Applications of the intramolecular nuclear Overhauser effect in structural organic chemistry

Bachers, Gary E.; Schaefer, Ted

doi:10.1021/cr60274a002

Cited by 47 publications

(14 citation statements)

References 2 publications

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“…More recent reviews (22,30) also give a description of the theory and describe its application and the pitfalls of its interpretation. In brief, it is assumed that for small molecules that tumble fast (typical oforganic molecules dissolved in a nonviscous solvent) only direct dipole-dipole interactions between protons contribute to the proton relaxation mechanisms and hence to the NOE, so that in a three-spin system (Fig.…”

Section: Introductionmentioning

confidence: 99%

Conformational analysis of 9β,19-cyclopropyl sterols: Detection of the pseudoplanar conformer by nuclear Overhauser effects and its functional implications

Wd¹,

Benson²,

Re³

et al. 1988

Proc. Natl. Acad. Sci. U.S.A.

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Nuclear Overhauser difference spectroscopy and variable temperature studies of the 9.3,19-cyclopropyl sterols 24,25-dehydropollinastanol (4,4-desmethyl-5a-cycloart-24-en-3fi-ol) and cyclolaudenol [(24S)-24-methyl-5a-cycloart-25(27)-en-3,3-ol] have shown the solution conformation ofthe B/C rings to be twist-chair/twist-boat rather than boat/ chair as suggested in the literature. This is very similar to the known crystal structure conformation of 9,3,19-cyclopropyl sterols. The effect of these conformations on the molecular shape is highly significant: the first conformation orients into a pseudoplanar or flat shape analogous to lanosterol, whereas the latter conformation exhibits a bent shape. The results are interpreted to imply that, for conformational reasons, cyclopropyl sterols can be expected to maintain the pseudoplanar shape in membrane bilayers.For reasons that are still unknown, organisms having a photosynthetic lineage cyclize squalene oxide to the 9f3,19-cyclopropyl sterol (CS) cycloartenol, while organisms having an evolutionary history that is completely nonphotosynthetic cyclize squalene oxide to the isomeric tetracycle lanosterol (1, 2). Cycloartenol has been shown to isomerize to lanosterol under acidic conditions (3) and during routine metabolism in plants (2). In lanosterol the 8,9 double bond approximates a trans structure for the B/C ring junction, maintaining a flat conformation of the molecule. However in CSs the 9,10 cyclopropyl bridgehead and the ,B oriented hydrogen at C-8 approximate a syn-cis configuration at the A/C and B/C ringjunctions. Some investigators believe (3-8) that this configurational disposition bends the plane of the molecule at the B/C ring junction through almost 90°. Dreiding models also show the molecule is no longer flat in the bent shape but neither they nor stereochemical considerations of polycyclic systems show that CSs could orient into the exaggerated "curvilinear belt" described by Bloch (4). That the configuration of the ring junctures influences the overall conformation of the molecule is known (8) in the isomeric pair of saturated sterols cholestanol and coprostanol. Here, inversion of the configuration of H-5 changes the A/B ring juncture stereochemistry from flat

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Section: Introductionmentioning

confidence: 99%

Conformational analysis of 9β,19-cyclopropyl sterols: Detection of the pseudoplanar conformer by nuclear Overhauser effects and its functional implications

Wd¹,

Benson²,

Re³

et al. 1988

Proc. Natl. Acad. Sci. U.S.A.

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“…Dreiding models show that the C-5 methine and C-7a P protons in conformer 9A are approximately 1.3 A apart, ideally disposed to exhibit an nOe effect. The C-7a methylene protons in conformer 9B were each estimated to be -3.5 A distant from the C-5 methine proton, a distance considered too remote for any nOe enhancement between these protons (43). Irradiation of the doublet of doublets at 6 2.96 induced a 12% enhancement of the area of the C-5 methine proton signal in the 'H nmr spectrum of 9 , thereby confirming that 9A is the preferred confornlation.…”

Section: Irradiation Ofmentioning

confidence: 90%

Photochemistry of cyclic α-hydroxy ketones. I. The nature and genesis of the photoproducts from steroidal 5-hydroxy 6-keto steroids and related compounds

Stiver¹,

Yates²

1988

Can. J. Chem.

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14 (1988). Photolyses of 5-hydroxy-5a-and 5P-cholestan-6-one and their 3P-acetoxy-and 30-benzyloxy derivatives in benzene or ethanol proceed stereospecifically with retention of configuration at C-5 to give the corresponding lactones, 6-oxa-Bhomocholestan-7-ones. Photolyses of 3P-acetoxy-7a-deutero-5-hydroxy-5a-and 50-cholestan-6-ones also proceed stereospecifically to give the corresponding 5-deutero lactones. 3~-Acetoxy-5-deuteroxy-5a-and SP-cholestan-6-one give on irradiation 1:3 and 7 : l mixtures, respectively, of the corresponding 7 a a -and 7aP-deutero lactones. Irradiation of 3~-acetoxy-5-methoxy-5a-cholestan-6-one in ethanol leads to the stereoselective formation of ethyl 3P-acetoxy-5-methoxy-5,6-seco-5a-cholestan-6-oate, while that of 3P-acetoxy-Sa-cholestan-6-one gives mainly photoreduction products. These observations are interpreted in terms of a-cleavage of the C-5-C-6 bond of the ketols to give alkyl acyl diradicals that undergo hydrogen transfer to give hydroxy ketenes, which then form the lactones. It is proposed that retention of configuration at C-5 results from two major factors-the nonplanar geometry of the hydroxyalkyl radical center, and fast hydrogen transfer in the diradical, the latter resulting from restricted rotation about the C-9-C-10 bond. The specific transfer of the 7a-deuterium atom in the 7a-deutero ketols is attributed to these factors and to the preferred direction of opening to the diradical on a-cleavage. The 0-deuterium labelling results are interpreted in terms of product development control in the conversion of the hydroxy ketenes to the lactones and are in accord with restricted rotation about the C-9-C-I0 bond. The photolysis of 3P-acetoxy-5-amino-5a-cholestan-6-one proceeds stereoselectively to give the SP-lactam analogue of the SP-lactone formed from the analogous Sa-ketol.SHIRLEY STIVER et PETER YATES. Can. J. Chem. 66, 214 (1988). La photolyse des hydroxy-5 5 a -et SP-cholestanones-6 et de leurs dCrivCs acCtoxy-3P et benzyloxy-3P, dans le benzkne ou I'tthanol, se produit d'une faqon stCrCospCcifique, avec rCtention de configuration en C-5, pour conduire aux lactones correspondantes, oxa-6 B-homocholestanones-7. La photolyse des acCtoxy-3P deutCro-7a hydroxy-5 5 a -et SP-cholestanones-6 se produit aussi d'une faqon stCrCospCcifique pour conduire aux lactones correspondantes deutkro-5. Par irradiation, les acCtoxy-3P deutCroxy-5 5 a -et 5P-cholestanones-6 conduisent respectivement a des melanges 1:3 et 7:l des lactones correspondantes deutCro-7aa et -7aP. L'irradiation de I'acCtoxy-3P mCthoxy-5 5a-cholestanone-6, dans I'Cthanol, conduit la formation stCrCosClective de I'acCtoxy-30 mCthoxy-5 sCco-5,6 5a-cholestanoate-6 d'Cthyle alors que celles de I'acetoxy-3P 5a-cholestanone-6 donne principalement des produits de photorCduction. On interprkte ces rCsultats en fonction d'un clivage-a de la liaison C-5/C-6 des cCtols qui fournit des diradicaux alkyle acyle qui subissent des transferts d'hydrogene conduisant des hydroxy-cCtenes qui forment alors des lactones. On suggkr...

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“…The propene unsaturation could be conjugated with ei ther of two possible aromatic phenolic chromaphores. Answers to re lated problems have been addressed through Dverhauser experiments (9) 9 but in this case the sample was too small to obtain sufficient sig nal-to-noise to detect the enhancements. Partially relaxed ^H-NMR proved to be a far more sensitive method of detecting regiochemical proximities in this system.…”

Section: Haustorial Inducing Factors From Gum Tragacanthmentioning

confidence: 99%

The Involvement of Allelochemicals in the Host Selection of Parasitic Angiosperms

Lynn

1985

ACS Symposium Series

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Several compounds have now been found which are capable of inducing the differentiation of the haustorium-a specialized organ in parasitic angiosperms which functions to attach the parasite to its host. A detailed description of the chemical nature of these compounds is presented. Measurements of the levels of these compounds exuded by a host have been made and correlated with levels required for haustorial induction. The data support a strict structural requirement for the factors involved in the induction of the haustorium. Although more than one structural class may be involved, the compounds appear to be linked with the host's allelochemicals and constitutive antibiotics. The evidence for synergistic activity among several different components in host exudate suggests the possibility of a more highly sophisticated mechanism of host selection through haustorial differentiation than previously anticipated.As so many natural products chemists before us, we have become intrigued with the seemingly endless array of organic structures which are produced in nature. Over the past thirty years, questions directed primarily at the structures of these compounds and at their effects on man's physiology have led to profound technological advances. Man can now synthesize virtually any organic compound which exists in nature and has used these talents to develop new methods to study and combat human disease. We are now in a position to explore a fundamentally different frontier-the physiological and ecological significance of these compounds to the organisms which produce them. The greater our understanding of the chemical communication and chemical defenses inherent in all living organisms, the better we will be able to understand and maintain the intricate chemical nature of the world which surrounds us (1). A need for this understanding in the plant kingdom, the area on which so much of our food and oxygen supply is dependent, is becoming particularly apparent. We have focused our interest on the mechanisms of host selection in parasitic angiosperms-plants which have evolved the capability of parasitizing other plants. An earlier proposal (2) that 0097-6156/85/0268-0055$07.75/0

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Applications of the intramolecular nuclear Overhauser effect in structural organic chemistry

Cited by 47 publications

References 2 publications

Conformational analysis of 9β,19-cyclopropyl sterols: Detection of the pseudoplanar conformer by nuclear Overhauser effects and its functional implications

Conformational analysis of 9β,19-cyclopropyl sterols: Detection of the pseudoplanar conformer by nuclear Overhauser effects and its functional implications

Photochemistry of cyclic α-hydroxy ketones. I. The nature and genesis of the photoproducts from steroidal 5-hydroxy 6-keto steroids and related compounds

The Involvement of Allelochemicals in the Host Selection of Parasitic Angiosperms

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