Catechols solid support

If the latter reaction proceeds through a closed transition state (e.g., 5 in Scheme 7.2), good diastereocontrol can be expected in the case of trans- and cis-CrotylSiCl3 (2b/2c) [14, 15]. Here, the anh-diastereoisomer 3b should be obtained from trans-crotyl derivative 2b, whereas the syn-isomer 3c should result from the reaction of the cis-isomer 2c (Scheme 7.2). Furthermore, this mechanism creates an opportunity for transferring the chiral information if the Lewis base employed is chiral. Provided that the Lewis base dissociates from the silicon in the intermediate 6 at a sufficient rate, it can act as a catalyst (rather than as a stoichiometric reagent). Typical Lewis bases that promote the allylation reaction are the common dipolar aprotic solvents, such as dimethylformamide (DMF) [8,12], dimethyl sulfoxide (DMSO) [8, 9], and hexamethylphosphoramide (HMPA) [9, 16], in addition to other substances that possess a strongly Lewis basic oxygen, such as various formamides [17] (in a solution or on a solid support [7, 8, 18]), urea derivatives [19], and catecholates [10] (and their chiral modifications [5c], [20]). It should be noted that, upon coordination to a Lewis base, the silicon atom becomes more Lewis acidic (vide infra), which facilitates its coordination to the carbonyl in the cyclic transition state 5. 

Fig. 4. The extent to which cyclic peptide synthetic strategies may be accomplished on solid support. Note that the catechol safety-catch linker requires the inversion of the order of synthesis where the protecting groups are removed followed by cyciization.

A different type of complex, which exhibits a characteristic band near 700 nm, is also formed when catechols are mixed with FeCl3 in various organic solvents [35]. Funabiki et al have isolated catecholatoiron complexes which are formulated either as [FeCl2(Cat)py] [H] (Cat H-Cat and 4-Me-Cat) or as FeCl(Cat)py2 (Cat 3-Me-Cat and DTBC) [32]. These complexes, which are dimeric in the solid state, become monomeric in THF and exhibit characteristic bands near 700 nm (H 630 nm, 4-Me 646 nm, 3-Me 658 nm, 3,5- Bu2 722 nm). The spectra have not, so far, been well explained. Although it is reported that various metal-semiquinonate complexes show a characteristic band in these regions [82], this is not a strong support for the formation of a stable complex of a Fe -SQ form in solution. Differences between M-DTBC and M-DTBSQ in electronic spectra have been discussed based on data for the Zn complexes [102]. 

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