Cobalt protecting group

The Nicholas reaction was used to synthesize the p-lactam precursor of thienamycin in the laboratory of P.A. Jacobi and thereby accomplish its formal total synthesis. The necessary p-amino acid was prepared by the condensation of a boron enolate (derived from an acylated oxazolidinone) with the cobalt complex of an enantiopure propargylic ether. The resulting adduct was oxidized with ceric ammonium nitrate (CAN) to remove the cobalt protecting group from the triple bond, and the product was obtained with a 17 1 anti.syn selectivity and in good yield. 

Tosyl chloride 361 can be applied similarly (entry 23) [404], Here the best cobalt complex was dependent on the structure of the starting olefin. For terminal alkenes 2 mol% of the (salen)Co complex 357a was preferred. Secondary alkyl chlorides were obtained with complete regioselectivity in 73-94% yield, while the catalyst derived from Co(BF4)2 and /V-salicylidene diphenylglycinate 353 proved to be better for the hydrochlorination of 1,1-disubstituted olefins (entry 22). Tertiary alkyl chlorides 362 were obtained in 67-96% yield. The reaction conditions are mild so that acid- and base-sensitive protecting groups are compatible. 

Our group has used a combined metathesis-PKR for the synthesis of tricyclic compounds in one step. The process starts from pure cobalt complexed dienynes 52. The cobalt cluster acts first as a protecting group to avoid undesired enyne metathesis processes. The methodology allows the formation of tricyclic [6.5.5] (53) and [7.5.5] (54) structures including, in some examples, oxygen or nitrogen. Tricycles 53 are obtained in a total stereoselective manner, while compounds 54 are formed as mixtures of two diastereomers (Scheme 17) [110]. 

The first [M Dm3(ttnFe)2](C104)2 complexes of this type were obtained [79] by the interaction of iron(II) acetate and copper, zinc, nickel, cobalt, and manganese acetates with dimethylglyoxime and 1,4,7-trimethyl-1,4,7-triazacyclononane ttn) in methanol in the presence of triethylamine (Reaction 20). In this case, a triazamacrocycle served as the protecting group in the octahedral capping nFe "03 fragment. 

Protection of amino groups. Amino acids are converted to TCBpC-protected amino acids by reaction with 1 under the usual Schotten-Baumann conditions (80-95 7o yield). The products are nicely crystalline and are readily soluble in organic solvents. The TCBOC group is stable to N NaOH (40°, 2 hours) and to TFA (20°, 2 hours). The protecting group can be cleaved, as expected, with zinc in acetic acid or by reaction with lithium cobalt(I)-phthalocyanine. 

Functional moieties for click reaction can either be incorporated in the polymer chain directly with the monomer, by quenching or via post-functionalization approaches. If the functional moiety is introduced with the monomer or the quencher, one has to consider the cross-reactivities between the metathesis catalyst and the functional moiety. In this case, the specific functional group can either be introduced by using protective groups, for example, complexing alkyne with Co2(CO)g, di-cobalt octacarbonyl or has to be introduced after the actual polymerization reaction. Post-functionalization approaches thereby often... 

The Nicolas reaction is that the triple bond coordinates to the cobalt atom, and the triple bond and the carbon atom at the a-position are stabilized. Therefore, with the cobalt compound, the triple bond is stabilized, and the other functional group is able to react. Hence, the cobalt compound is able to be used as the protecting group for a reactive triple bond. For example, the selective reduction of a double bond of a compound having a triple bond and in Friedel-Crafts acylation are shown in eqs. (17.25) and (17.26), respectively [45,68-70]. 

A cobalt-catalyzed C2-alkylation of indole was shown to be possible with alkyl chlorides however, the indole N had to be functionalized with a pyridyl or pyrimidyl directing group (both protecting groups are easily installable and readily... 

Haloethyl ester protecting groups can be efficiently removed using cobalt(i)-phthalocyanin anions. cf. 4, 352 and 353.) The cobalt(l) is oxidized to cobalt(ii) during the reaction and is converted back to cobalt(i) by borohydride hence only catalytic quantities ca. 0.01 eq.) of the phthalocyanin are required. (See also ref. 62.)... 

Kinetically inert cobalt(III) complexes have been employed as N-protecting and N-activating groups for peptide synthesis. " Pentaam-minecobalt(III) has now been shown to be a useful C-terminal protecting group for sequential peptide synthesis. The reaction of complexes of... 

Deoxynucleosides have been protected at 0-5 by a modified dimethoxytrityl group as in 182 the A -hydroxysuccinimidyl ester permits attachment to resins with pendant amine groups, and the nucleoside can be recovered by mild acid treatment. On the other hand, the l,l-dianisyl-2,2,2-trichloroethyl ether (DATE) protecting group, as in 183, is particularly stable towards both acids and bases, but easily removed by reductive fragmentation using the supemucleophilic lithium cobalt(I) phthalocyanine. ... 

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