1 Applications of Ethylboron Compounds in
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Carbohydrate Chemistry R. KÖSTER and W. V. DAHLHOFF Max-Planck-Institut für Kohlenforschung, Mülheim-Ruhr, Germany
The fundamental work of Böeseken (1) in the first half of this century showed that the element boron has different a f f i n i t i e s towards the various polyhydroxy compounds. The early investigations demonstrated the stereospecifity of the various polyols and saccharides towards boric acid. One analytical application that developed from this work was the quantitative determination of boron based on the interaction of boric acid with certain hydroxy compounds. However, it i s the preparative aspects that are of interest to carbohydrate chemists, and we will present here some of the uses of organoboron compounds in synthetic work and show the advantages they offer over conventional blocking groups. The most important and well known application of organoboron compounds in sugar chemistry was, and is still, the use of the bifunctional O-phenylboranediyl-ligand as a protective group. Some O-phenylboranediyl derivatives of monosaccharides have been described in the literature (2,3). They have been prepared from phenylboric acid, which is neither as easy to react nor as easy to remove as the O-ethylboron group. The products are often not volatile and cannot be purified by distillation. Usually crystallisation is used to purify the products, but this is often difficult to achieve. In the past three years we have discovered new methods for the preparation of boron derivatives of hydroxy compounds and in particular O-ethylboron compounds. We were thus able to apply our 20 years of experience in the f i e l d of organoboranes to carbohydrate chemistry. The combination of two separate fields of research often brings about new impetus to the development of science. We believe that by combining our expertese in the field of sugars and 1
El Khadem; Synthetic Methods for Carbohydrates ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
2
SYNTHETIC
METHODS
FOR
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Figure 7.
1-OAcyl-glycerols
via ethylboron intermediates
El Khadem; Synthetic Methods for Carbohydrates ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
1.
KOSTER AND DAHLOFF
Ethylboron
the u s u a l manner, then or 1-0-benzoylglycerol
7
Compounds
deborylated to i n high yields
give (see
1-0-acetylf i g . 7).
The O - e t h y l b o r o n a t i o n o f b u t a n - 1 , 2 , 4 - t r i o l i s t h e seconcT e x a m p l e ( V7). This a l k a n e t r i o l yielded on O - e t h y l b o r o n a t i o n a gas c h r o m a t o g r a p h i c a l l y (V7) pure, Tsomer-free compound i n h i g h y i e l d w h i c h h a d one 2-ethyl-1,3,2-dioxaborinan-ring ( 160'C)
0 b
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w
, HO
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OH HO
OH
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-3 EtH Et
route
D-Mannitol •
lu
• 6 BEt-
Β
N
B "
Et t
-«EtH
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,-3(BUC) 0 2
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Et
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route IV ,"|(tBiiC0e) O 2
)2° .00*0 3 H
2°
b = BEt 2 BEt *= act. B E t 3
Figure 10.
3
Five routes to prepare 1,2:3,4:5,6-tris-O-ethyïboranediyl-O-mannitol
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* MeOH(2Q-(^
oSÇ%/ûMe -1e v a c u u m d i s t i l l e d without decomposition and are quite often c r y s t a l l i n e . 6. S e v e r a l o f t h e b i s - O - e t h y l b o r a n e d i y l - d e r i v a t i v e s c a n be p a r t i a l l y a n d s e l e c t i v e l y deborylated w i t h methanol a t room t e m p e r a t u r e . Such s e l e c t i v e d e b o r y l a t i o n s have not been observed f o r any 0 - p h e n y l boranediyl derivatives. 7 . T h e s t r u c t u r e s o f many b u t n o t a l l o f the products resemble those of the analogous 0-isopropylidene compounds. 8 . The O - e t h y l b o r a n e d i y l g r o u p c a n be removed under f a r miTder conditions than the O-isopropylidene g r o u p . T h u s , f o r e x a m p l e , we h a v e b e e n a b l e t o prepare some 1 - 0 - a c y l d e r i v a t i v e s v i a O - e t h y l b o r a n e d i y l i n t e r mediates. Because of these f a c t s , the use of the O - e t h y l boron protective groups offers new p o s s i b i l i t i e s f o r t h e p u r i f i c a t i o n , t h e s e p a r a t i o n a n d many O - d e r i v a t i zations of polyhydroxy compounds.
Literature Cited 1. Böeseken, J., Adv. Carbohydr. Chem. (1949) 4, 189-210. 2. P e r r i e r , R . J . and Prasad, D . , J. Chem. Soc. (1965) 7429-7432. 3. Shafizadeh, F., McGinnis, G.D., and Chin, P . S . , Carbohydrate Res. (1971) 18, 357-361. 4. Köster, R . , B e l l u t , H . , and Fenzl, W., Liebigs Ann. Chem. (1974) 54-68. 5. Köster, R., Binger, P . , and Dahlhoff, W.V., Synth. Inorg. Met.-Org. Chem. (1973) 3, 359-367. 6. Köster, R . , Amen, K.-L., and Dahlhoff, W.V., Liebigs Ann. Chem. (1975) 752-788. 7. Köster, R., F e n z l , W., and Seidel, G., Liebigs Ann. Chem. (1975) 352-372. 8. Dahlhoff, W.V. and Köster, R., Liebigs Ann. Chem. (1975) 1625-1636. 9. Dahlhoff, W.V. and Köster, R., Liebigs Ann. Chem. (1975) 1914-1925. 10. Dahlhoff, W.V. and Köster, R . , Liebigs Ann. Chem. (1975) 1926-1933.
El Khadem; Synthetic Methods for Carbohydrates ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
I.
KÖSTER AND DAHLOFF
Ethylboron Compounds
11. Dahlhoff, W.V., Schüßler. W., and Köster, R., Liebigs Ann. Chem. (1976) 387-394. 12. Dahlhoff, W.V. and Köster, R . , J. Org. Chem. (1976) 98 in press. 13. Köster, R. and Dahlhoff, W.V., Liebigs Ann. Chem. (1977), in press. 14. Hoffmann, E.G. and Schroth, G . , Max-Planck-Insti tut für Kohlenforschung, Mülheim-Ruhr. 15. Mynott, R . , Max-Planck-Institut für Kohlenfor schung, Mülheim-Ruhr. 16. Hoffmann, E.G. and Mynott, R . , Max-Planck-Institut für Kohlenforschung, Mülheim-Ruhr. 17. Schomburg, G. and Kandziorowski, F., Max-Planck - I n s t i t u t für Kohlenforschung, Mülheim-Ruhr. 18. cf. Schmidt. O.Th., i n Methods i n Carbohydrate Chem. (1963) 2, 318-325; E d i t o r s : R . L . Whistler and M.L. Wolfrom.
El Khadem; Synthetic Methods for Carbohydrates ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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