Oxygen after intermediate

Rh2(DPB). In contrast to the dimers [Rh(P)]2, this compound reacted with dihydrogen only after intermediate addition of CO to yield a hydride (RhH)2(DPB) with two Rh-H bonds probably located inside the cavity, An insertion of CO into the Rh-H bonds, as with RhH(P) (see Sect. 4.2), was not observed. The dihydride was able to be dehydrogenated with molecular oxygen to reform Rh2(DPB) the formation of a dioxygen adduct (p-peroxo derivative) of the dirhodium(II) species which might be expected according to Scheme 3 (paths — q, t, u) was not observed. 

Fig. 5.4 Schematic representation of the Poulos-Kraut peroxidase mechanism in which the conserved distal histidine serves as an acid-base catalyst that transfers a proton from the H202 to the terminal oxygen after formation of the [Fe(III)-OOH] intermediate. The proximal histidine iron ligand and the catalytic histidine and arginine are shown. In HRP, these residues are His 170,...

As inert as the C-25 lactone carbonyl has been during the course of this synthesis, it can serve the role of electrophile in a reaction with a nucleophile. For example, addition of benzyloxymethyl-lithium29 to a cold (-78 °C) solution of 41 in THF, followed by treatment of the intermediate hemiketal with methyl orthoformate under acidic conditions, provides intermediate 42 in 80% overall yield. Reduction of the carbon-bromine bond in 42 with concomitant -elimination of the C-9 ether oxygen is achieved with Zn-Cu couple and sodium iodide at 60 °C in DMF. Under these reaction conditions, it is conceivable that the bromine substituent in 42 is replaced by iodine, after which event reductive elimination occurs. Silylation of the newly formed tertiary hydroxyl group at C-12 with triethylsilyl perchlorate, followed by oxidative cleavage of the olefin with ozone, results in the formation of key intermediate 3 in 85 % yield from 42. 

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