Stereoselective synthesis of 2,3-unsaturated-aza-O-glycosides via new diastereoisomeric N-Cbz-imino glycal-derived allyl epoxides

Organic Letters

Volumn 9, Issue 22, 2007, Pages 4479-4482

  Di Bussolo, Valeria ^a   Fiasella, Annalisa ^a   Romano, Maria Rosaria ^a   Favero, Lucilla ^a   Pineschi, Mauro ^a   Crotti, Paolo ^a  

^a UNIVERSITY OF PISA   (Italy)

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Indexed keywords

EID: 35949000419     PISSN: 15237060     EISSN: None     Source Type: Journal    
DOI: 10.1021/ol701836a     Document Type: Article

References (17)

1. (a) Di Bussolo, V.; Caselli, M.; Romano, M. R.; Pineschi, M.; Crotti, P. J. Org. Chem. 2004, 69, 7383.
2. (b) J. Org. Chem. 2004, 69, 8702.
3. Afarinkia, K.; Bahar, A. Tetrahedron: Asymmetry 2005, 16, 1239.
4. As they were not necessary to our study as pure enantiomers, epoxides 5 and 6 were prepared in a racemic form. A ciano-substituted epoxide, structurally related to 5, has been previously described and its reaction with MeOH partially examined: Natsume, M.; Wada, M.; Ogawa, M. Chem. Pharm. Bull. 1978, 26, 3364.
5. 2OBn group axial (Scheme 2) (see Supporting Information).
6. (a) Dransfield, P. J.; Gore, P. M.; Prokes, I.; Shipman, M.; Slawin, A. M. Z. Org. Biomol. Chem. 2003, 1, 2723.
7. (b) In a closely related imino glycal system and conditions, the corresponding β-attack was up to 99%: Craig, D.; McCague, R.; Potter, G. A.; Williams, M. R. V. Synlett 1998, 55.
8. Comins, D. L.; Fulp, A. B. Tetrahedron Lett. 2001, 42, 6839.
9. 1H NMR spectrum of 5.
10. The monotosylate 23 was obtained in an unsatisfactory yield (36%), reasonably due to the concomitant formation of the very reactive regio-isomeric allyl tosylate 23-al, which rapidly decomposes in the reaction medium, determining the loss of the corresponding portion of the starting diol 17. The structure of 23 was confirmed by NOE experiments on the corresponding acetate 23-Ac.
11. Luche, J.-L.; Gemal, A. L. J. Am. Chem. Soc. 1979, 101, 5848.
12. The characterization by NMR spectroscopy of many of the compounds obtained was hampered by the presence of rotamers around the N-CO bond. This necessitated the use of variable-temperature NMR spectroscopy to complete structural assignments.
13. 2OBn group axial is largely the more stable one (see Supporting Information).
14. 2OBn side chain (Scheme 9).
15. Particularly significant is the result from epoxide 5, where a product-dependent selectivity of the glycosylation process would have led to the corresponding β-anomer instead of the observed α-anomer. In 6, the product- and the substrate-dependent selectivity lead to the same compound, the corresponding β-anomer.
16. Alternatively, the intermediate occurrence of an N-acyliminio carbocation, pseudoaxially attacked by MeOH, cannot be excluded.
17. Analogously, under the same reaction conditions, 29 completely epimerizes to 30, whereas 28 and 30 are stable.