CARBOCYCLES FROM MONOSACCHARIDES .3. CONCERNING THE DIASTEREOSELECTIVE FORMATION OF CYCLOPENTANE DERIVATIVES - TRANSFORMATIONS IN THE GALACTOSE SERIES

Carbocycles from monosaccharides. III. Concerning the diastereoselective formation of cyclopentane derivatives. Transformations in the galactose series. The diastereoselectivity of the intramolecular nitrone‐olefine cycloaddition of 1, 3 and 4 Scheme 1, yielding only 2, 5 and 6 but none of the isomers 8, 9 and 10 is explained by assuming a kinetic control and postulating that the relative activation energies of the two relevant transition states in the cyclization of e.g. 1 can be estimated from the conformers A and B, the latter being destabilized by a synperiplanar arrangement of the nitrone function and the 2‐alkoxy‐group Scheme 2. It is further postulated, that this destabilization is responsible for the formation of (2,3)‐trans configurated products. Since 2, 5 and 6 are presumably thermodynamically more stable than 8, 9 and 10, a case was investigated, where the cycloaddition can either give thermodynamically less stable (2,3)‐trans‐product such as 12 or a thermodynamically more stable (2,3)‐cis‐product such as 13. 12 and 13 could both be formed from the aldehyde 25 via the nitrone 11 (Schemes 3 and 5). Treatment of the galactoside 16 first with Zn in aqueous butanol (forming among other products 25 and its 2‐debenzyl‐oxy‐derivative) and then with N‐Methyldroxylamine yielded the isoxazolidines 12 (72%), 13 (2%) and 27 (7%) (Schemes 4 and 6). Similarily, the anomeric silylated galactosides 17 and 23 gave 29 (78% from 17, 77% from 23) and 27 (5% and 3%). Upon desilylation, 29 gave 32, which was converted into 12. The structure of the isoxazolidines was unambiguously deduced from their NMR. spectra and those of their derivatives 33 and 34. Compound 32 was further transformed into its deoxyderivative 36. The high diastereoselectivity of the cycloaddition restricts the number of diastereomeric, pentasubstituted cyclopentanes available by this method. However, cyclization of the 2‐Hydroxy‐aldehyde 37 (Scheme 8) gave the kinetically less favoured isomer 40 in a higher proportion, showing the differential influence of hydrogen‐bonds on the relevant activation energies. Thermolysis of 32 gave 40 (79%) and 41 (11%). The structure of 41 was deduced from its NMR. spectra and those of its derivatives 42 and 43. Thermolysis of 29 gave, after desilylation, 41 (42%), 40 (22%) and 32 (13%) and thermolysis of 6 lead to a 25 : 75 equilibrium with 44 (combined yield 90%). These transformations illustrate means leading to additional isomers and are in agreement with the proposed explanation of the diastereoselectivity in question. Copyright © 1979 Verlag GmbH & Co. KGaA, Weinheim