Reinvestigation of the Synthesis of Per-benzylated Glycosylethenes: A New Result
SCHEME 1
Juan Xie,* Franc¸ ois Durrat, and Jean-Marc Vale´ry Laboratoire de Chimie des Glucides, CNRS UMR 7613, Universite´ Pierre et Marie Curie, 4 Place Jussieu, 75005 Paris, France
TABLE 1.
[email protected] Received June 10, 2003
Abstract: Addition of vinylmagnesium bromide on the perbenzylated glucono-1,5-lactone gave, after reduction with Et3SiH/BF3‚Et2O, a mixture of the desired β-C-vinyl glucoside 1 and the unexpected β-C-but-3-enyl glucoside 3 resulting from double addition of vinylmagnesium bromide on the lactone. Similar results have been obtained with the perbenzylated galactono-1,5-lactone. This side reaction was then been explored to prepare β-C-but-3-enyl glycosides and other β-C-glycosyl derivatives by employing different Grignard reagents. An alternative approach to the per-benzylated glycosylethenes has been studied and compared.
Glycosylethenes (e.g. (2,3,4,6-tetra-O-benzyl-β-D-glucopyranosyl)ethene, 1) are versatile synthetic intermediates which have been used for the synthesis of C-glycosyl amino acids1,2 and carbasugars.3 During the course of studies on the synthesis of sugar amino acids, we needed to prepare the β-C-vinyl compound 1 that could be readily transformed into desired sugar amino acid derivatives.4 The first synthesis of this compound was reported by Kraus and Molina, by treating the per-benzylated lactone 2 with 1.5 equiv of vinylmagnesium bromide at -78 °C followed by reduction with Et3SiH and BF3‚Et2O, in 60% yield with two steps (Scheme 1).5 However, this reaction was found quite tricky in our hands. Our first attempt using the described procedure led to two products: the desired β-C-vinyl compound 1 together with the β-C-but3-enyl derivative 3 in 32.6% and 17.9% isolated yield (Table 1, entry 2). The physical and spectral properties of 3 were essentially identical with those reported, since compound 3 has already been prepared by nucleophilic addition of but-3-enylmagesium bromide on the lactone 2 followed by reduction.6,7 It is to be noticed that Dondoni and colleagues have obtained the same yield of 1 (32.5%) using similar reaction conditions.1 Nevertheless, the formation of 3 has never been reported under these conditions. * To whom correspondence should be addressed. Phone: 33-1-4427-58-93. Fax: 33-1-44-27-55-13. (1) Dondoni, A.; Giovannini, P. P.; Marra, A. J. Chem. Soc., Perkin Trans. 1 2001, 2380-2388. (2) Westermann, B.; Walter, A.; Flo¨rke, U.; Altenbach, H.-J. Org. Lett. 2001, 3, 1375-1378. (3) Wang, W.; Zhang, Y.; Zhou, H.; Ble´riot, Y.; Sinay¨ , P. Eur. J. Org. Chem. 2001, 1053-1059. (4) Xie, J. Eur. J. Org. Chem. 2002, 3411-3418. (5) Kraus, G. A.; Molina, M. T. J. Org. Chem. 1988, 53, 752-753. (6) Best, W. M.; Ferro, V.; Harle, J.; Stick, R. V.; Tilbrook, D. M. G. Aust. J. Chem. 1997, 50, 463-472. (7) Cipolla, L.; Nicotra, F.; Vismara, E.; Guerrini, M. Tetrahedron 1997, 53, 6163-6170.
products (yield) entry
lactone
CH2dCHMgBr (equiv)
mono-adduct (1,5)
bis-adduct (3,6)
1 2 3 4 5 6
2 2 2 4 4 4
1.2 1.5 4 1.2 1.5 4
1 (49.3%) 1 (32.6%)
3 (13.8%) 3 (17.9%) 3 (89%)
5 (50%) 5 (25%)
6 (28.4%) 6 (47%)
To improve the yield of the desired product 1, we tried to modify the reaction conditions. The 1/3 ratio was insensitive to the reaction concentration. Raising the reaction temperature led to a complex mixture. When conducted at lower temperature (-100 °C), the reaction was extremely slow, and the majority of the starting lactone was recovered. The best yield of 1 (49.3%) was obtained with 1.2 equiv of nucleophile, along with 13.8% of 3 (entry 1). A lower conversion of the starting lactone was observed when using
