Barrett’s synthesis

Another interesting use of TEMPO has been in free-radical substitution of alkyl halides. In this reaction, halides react with tributyltin hydride and TEMPO to yield A-alkoxyamine substitution products [18. This reaction is especially attractive in cases where anionic nucleophiles are sterically prevented from carrying out substitution reactions. An example of this can be seen in Barrett s synthesis of sucrose [18b], in which a stereoselective iodoetherification reaction was used to produce neopentyl alkyl iodide 13 (Scheme 5). Free radical substitution mediated by tributyltin hydride and TEMPO yielded A-alkoxyamine 14. The mechanism [19] involves TEMPO abstraction of hydrogen from tributyltin hydride [20] the stannyl radical then abstracts iodide from the substrate, and a second equivalent of TEMPO traps the resulting carbon radical. 

Scheme 5. Free-radical substitution of an alkyl iodide in Barrett s synthesis of sucrose...

In Barrett s synthesis of e t-clavilactone B 385 (Scheme 1.70), the benzyne intermediate generated from 386 was allowed to react with methallyhnagnesium chloride and epoxy-aldehyde 387, leading to compound 388, which was converted to the RCM precursor 389. As expected, RCM proved to be difficult and had to be performed under special conditions the catalyst [Ru]-II (40 mol%) was slowly added, along with tetrafluoro-l,4-BQ (to prevent olefin isomerization), and the formed ethylene was removed during the reaction. Under these conditions, the RCM product 390 was obtained in a satisfactory yield (65%) and converted to e t-clavilactone B [103]. 

Shortly after Barrett s publication of the successful approach to U-106305, Charette and Lebel reported the enantioselective synthesis of its non-natural enantiomer [12], and thereby further emphasised the power and the generality of their method. The same methodology was used to make the unnatural, all-trans septicyclopropane derivative 14 [10], the most highly cyclopropanated linear structure prepared to date. 

Sutherland and his co-workers developed a concise route for the synthesis of Barrett s intermediate 148, also using hydrogenation of a cyclic intermediate to establish the C-6 and C-8... 

Synthesis of Disaccharides and their Derivatives.- In the area of non-reducing disaccharides Barrett s very elegant synthesis of sucrose, which depends on an intramolecular redox cyclization process, has been published in detail. See Vol. 24, p. 29 for a preliminary report. Enzymic transfer from UDP-Gal can be used to obtain (3-d-galactopyranosyl 3-acetamido-3-deoxy-(S-D-xylopyranoside. ... 

J. Jaques, A. CoUet, and S. WiUen, Enantiomers, Racemate, and Resolutions,]o m Wiley Sons, Inc., New York, 1981 The Chemical Society of Japan, eds., Kikan Kagaku Sosetsu (No. 6, Resolution of Optical Isomers), Gakkai Shuppan Senta, Tokyo, Japan, 1989 G. C. Barrett ia Ref. 1, Chapt. 10, pp. 338—353 S. Otsuka and T. Mukaiyama, Progress of ylsymmetric Synthesis and Optical Resolution (ia Japanese), Kagaku Dojia, Kyoto, Japan, 1982. 

LBPs are likely to have conventional roles in the energy metabolism and transport of lipids in nematodes for membrane construction, etc. Many parasitic helminths have deficiencies in the synthesis of some lipids and so their lipid acquisition, transport and storage mechanisms clearly need to be specialized and therefore pertinent to the host-parasite relationship (Barrett, 1981). From a practical point of view, lipid transporter proteins may also be important in the delivery of anthelmintic drugs to their target most anthelmintics are hydrophobic and if they do not distribute to their site of action within the parasites by simple diffusion across and along membranes, then the parasite s own carrier proteins may be involved. 

Acyloxybutenolides obtained by / -elimination from ribonolactone (see Section IX.2) have served as appropriate chiral intermediates in several synthesis of antibiotics. Barrett and Sheth (205) reported a seven-step synthesis of racemic terI-butyl-8-0-ter/-butyldimethylsilylnonactate, a monomeric moiety of the antibiotic nonactin, from 157. Also, (4,S,6[Pg.189]

Gordon, E. M. Barrett, R. W. Dower, W. J. Fodor, S. P. A. Gallop, M. A. 1994. Applications of combinatorial technologies to drug discovery. 2. Combinatorial organic synthesis, library screening strategies, and future directions. /. Med. Chem.,37,1385-1401. 

With regard to the synthesis of the oligo-cyclo-propane natural product FR-900848 (1) multiple and consecutive cyclopropanation reactions using zinc carbenoids have been applied. Thus, in the total synthesis of 1 by Barrett et al. the Furu-kawa-procedure was used for the conversion of 52 into the biscyclopropane 53 (Scheme 11). [37] After bidirectional elongation of the molecule, another double cyclopropanation of diene 54 using Charette s catalyst gave tetracyclopro-pane 55 in 93 % yield as one stereoisomer only. Finally, the olefin 56 is cyclopropanated at -40 °C to yield the desired pentacyclopropane alcohol 57. Thus, all five cyclopropane rings of... 

Yuan Y, Barrett D, Zhang Y, Kahne D, Sliz P, Walker S. Crystal structure of a peptidoglycan glycosyltransferase suggests a model for processive glycan chain synthesis. Proc. Natl. Acad. Sci. U.S.A. 2007 104 5348-5353. 

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