Abstract:mg of the product by vpc gave 7.4 mg of partially crystalline material having mp 93.5-95.5°(softens at 91°). Sublimation gave ( + )-occidentalol (2) having mp 94.2-95.0°; uv max 263.5 nm (« 4300, 3940); [«]"d +364°, +360°(c 0.36, CHCl«j. The nmr (microcavity tube) and ir spectra of synthetic ( + )-occidentalol were essentially identical with spectra determined on a sample of authentic (+ )-occidentalol (kindly supplied by Dr. E. von Rudloff) which exhibited mp 94-95.5°; [ ] 6 +341°(c0.53); uv max 263.5 nm (e 4… Show more
“…Removal of the TES protecting group in 34 was achieved by treatment with TBAF in THF, and the following purification by aqueous AgNO 3 extraction afforded pure 15-hydroxyhedycaryol ( 35 ) in good yield. Just as found for hedycaryol, the 1 H NMR spectrum of 35 indicates the presence of at least three distinct conformers at room temperature. 10 …”
An efficient method has been developed for the synthesis of (E,E)-germacrane sesquiterpene alcohols. The key step in these syntheses involves the enolate-assisted 1,4-fragmentation of properly functionalized perhydro-1-naphthalenecarboxaldehydes with 1 equiv of sodium tert-amylate as base, to give the corresponding (E,E)-germacrane aldehydes. These aldehydes are not very stable, and in situ reduction of the aldehyde function with Red-Al is required to obtain high yields of the desired germacrane alcohols. This procedure has led to the successful synthesis of 15-hydroxygermacrene B (4) and 15-hydroxyhedycaryol (35) from the mesylates 6 and 33, respectively. When KHMDS is used instead of sodium tert-amylate in the fragmentation reaction of 6, isomerization of the initially formed C(4)-C(5) E double bond cannot be avoided and results, after in situ reduction with RedAl, in the formation of the (E,Z)-germacrane alcohol 24. The 15-hydroxy-(E,E)-germacranes are excellent starting materials for the selective synthesis of the corresponding 4,5-epoxides, which in turn can perfectly well be used in studies on the biomimetic formation of guaiane sesquiterpenes.
“…Removal of the TES protecting group in 34 was achieved by treatment with TBAF in THF, and the following purification by aqueous AgNO 3 extraction afforded pure 15-hydroxyhedycaryol ( 35 ) in good yield. Just as found for hedycaryol, the 1 H NMR spectrum of 35 indicates the presence of at least three distinct conformers at room temperature. 10 …”
An efficient method has been developed for the synthesis of (E,E)-germacrane sesquiterpene alcohols. The key step in these syntheses involves the enolate-assisted 1,4-fragmentation of properly functionalized perhydro-1-naphthalenecarboxaldehydes with 1 equiv of sodium tert-amylate as base, to give the corresponding (E,E)-germacrane aldehydes. These aldehydes are not very stable, and in situ reduction of the aldehyde function with Red-Al is required to obtain high yields of the desired germacrane alcohols. This procedure has led to the successful synthesis of 15-hydroxygermacrene B (4) and 15-hydroxyhedycaryol (35) from the mesylates 6 and 33, respectively. When KHMDS is used instead of sodium tert-amylate in the fragmentation reaction of 6, isomerization of the initially formed C(4)-C(5) E double bond cannot be avoided and results, after in situ reduction with RedAl, in the formation of the (E,Z)-germacrane alcohol 24. The 15-hydroxy-(E,E)-germacranes are excellent starting materials for the selective synthesis of the corresponding 4,5-epoxides, which in turn can perfectly well be used in studies on the biomimetic formation of guaiane sesquiterpenes.
“…On the other hand, in the case of the 1Cl crystal structure, the conformations of the two molecules are similar to the former 1Br molecule and the third one is similar to the latter 1Br molecule. Hence, this is a case of conformational isomorphism, the occurrence of different conformers in the same crystal structure . Interestingly, 1I, 1Br, and 1Cl are isostructural in spite of different molecular conformations, and the Z′ value also increases gradually from 1 to 3.…”
Crystal structures of the guest free forms and some solvates
of
1,3,5-triethyl-2,4,6-tris(4-halophenoxy)methylbenzenes (1X, X = I,
Br, Cl, F) have been studied. The guest free forms of 1I, 1Br, and
1Cl are isostructural, but the crystal structure of 1F is different
from the heavier halogen analogues. An entirely different crystal
structure of the Me analogue, which is known to be isosteric to the
corresponding bromo compound, shows the importance of the electronic
factors of the halogens in this series of structures. 1I and 1Br form
four types of architectures in their solvates depending upon the inter-halogen
interaction geometries. All these solvates are two dimensionally isostructural
to the guest free form. 1Cl forms three different types of frameworks
in its solvates, and the ethylacetate solvates of the 1I, 1Br, and
1Cl are isostructural. 1F forms only one type of solvate which is
isostructural to the corresponding 1Cl solvate.
“…For (+)-hedycaryol ((10R)-11-hydroxy-germacrene A) the up-up configuration predominated at all temperatures according to NMR studies (75 % at À30 8C). [15] It is reasonable to suggest that the down-down orientation of the methyl groups is the dominant orientation in 4 b since AS produces (À)-germacrene A; [16] hence, the isopropylidene group will predominantly adopt the pseudoequatorial configuration. Analysis of the relative intensities of the resonances for the proton on C6 in the À60 8C …”
Section: Variable Temperature Nmr Spectroscopic Analysis Of 4 Bmentioning
The mechanism of the conversion of (E,E)-farnesyl diphosphate (FPP, 1a) to aristolochene (6) catalyzed by aristolochene synthase from Penicillium roqueforti has been proposed to proceed through the neutral intermediate germacrene A (4a). However, much of the experimental evidence is also in agreement with a mechanism in which germacrene A is not an intermediate in the predominant mechanism that leads to the formation of aristolochene, but rather an off-pathway product that is formed in a side reaction. Hence, to elucidate the mechanism of FPP cyclisation the substrate analogue 2-fluoroFPP (1b) was synthesized, and upon incubation with aristolochene synthase was converted to a single pentane extractable product according to GC-MS analysis. On the basis of NMR analyses this product was identified as 2-fluorogermacrene A (4b). Variable temperature (1)H NMR spectroscopy indicated the existence of two conformers of 4b that were in slow exchange at -60 degrees C, while at 90 degrees C the two isomers gave rise to averaged NMR signals. In the major isomer (approximately 75%) the methyl groups on C3 and C7 were most likely in the down-down orientation as had been observed for other (E,E)-germacranes. This work suggests that after an initial concerted cyclisation of FPP to germacryl cation deprotonation leads to the formation of germacrene A, and provides compelling evidence that germacrene A is indeed an on-pathway product of catalysis by aristolochene synthase.
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