Complete Stereoselective Synthesis of Quasi-Enantiomeric Pseudo Imino-C-disaccharides: Parallel Kinetic Resolution of a Racemic cis-Dihydroxypyrroline N-Oxide by 1,2-Glycals
✍ Scribed by Francesca Cardona; Silvia Valenza; Andrea Goti; Alberto Brandi
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 260 KB
- Volume
- 1999
- Category
- Article
- ISSN
- 1434-193X
No coin nor oath required. For personal study only.
✦ Synopsis
Cycloadditions of hydroxylated enantiopure pyrroline N-with a racemic cis-dihydroxypyrroline N-oxide allowed the synthesis of two different pseudo imino-C-disaccharides oxides to 1,2-glycals display high double-asymmetric induction. The selectivity is controlled by the stereochemistry precursors in a completely enantiopure form, totally avoiding the formation of minor cycloadducts. at C-3 of the glycal. A parallel kinetic resolution experiment
We have recently reported a novel intermolecular cyclo-to Haworth convention. From an approach of this type, "matched" or "mismatched" interactions derive, depending addition of enantiopure pyrroline N-oxides 1 to glycals 2 (Scheme 1) [1] [2] aimed at the straightforward synthesis of a on the enantiomorphic form of the nitrone employed (Scheme 2). This double asymmetric induction is respon-new class of (1Ǟ2)-linked pseudo imino-C-disaccharides 4. [3] The interest of these compounds rests in their pre-sible for the quite different reactivity of nitrone 5 [7] derived from -tartaric acid with -glucal 6 and with -rhamnal sumed selectivities in inhibition of glycosidases, [4] since incorporation of a common sugar by a non-hydrolizable car-7. [1] The "mismatched" interaction occurs when the exo approach, anti to the C-3 group of -rhamnal 7, forces the bon link might improve the enzyme-selectivity of simple iminosugars. [3d][5] nitrone to react syn with respect to the vicinal O-tert-butyl group. [1c] Vice versa, the enantiomorphic nitrone 10 [7] derived from -tartaric acid affords with -rhamnal 7 the "matched" top-anti cycloadduct 11 in high yield, [1c] while the "mismatched" bottom-syn adduct 12 to -glucal 6 is formed much more slowly and with definitely more moderate yield (Scheme 2). [1b] The different reactivity of nitrones approaching syn or anti with respect to vicinal OR protecting groups, and the high selectivity of the bottom face of -glucal 6 and the top face of -rhamnal 7, appears nicely suited for a kinetic resolution of racemic nitrones. [8] In fact, reaction of racemic nitrone 13 (synthesized in 5 steps from or -arabinose) [9] with an excess of glucal 6 afforded a 3.3:1 mixture of the two bottom-exo adducts 14 and 15 with recovery of enantio-Scheme 1. Synthetic plan cycloadduct were detected in the 1 H-NMR spectra of the