. Can. J. Chem. 59, 1105 (1981).Several isomeric compounds derived from the spiro systems 5 to 9 (Scheme 4) were obtained from the acid cyclization of the appropriate dihydroxy ketone precursor.The configuration and the conformation of the products obtained was determined by I3C nmr analysis and equilibration studies. The experimental results can be rationalized by taking into account the anomeric and the exo-anomeric effects and the usual steric interactions.PIERRE DESLONGCHAMPS, DARYL D. ROWAN, NORMAND POTHIER, TILLES SAUVE et JOHN K. SAUNDERS. Can. J. Chem. 59, 1105 (1981).Plusieurs composes isomeres derives des systemes spiranniques 5 a 9 (Schema 4) furent obtenus par cyclisation en milieu acide des precurseurs dihydroxycetoniques appropries.La configuration et la conformation des produits obtenus ont ete dkterminees par la rmn du I3C et par des etudes d'equilibration. Les resultats exptrimentaux sont expliques en tenant compte de I'effet anomere, de I'effet exo-anomtre et des interactions steriques habituelles.Stereoelectronic effects have been recognized for a long time to influence the configuration and the conformation of acetals, particularly in carbohydrates where these effects were first discovered and discussed in terms of the anomeric and exoanomeric effects (I).,,, a-And P-glycosides can theoretically take conformations A , , A,, A, and E,, E,, E, respectively (Scheme 1). The relative proportions of these various conformers in a and P-glycosides should be influenced by the usual steric interactions and by stereoelectronic effects. For instance, if stereoelectronic effects (anomeric and exo-anomeric effects) are important, the following predictions can be made: (a) conformer A, has only one anomeric effect and conformers A , and A, have two each,4 and since A, must be eliminated because of strong 'This work has been presented at the 8th Natural Products Symposium, University of West Indies, Mona, Jamaica, January 1980, and at the 1st International Symposium on Stereoelectronic Effects, University of St. Andrews, Scotland, July 1980.2The preference for an alkoxy group to be axially oriented is due to the anomeric effect, whereas the preference for the R group of the side chain at the anomeric center to be in the A , or the E, conformation in a and P-glycosides respectively is due to the exo-anomeric effect (Scheme 1).3The anomeric (or the exo-anomeric) effect is considered to be a stabilizing effect which occurs when an electron pair of an oxygen atom is oriented antiperiplanar to a C-OR bond. The stabilization would be gained through an electronic delocalization due to the overlap of an electron lone pair orbital of an oxygen with the antibonding orbital of a C-OR sigma bond (2, 3).41n this article, we use the term anomeric effect to describe either an anomeric or an exo-anomeric effect, assuming that both effects are of the same order of magnitude. steric interactions, A , must correspond to the most stable conformer of an a-glycoside; (b) conformer E, has no anomeric effect whereas El and E, have ...
Acid-catalyzed cyclizations under thermodynamically and kinetically controlled conditions of the hydroxyenol ethers 18-21 are reported. Thermodynamically controllcd cyclizations of 18, 19, and 21 produced only the more stable corresponding spiroacetals 22 and 27. Thermodynamically controlled cyclization of compound 20 produced a 1:l mixture of non-epimerisable spiroacetals 24 and 26. On the other hand, kinetically controlled cyclizations of the same four hydroxyenol ethers produced, along with the more stable spiroacetals mentioned above, the less stable spiroacetals 23,25, and 28. These results show that the kinetically controlled cyclizalion takes place via an early transition state which produces a mixture of the less stable and the more stable isomers. These results are explained by an early transition state taking into account the principle of stereoelectronic control while following the antiperiplanar lone-pair hypothesis (Burgi-Dunirz angle of attack of a nucleophile on a n systcm).Introduction. -We previously reported [ 11 a study which revealed that the unsubstituted 1,7-dioxaspiro[5.5]undecane exists exclusively in conformation A, (Scheme I ), even at room temperature. This experimental observation was explained by the fact that conformation A, is stereoelectronically and sterically more stable than conformations A, and A, which were estimated to be less stable by a value of 2.4 and 4.8 kcal/mol, respectively'). This result was further confirmed experimentally by comparing the behavior of l-oxaspiro[5.5]undecane which was shown to exist as an equilibrium mixture of two conformers [2].
. Can J. Chem. 59, 1132 (1981). The carbon-13 chemical shift data for a series of 1,7-dioxa-, 1-oxa-7-thia-, and 1,7-dithiaspiro[5,5]undecanes and related compounds are presented. Analysis of this data led to structural and stereochemical assignments. The results show that the unsubstituted spiro bicyclic compounds exist in the conformation shown as conformation a . It is shown that the 13C chemical sbift parameters developed for cyclohexane derivatives can be applied to the sulfur but not to the oxygen heterocycles.NORMAND POTHIER, DARYL D. ROWAN, PIERRE DESLONGCHAMPS et JOHN K. SAUNDERS. Can. J. Chem. 59, 1132 (1981). Les deplacements chimiques en rmn de 13C de plusieurs derives de dioxa-1,7, oxa-1-thia-7 et dithia-1,7 spiro [5,5]undecanes sont presentes ainsi que ceux de quelques composes modkles. Les structures et la stereochimie des composes ont tte attribuees a partir de I'analyse de ces donnees. L'analyse conformationnelle demontre que les composes spiro-bicyclique non substitues existent dans la conformation a . On demontre que les parametres de deplacement chimique de 13C developpes pour les derives du cyclohexane peuvent &tre appliques aux heterocycles sulfures mais ils ne peuvent pas &tre utilises pour les composes oxygenes.
PIERRE DESLONGCHAMPS and NORMAND POTHIER. Can. J. Chem. 68, 597 (1990). A I3C nuclear magnetic resonance spectroscopy study of 1-oxaspiro[5.5]undecanes 1 , 4 , and 5 as a function of temperature is reported. Compound 1 exists as a rapidly equilibrating mixture of a major and a minor conformer ( l a and 1 b) at room temperature that can be observed at low temperature. By comparison, 1,7-dioxaspiro[5.5]undecane 2 is conformationally rigid in a single conformation (2a) at room temperature. This completely different behavior demonstrates the importance of the endo and the exo anomeric effects in the acetal function.Key words: spiro ethers, conformational analysis, NMR.PIERRE DESLONGCHAMPS et NORMAND POTHIER. Can. J. Chem. 68,597 (1990). Une ktude par spectroscopie de rksonance magnktique nuclkaire du 13C en fonction de la tempkrature des 1-oxaspiro[5.5]undkcanes 1 , 4 et 5 est rapportfie. Le composfi 1 est un mklange de deux conformkres, l'un majoritaire (la) et l'autre minoritaire (lb) a la tempkrature ambiante que l'on peut observer a basse tempkrature. Par comparaison, le 1,7-dioxaspiro[5.5]undkcane 2 est complktement rigide existant dans une seule conformation (2a) a la tempkrature ambiante. Ce comportement diffkrent dkmontre clairement l'importance des effets endo et exo anomkres de la fonction acktale.
In Table 1 a , in water should read 0.018 and QF(Sl) in cyclohexane 0.08. In Fig. 2 the scale is to be divided by a factor of 10 for Q , and for QH. In Fig. 3 the scale is to be divided by a factor of 10 for a , and by a factor of 3 for a(, + H,.Can. J. Chem. Downloaded from www.nrcresearchpress.com by 34.215.51.103 on 05/11/18For personal use only.
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