Synthetic strategies cascade

To rapidly construct complex structures, a recent synthetic strategy uses the Diels-Alder cycloaddition in sequence with another Diels-Alder reaction or with other reactions without isolating the intermediates (domino, tandem, cascade, consecutive, etc., reactions) [4-6]. Scheme 1.2 illustrates some examples. 

On the other hand, branched building blocks [30] possess inherent branching points remote from the site of connection. These monomers can be simple or complex in design and have been used in both divergent and convergent synthetic strategies. In addition, branched monomers can also be used to install utilitarian functionality within cascade molecules. 

Nature accomplishes many syntheses-even those of complex molecules-by sequences of elementary steps. In the last few decades, the blueprint of catalyzed cascade reactions has found fertile soil through the advent of transition metal catalysis in laboratories. Scrutinizing catalytic cycles and mechanistic insight has paved the way for designing new sequential transformations catalyzed by transition metal complexes in a consecutive or domino fashion. In particular, transition metal-catalyzed sequences considerably enhance structural complexity by multiple iterations of organometalhc elementary steps. All this has fundamentally revolutionized synthetic strategies and conceptual thinking. 

More recently, receptors derived from simple Schiff base synthetic strategies were found to bind fluoride and water in the cavity 8 (Ar=m-xylyl). " In a slightly larger cavity derived from a p-xylyl spacer, two fluorides were bound inside the cavity, bridged by a single water molecule in a type of cascade complex (Ar=p-xylyl). The latter motif is reminiscent of the ditopic metal ion cascade complexes observed for these ligands. 

Liu, W, Khedkar, V., Baskar, B., Schurmann, M., and Kumar, K. (2011) Branching cascades a concise synthetic strategy targeting diverse and complex molecular frameworks. Angew. Chem. 

Multicatalytic domino processes are synthetic strategies in which two or more catalytic domino reactions are sequentially used in the synthesis of a compound Hbrary [8-10]. The product obtained from the first domino reaction is the substrate for subsequent cascades, which yield products with either a different scaffold or modified functional group formation. 

This one-step synthesis of tricyclic systems has also been extended to provide a synthetic strategy for both cis- and tra 5 -hexahydro-l//-benz[/]indene starting from a common acetylenic precursor (44) (Scheme 21). Hydrogenation of 44 over Lindlar s catalyst gave 45. This compound would then be involved in a Pd-catalyzed cascade process, resulting in a bis-cyclization reaction. This reaction, performed in THF, at 55 °C, with potassium t-butoxide and 5% Pd(dppe) led to the exclusive formation of trans-hexa-hydro-l//-benz[/]indene. The stereochemical control of this reaction may be explained if it were to pass through a transition state in which the benzylic substituent lies in a pseudoequatorial orientation. 

Reaction conditions Chloroform-acetonitrile (1 1), room temperature Synthetic strategy One-pot cascade annulation via Diels-Alder reaction Cataiyst Trifluoroacetic acid (TFA)... 

Synthetic strategy Regioselective terminal aziridine opening - aminoacetylative cyclization cascades... 

Type of reaction C-N bond formation Reaction conditions Toluene, room temperature Synthetic strategy Base-catalyzed cascade reaction Catalyst l,8-Diazabicycl[5.4.0]undec-7-ene (DBU)... 

Synthetic strategy One-pot DMAP-promoted cascade C-S/C-N bonds formation... 

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