenes having available... 

Two strategies for preparation of neutral and monocationic porphyrin models were used. The presence of a spacer arm directly attached to the meso phenyl position of the porphyrins gave access to a series of dimers with different substituents on the meso position and with different geometries. 

Another efficient strategy, the armed-disarmed approach, developed by Fraser-Reid is based on the chemoselectivity principle [193,202]. According to this... 

The first (phenoxyl)zinc(II) complexes have been prepared by using a similar strategy (143-145). Ligands containing a 1,4,7-triazacyclononane backbone and one, two, or three phenol pendent arms form very stable (phenolato)zinc(II) precursor complexes (Table II). Accordingly, their cyclic voltammograms display one, two or three reversible one-electron oxidation waves that in all cases have been shown by spectroelectrochemistry and/or EPR spectroscopy to be ligand based. 

A different strategy for the development of carbohydrate mimics has focused on the replacement of the non-pharmacophoric parts of active oligosaccharides by conformationally stable cyclic diols. Thus, looking for an analogue of the a-o-Man/)-(l — 2)-a-D-Man/>-(l — 6)-a-D-Man/)-mannotrioside, one of the terminal arms of the MangGlcNAc2... 

Wooley and coworkers have cross-linked the micellar corona and obtained the so-called shell cross-linked knedellike micelles [83,84]. This strategy was further applied to a wide variety of block copolymer micelles. Armes and coworkers have used a similar approach for the preparation of shell cross-linked micelles with hydrophilic core and shell [85]. Many other related examples can be found in the literature. 

The concept of armed/disarmed glycosyl donors was subsequently extended by other groups to thioglycosides21 and selenoglycosides.64 A similar strategy has been used by Friesen and Danishefsky to achieve chemoselectivity in electrophilic addition to glycal double bonds.65... 

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