Bartons protocol

Musso has reported the synthesis of diasterane (tricyclo-[3.1.1.I2 4]octane) 15. For this first member of the series of asteranes, the decarboxylation of 16b -> 16c was best achieved via the photolysis of the Barton ester of 16a in the presence of BuSH, as shown in Scheme 5.14 Fukumoto has accomplished asymmetric total synthesis of atisine 17, where the bridged pentacyclic intermediate 18, a precursor for atisine, was synthesized via an intramolecular double Michael reaction starting with 19, Scheme 6.15 Barton protocol was favored during the late stages of the synthesis and the presence of various functionalities was easily accommodated. 

Barton protocol to remove the bridgehead carboxyl group as shown in Scheme 12.23... 

In this unusual and difficult to synthesize polyhydroxylated derivative of agarofuran there are six axial substituents. Barton protocol was successfully employed at the later stages of the synthesis. Huffman, J. W. Raveendranath, P. C. J. Org. Chem. (1986), 51, 2148. 

In the early 1990s, the Bartons protocol was widely applied to the preparation of deoxygenated derivatives at many sites on taxanes. Enhancement of cytotoxicity was observed for many 7-deoxy paclitaxel and docetaxel analogs, along with a... 

The remote functionalization in alkaloids is usually associated with the Hofmann-Loffler-Frej ag reaction however the Barton protocol may also be useful. Two examples are shown below, where photolysis of the nitrite yielded the corresponding oximes in good yields. ... 

Ono and Lash have been the two pioneers in applying the BZ reaction to the synthesis of pyrroles and, particularly, with applications to the synthesis of novel fused and other porphyrins. Although the concept was recognized by Barton and Zard, Ono and Lash independently discovered the conversion of 2-pyrrolecarbo ylates, prepared by the BZ reaction, into porphyrins by what is now a standard protocol (1. LiAlfL 2. 3. 

The stereoselective total synthesis of (+)-epiquinamide 301 has been achieved starting from the amino acid L-allysine ethylene acetal, which was converted into piperidine 298 by standard protocols. Allylation of 297 via an. V-acyliminium ion gave 298, which underwent RCM to provide 299 and the quinolizidine 300, with the wrong stereochemistry at the C-l stereocenter. This was corrected by mesylation of the alcohol, followed by Sn2 reaction with sodium azide to give 301, which, upon saponification of the methyl ester and decarboxylation through the Barton procedure followed by reduction and N-acylation, gave the desired natural product (Scheme 66) <20050L4005>. 

Organolithium reagent 35 was added to aldehyde 31 (Scheme 7.6) to obtain alcohol 36 as an inconsequential 1 1 mixture of diastereomers. The benzylic alcohol was removed using a Barton two-step radical deoxygenation protocol, followed by electrophilic aromatic bromination to provide the desired coupling partner 37. 

S. Grant, F. Davis, K.A. Law, A.C. Barton, S.D. Collyer, S.P.J. Higson, and T.D. Gibson, Label-free and reversible immunosensor based upon an AC impedance interrogation protocol. Anal. Chim. Acta 537, 163-168 (2005). 

The transformation of endocyclic nitrone 56 (made from N,0-bis-protected hydroxylamine 55) to lactam 20 can be carried out by photochemical activation or by a two-step modification of Barton s protocol, that is, by trapping the nitrone oxygen followed by an alkali-promoted, semi-pinacol-like rearrangement (03JOC8065). 

The keto group of the trisubstituted dioxanone 133 generated by ozonolysis was removed by radical deoxygenation according to the Barton-McCombie protocol [80] via the alcohols 134 and the corresponding xanthate, leading after deben-zylation to the dioxane 135 in excellent yield. After conversion to the tosylate, cleavage of the acetonide and protection of the secondary alcohol function as a TBS ether provided access to oxirane 128 by cyclization with NaH in 99% yield and in virtually diastereo- and enantiomerically pure form (de, ee > 96%). 

To demonstrate the versatility of his S3mthesis strategy Yamada used ketoester 151 as relais substance to S3mthesize two further picrotoxane alkaloids isolated from Dendrobium species, nobilonine (90) and 2-hydroxydendrobine (87) (Scheme 14) (84). Monobromination of 151 with bromine in dioxane and subsequent treatment with water resulted in hydroxy-y-lactam 152, whereas attempts to hydroxylate 151 by Barton oxidation led to rearrangements. Chemo- and stereoselective reduction with zinc borohydride converted 152 into the en fo-alcohol. To counterbalance the unfavorable conformational equilibrium this alcohol had to be converted into the alcoholate to achieve lactonization. Chemoselective reduction of the hydroxylac-tam moiety of lactone 153 again followed Borch s protocol, which led in this case to boron complexed amino compounds necessitating successive acid treatment to obtain racemic 2-hydroxydendrobine (87) in low yield accompanied by dendrobine (82). 2-Hydroxydendrobine (87) was converted into nobilonine (90) by Eschweiler-Clark methylation. 

Jackson BA, Barton JK. Probing nucleic acid structure with shape-selective rhodium and ruthenium complexes. In Current Protocols in Nucleic Acid Chemistry. Beaucage SL, Bergstrom DE, Click CD, Jones RA, eds. 2000. John Wiley Sons, Inc., New York. 

In the asymmetric synthesis of the C1-C19 fragment of kabiramide C, to complete the stereochemical array, J. Panek and co-workers used, among other methods, the Barton-McCombie deoxygenation protocol. ... 

F. Luzzio and co-workers devised a total synthesis for both antipodes of the (-)-Kishi iactam, which is a versatiie intermediate for the synthesis of the perhydrohistrionicotoxin (pHTX) aikaioids. in the final stages of the synthesis of the (-)-Kishi lactam, it was necessary to remove one of the secondary alcohol groups. The Barton radical deoxygenation protocol was utilized for this operation. 

Chiral auxiliaries have also been applied to the radicals themselves in the formation of chiral hydroxyalkyl radical equivalents [59]. Once again, stereocontrol is accessed through the use of chiral acetals, which are readily available in the form of sugars. Typical reactions of this type are shown in Eq. (13.47). First, the thiohydroxamate ester 148 is prepared so that radical intermediate 149 can be formed photolytically via Barton s radical decarboxylation protocol [60]. The chiral radical 149 can then be trapped by methyl acrylate in a 61% yield with an 11 1 diastereomeric preference for y-substituted 150. 

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