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2. Azobisisobutyronitrile (AIBN) and tributylstannane were obtained from the Aldrich Chemical Company, Inc. The amount of AIBN can be reduced to 0.82 g (5 mmol) without affecting yields. A small excess (1.1 to 1.2 equiv) of tributylstannane must be used to ensure total consumption of starting material.
3. This method ensures that AIBN is not thermolized in the needle and that tributylstannane is diluted by the refluxing solvent before reaching the reaction mixture. It is also possible to add the tributylstannane solution by a dropping funnel (1 drop every 2 seconds) which replaces septum and syringe pump. This method gives
only slightly lower yields (75%) if the stannane solution runs down slowly on the glass surface of the condenser and does not enter the reaction mixture undiluted.
4. By this procedure most of the tributylbromostannane, and other stannyl compounds are removed. It is important to wait for complete separation of the phases.
5. The product is analytically pure, Anal., Calcd for C14H20O9: C, 50.60; H. 6.07. Found: C, 50.71; H, 6.25. 1H NMR (300 MHz, CDCI3) 8: 1.97 (ddd, 1 H, H-2a; J1,2a = 3.7, J2a,2c = 13.6, J2a,3 = 11 6); 2.04, 2.05, 2.09, 2.14 (4 s, 12 H, acetyl); 2.28 (ddd, 1 H, H-2e, Ji,2e = 1-4, J2e,3 = 5.3); 4.00-4.11 (m, 2 H, H-5, H-6); 4.36 (m, 1 H, H-6"); 5.08 (t, 1 H, H-4, J3(4 = J4,5 = 9.7); 5.32 (ddd, 1 H, H-3); 6.26 (br d, 1 H, H-1 ).
6. Concentration of the mother liquors gives another 0.4-0.6 g of impure product which can be recrystallized from tert-butyl methyl ether/hexane to give another
0.3-0.5 g (2-3%) of analytically pure product.
The main reaction step of this synthesis of 2-deoxy sugars is a radical rearrangement (2 -> 3).2 Bromine abstraction from the glucosyl bromide 1 by tributyltin radicals yields glucosyl radical 2 that undergoes acetoxy migration and gives the rearranged radical 3. This rearrangement is a stereoselective one-step reaction that occurs with rate coefficients of about 103 at 75°C in benzene.3 The driving force of the rearrangement 2 -> 3 is the formation of the acetal structure at C-1 of 3.4 Hydrogen abstraction from tributyltin hydride yields 2-deoxy sugar 4 and the tributyltin radical that starts another chain.
This rearrangement offers a general synthesis of a- and ß-2-deoxy sugars with pyranoid and furanoid ring systems (Table).5
1. Institut für Organische Chemie, TH Darmstadt, Petersenstrasse 22, D-6100 Darmstadt, Germany.
2. Giese, B. In "Stereochemistry of Organic and Bioorganic Transformations;” Bartmann, W.; Sharpless K. B., Ed.; VCH; Weinheim and New York: 1987; p. 261; Giese, B.; Gröninger, K. S.; Witzel, T.; Korth, H.-G.; Sustmann, R. Angew. Chem. 1987, 99, 246; Angew. Chem., Intern. Ed. Engl. 1987, 26, 233.
3. Korth, H.-G.; Sustmann, R.; Gröninger, K. S.; Leisung, M.; Giese, В. J. Org. Chem. 1988, 53, 4364.
4. Schleyer, P. v. R.; Jemmis, E. D.; Spitznagel, G. W. J. Am. Chem. Soc. 1985, 107, 6393.
5. Giese, B.; Gilges, S.; Gröninger, K. S.; Lamberth, C.; Witzel. T. Liebigs Ann. Chem. 1988, 615.
Synthesis of 2-Deoxy Sugars via Reductive Rearrangement of Glycosyl Derivatives5
Glycosyl Bromide 2-Deoxy Sugar Yield(Si
AcO*"/ V'"Br AcO«»/ V-OAc
AcO 't)Ac AcO
■u.Br AcO-/ >""OAc
AcO'' bAc AcO'
AcO OAc AcO
BzO'' ”'OBz BzO*
)"-Br AcO"-/ y—OAc 81
AcO-/ V-Br AcO—( )—OAc 65
/ \ J
AcO OAc AcO
BzO^y^ BzO^)-°BZ 75
Chemical Abstracts Nomenclature (Collective Index Number); (Registry Number)
188.8.131.52-Tetra-0-acetyl-2-deoxy-a-D-glucopyranose: D-arabino-Hexopyranose, 2-deoxy-, tetraacetate, a- (8); a-D-arabino-Hexopyranose, 2-deoxy-, tetraacetate (9); (16750-06-4)
184.108.40.206-Tetra-O-acetyl-a-D-glucopyranosyl bromide: Glucopyranosyl bromide, tetraacetate, cx-D- (8); a-D-glucopyranosyl bromide, 2,3,4,6-tetraacetate (9); (572-09-8)
Azobisisobutyronitrile: Propionitrile, 2,2'-azobis[2-methyi- (8); Propanenitrile, 2,2'-azobis[2-methyl- (9); (78-67-1)
Tributylstannane: Stannane, tributyl- (8,9); (688-73-3)
THE SYNTHESIS OF ENOL ETHERS BY METHYLENATION OF ESTER 1-PHENOXY-1-PHENYLETHENE AND 3.4-DIHYDRO-2-METHYLENE-2H-1-BENZOPYRAN (Ether, phenyl 1-phenylvinyl and 2H-1-Benzopyran, 3,4-dihydro-2-methylene-)
Submitted by Stanley H. Pine,1a Gia Kim,1b and Virgil Lee.
Checked by Roger B. Ruggeri and Clayton H. Heathcock.
A. 1-Phenoxy-1-phenylethene. To a 250-mL round-bottomed flask (Note 1) equipped with a magnetic stirring bar is added 5.0 g (20.0 mmol) of titanocene dichloride [bis(cyclopentadienyl)titanium dichloride] (Note 2). The flask is fitted with a rubber septum through which a large-gauge needle is passed to flush the system with dry nitrogen. After the vessel has been thoroughly purged, the nitrogen line flowing to the needle is opened to a mineral oil bubbler and 20 mL of a trimethylaluminum solution (2.0 M in toluene, 40 mmol) is added by a nitrogen-purged syringe (Note 3). Methane gas evolved by the reaction is allowed to vent as the resulting red solution is stirred at room temperature for 3 days. The Tebbe reagent2 thus formed is used In situ