Ansa-bridging

Synthesis of the aromatic portion of rifamycia S has been accompHshed (161) as has the ansa-bridge portion (162—164). Joining of these fragments resulted in rifamycia S total synthesis (165,166). 

Roseophilin (273), a deeply red-colored pentacyclic compound isolated from the culture broth of Streptomyces griseoviridis, is a novel antitumor antibiotic. Compound 273 possesses a topologically unique pentacyclic skeleton, consisting of a 13-membered macrocycle incorporated in an ansa-bridged azafulvene, which in turn is linked to a conjugated heterocyclic ring system. The absolute stereochemistry of roseophilin, as depicted in Fig. 9, was unknown until the first total synthesis published by Tius and Harrington in 2001 [125]. All syn-... 

Samarium-catalyzed pinacol coupling of 27 is used for ansa-bridge formation to give [n]paracyclophanediols 28 (Scheme 15) [49]. 

Chien already postulated that C,-symmetric ansa-bridged complexes exist in two isomeric states, which interconvert during the course of the polymerization reaction [14, 15, 21, 22], Different stereoselectivities for monomer coordination and insertion are found for the two coordination sites of the asymmetric metallocene catalysts (Fig. 6,1 and IV). The migration of the polymer chain to the monomer, coordinated at the isoselective site f I—>11), followed by a consecutive chain back-skip (at higher temperatures) to the sterically less hindered side (II >111) leads to isotactic [mmmm] sequences [11],... 

It should be noted that, in the case of [(R)(Me)SiCp2YR], the TOF of the hydrogenation of 1-hexene decreases dramatically, from 11 100 h 1 (R=Me) to 200 h 1 when an ether functional group is bound on the ansa bridge (R=(CH2)5OMe), which shows the sensitivity of Group III metal complexes towards polar functionalities [121]. 

Pyrrolotropone is formed by a thermal retro-Grignard reaction from a 4-aryl-4-hydroxycyclohepta[i]pyrrole derivative (72CB1224). Finally, cycloadditions of 8-styryl-l-azaazulenes with dimethyl acetylenedicarboxy-late and subsequent oxidation yield 1,7-ansa-bridged pyrrolotropones (80BCJ1406). 

Me2Si] ansa bridges, relative to that of a single bridge, is a result of it enforcing a ligand conformation that reduces backdonation from the metal [16]. 

ANTIBIOTICS ANSAMACROLIDES. The ansamacrolides or ansa-tnycins are a family of antibiotics characterized by an aliphatic ansa-bridge that connects two nonndjacent positions of an the aromatic nucleus. Ansamacrolides can be divided into two groups based on the nature of the aromatic nucleus. One group contains a naphthoquinoid nucleus and... 

This stereoselective hydroboration can be used to obtain the nine chiral centers of the polyol 4, which is present in the ansa bridge of rifamycin. 

Compounds of the first type can be obtained starting Ifom butadiene zirconocene (with or without an ansa bridge). In the formation of (52), a pronounced property... 

Conformation polymorphism has been observed for a chiral s ivo-cis-ansa-bridged spermidine derivative of NPPh2(NPCl2)2 (121). ... 

The ansa-bridged homobinuclear titanium and heterobinuclear titanium-zirconium complexes have been reported. For more examples of this type of complexes, see Sections 4.05.4.1.1.(i).(g) and 4.05.4.1.3.(i)... 

The ansa-bridged group 4 titanium derivatives containing boron bridges have been reviewed.394,678,1729 The synthesis of the PhB-bridged homobimetallic titanium compound (Scheme 683) has been described using the dehalosilylation method.1729... 

Evidence for the initial steps of rifamycin B biosynthesis can be seen in the accumulation of a tetraketide chain elongation intermediate P8/1-0 58 from a mutant of A. mediterranei [92]. This evidence and the demonstration from feeding experiments that the carbons on either side of the ether link in the ansa bridge derive from the same propionate unit [93] indicate that this ether moiety is formed after the initial biosynthesis of a fully extended polyketide chain. 

---