Cleavage selective

Application of the Curtius reaction to the 3-carboxyl of a penicillin has provided intermediates which have been used for the construction of cephem derivatives. As can be seen in Scheme 23, this route allows the selective cleavage of the C(3)—N(4) bond of the thiazolidine ring, thereby allowing a reconstruction of that ring in a different form (72HCA388 and the following three papers). The preparation of a related intermediate is shown in Scheme 24 (76HCA2298). 

Selective cleavage of one secondary TBDMS ether in the presence of a somewhat more hindered one was achieved with Bu4N F in THF. ... 

Selective cleavage of a primary TBDMS group was achieved with acid in the presence of a secondary TBDMS group. 

The following example illustrates the selective cleavage of a 2 -benzoate in a nucleotide derivative. 

Selective cleavage to give either the more or less substituted derivativi possible with the proper choice of reagents. 

Zn, THF-H2O, pH 4.2, 25°, 4 h. The authors suggest that selective cleavage should be possible by this method since at pH 4.2, 25°, 2,2,2-trichlo-roethyl esters are cleaved in 10 min, 2,2,2-trichloroethyl carbamates are cleaved in 30 min, and the 2,2,2-trichloroethyl carbonate of estrone, formed in 87% yield from estrone and the acid chloride, is cleaved in 4 h (97% yield). 

Selective cleavage of an aiyl methylenedioxy group, or an aryl methyl ether, by boron trichloride has been investigated. 

For (n = 2, 3) T1(N03)3, CH3OH, 25°, 5 min, 73-99% yield.These conditions have been used to effect selective cleavage of a, 3-unsaturated thioketals. ... 

NO BF )" have also been used to cleave amides. Since only tertiary amides are cleaved by potassium r-butoxide (eq. 6), this method can be used to effect selective cleavage of tertiary amides in the presence of primary or secondary amides. " (Esters, however, are cleaved by similar conditions.) Photolytic cleavage of nitro amides (eq. 7) is discussed in a review. ... 

H2/Pd-C, 10 h, 87% yield. A nitrobenzyl carbamate is more readily cleaved by hydrogenolysis than a benzyl carbamate it is more stable to acid-catalyzed hydrolysis than is a benzyl carbamate, and therefore selective cleavage is possible. 

Hemithioketals can be prepared from 12,20-diketones [e.g., (81)] by reaction with mercaptoethanol, zinc chloride and sodium sulfate in dioxane. The 12-hemithioketal of triketocholanic acid is prepared by selective cleavage of the 3,7,12-trihemithioketal. " ... 

An excellent review is available that discusses the selective cleavage of numerous silyl derivatives. ... 

AcOH, H2O, THF (3 1 1), 25-80°, 15 min to 5 h2 Selective cleavage of a primary TBDMS group was achieved with acetic acid in the presence of a secondary TBDMS group. 

Sodium ethanethiolate has been examined for the selective cleavage of aryl methyl ethers. Methyl ethers para to an electron-withdrawing group are cleaved preferentially. ... 

Selective cleavage of the DMTr group from oxygen is accomplished with 80% aq. AcOH (rt, 10 min), whereas selective cleavage of the DMTr group from the thiol is effected with AgN03/NaOAc buffer (rt, 1 min). ... 

Interestingly, some unsymmetric ethers undergo selective cleavage and give only one of the two possible product combinations. 

Selective ether cleavage comes about during the substitution step, which obeys an Sn2 mechanism. Therefore, selective cleavage requires selective attack by Y on one of the electrophilic carbons in the protonated ether. Determine if selective attack is likely by examining the shape of the lowest-unoccupied molecular orbital (LUMO) in protonated ethyl propyl ether. Is this orbital larger near one carbon than the other If so, what product combination will result What other atom(s) contribute to the LUMO What would happen if 1 attacked this atom(s) ... 

Acidic ether cleavages are typical nucleophilic substitution reactions, either SN1 or Sn2 depending on the structure of the substrate. Ethers with only primary and secondary alkyl groups react by an S 2 mechanism, in which or Br attacks the protonated ether at the less hindered site. This usually results in a selective cleavage into a single alcohol and a single alkyl halide. For example, ethyl isopropyl ether yields exclusively isopropyl alcohol and iodoethane on cleavage by HI because nucleophilic attack by iodide ion occurs at the less hindered primary site rather than at the more hindered secondary site. 

Table 9.1 The use of modified sterols to allow selective cleavage of the side chain (based on Martin, CKA Sterols in Biotechnology Volume 6a Edited by Kieslich K1984 Verlag Chemie, Weinheim).

The L-threonine (EC 4.1.2.5), D-threonine (EC 4.1.2.-) or L-allothreonine aldolases (EC 4.1.2.6 synonymous to S1IMT) can be used for resolution of racemic (allo)threonine mixtures by highly selective cleavage of the unwanted isomers42, but can also efficiently direct the anabolic pathways. The substrate spectrum includes propanal, butanal and dodecanal43. 

In order to explain the selective cleavage of the C O bond, one has to take into account two factors (i) the oxonium is a better leaving group than the sulfonium (ii) for a nucleophilic substitution, weak bases are better leaving groups. The proposed mechanism follows Scheme XXI [99, 104] ... 

Tracer Studies on the Nitro lysis of Hexamine to RDX and HMX. The formation of RDX and/or HMX molecules from the nitration or nitrolysis of Hexamethylenetetramine (Hexamine) is a complex process and has been postulated to take place via two separate paths. One involves the selective cleavage of the Hexamine molecule to the appropriate cyclic nitramine (RDX, HMX or both) depending on the specific... 

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