Pyranose-derived dienes

Lopez J. C., Lukacs G. Pyranose-Derived Dienes and Conjugated Enals. Preparation and Diels-Alder Cycloaddition Reactions ACS Symp. Ser. 1992 494 33-49 Keywords carbohydrate, befera-Diels-Alder reactions, stereoselectivity... 

J. C. Lopez and G. Lukacs, Pyranose-derived dienes and conjugated enals—preparation and Diels-Alder cycloaddition reactions, in Cycloaddition Reactions in Carbohydrate Chemistry, 1992, pp. 33-49. 

Fraser-Reid s stereocontrolled synthesis of the Woodward reserpine precursor 195 relied upon a tandem 5-exol6-exo radical cyclization of pyranose-derived dienes [76-77]. As outlined in Scheme 36, a,P-unsaturated ester 188 was prepared by free radical coupling of iodide 187 with a tin acrylate. After hydrolysis of 188 (MeONa, MeOH, 100%) to give primary alcohol 189, the silicon tethered diene 190 was installed by silylation. Treatment of 190 with n-BujSnH led to the desired cage molecule 192 in high yield via a 5-exo-trig cyclization to intermediate 191 followed by a 6-exo cyclization. Tamao oxidation of tricycle 192 led to diol... 

Gomez, A M, Lopez, J C, Fraser-Reid, B, Serial radical cyclization of pyranose-derived dienes in the stereocontrolled synthesis of Woodward s reserpine precursor, J. Org. Chem., 60, 3859-3870, 1995. 

Several pyranose derived dienes and pyranose derived conjugated enals were prepared and the stereochemical outcome of their cycloaddition reactions studied. The diene and the enal moiety were incorporated into the carbohydrate ring by different methods depending on the substitution pattern of the pyranose. The pyranosidic ring might act as a diene or dienophile in cycloaddition reactions, and as a 1-oxabutadiene system in hetero Diels-Alder reactions. The factors governing the stereoselectivities for each type of cycloaddition are rationalized. The role of die steieogenic allylic alkoxy substituents in the relative topicity of the reactions is analyzed. 

LOPEZ LUKACS Pyranose-Derived Dienes and Qngugated Enals... 

Prior to this study there were two precedents in the literature of cycloaddition reaction of carbohydrate derived dienes where the diene moiety was incorporated into the ring. The first example came from Fraser-Reid s laboratories (30) on a furanoid derived diene 38 in Figure 12, and shortly after Giuliano and Buzby reported on a pyranose diene 39 (17), Of special interest to us was that in both reports just one single adduct was... 

The first two completed total syntheses of zaragozic acids have been reported this year. Carreira s synthesis of zaragozic acid C 122 commenced from D-erythronolactone.52 The synthesis of zaragozic A/squalestatin SI 123 by the Nicolaou group was based on sequential asymmetric dihydroxylations of the achiral diene 126, prepared by Sdlle coupling of 124 and 125 (Scheme 32).S3,54 The dihydroxylation was not completely diastereoselecdve, and elaboration led to the two epimeric pyranose derivative intermediates 127 and 128 en route to the natural product. 

As its methyl thioglycoside, lincosamine (6-amino-6,8-dideoxy-D- /7r/tro-D-ga/acto-octo-pyranose) is the saccharide portion of the clinically important antibiotic lincomycin. Racemic 3-methyl lincosaminide was prepared by Danishefsky and coworkers [140] who utilize the hetero Diels-Alder addition of crotonaldehyde to the Danishefsky s diene 268 to afford E) cA-2-(l-propenyl)-3-(benzyloxy)-2,3-dihydro-4-pyrone. The ZnBr2-catalyzed cycloaddition of 268 to A-carbobenzoxy-0-protected-D-a//(9-threoninal gives a 2 1 mixture of adducts 270 and 271 that has been converted into 272, a protected form of 6-amino-6,8-dideoxy-D- 77f/iro-L-ga/acto-octopyranose (Scheme 13.78). Analogously, the ZnBr2-promoted cycloaddition of the D-threoninal derivative 273 to 268 leads to a 3 1 mixture of adducts. The major adduct... 

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