Carbon framework of the

This C2Q pyrophosphate (122), thought to provide the carbon framework of the diterpene alkaloids such as veatchine (123), atisine (124),... 

The NMR techniques discussed so far provide information about proton-proton interactions (e.g., COSY, NOESY, SECSY, 2D y-resolved), or they allow the correlation of protons with carbons or other hetero atoms (e.g., hetero COSY, COLOC, hetero /resolved). The resulting information is very useful for structure elucidation, but it does not reveal the carbon framework of the organic molecule directly. One interesting 2D NMR experiment, INADEQUATE (Incredible Natural Abundance Double Quantum Transfer Experiment), allows the entire carbon skeleton to be deduced directly via the measurement of C- C couplings. 

The HMQC spectrum, H-NMR chemical shift assignments, and C-NMR data of vasicinone are shown. Consider the homonuclear correlations obtained from the COSY spectrum in Problem 5.14, and then determine the carbon framework of the spin systems. 

It is interesting to note that all previous attempts to utilize the Claisen rearrangement within the carbon framework of the fenestrane system as well as all efforts to prepare a fenestrane in which one of the ring fusions is trans, had not been successful. In Eq. 12.74, a facile rearrangement of fenestrene took place in aqueous pyridine to form a... 

The information on carbon chemical shifts and multiplicities is invaluable for structure determination. It would be ideal if we also had a method for obtaining information directly on carbon-carbon bonding in the compound under study, since this would allow us to draw on paper at least parts of the carbon framework of the molecule. 

Very often the carbon framework of the future allene is already present in the substrate and often it is propargylic in nature. For example, base-catalyzed isomeriza-tions of acetylenic hydrocarbons - with the triple bond in a non-terminal (40) or terminal position - were often used to prepare allenic hydrocarbons in the early days of allene chemistry [8]. The disadvantage of this approach consists in the thermodynamic instability of the allenes produced if not prohibited for structural reasons, the isomerizations do not stop at the allene but proceed to the more stable conjugated diene stage. In practice, complex mixtures are often formed [9] (see also Chapter 1). 

A B-ring-last-strategy was applied by Majetich and coworkers for their synthesis of the natural ( )-l(15),8-dolastadien-2-ol ( )-107 (desoxyisoami-jiol, 1991) [87, 88]. A Lewis acid mediated intramolecular allylsilane 1,6-ad-dition of the dienone 157 afforded the 5,7,6-tricyclic carbon framework of the dolastanes (158) (Scheme 25). The missing carbon atom was intro-... 

In a first apphcation of general interest, fluorenyllithium complexes (1, Scheme 1) were studied by solid state NMR spectroscopy. One reason for the choice of this system was that the results from the X-ray investigation presented at that time and solution NMR investigations were in conflict. The bis-quinuchdine complexes investigated in the solid state by X-ray analysis show that the lithium cation is asymmetrically positioned relative to the carbon framework of the anion, mainly interacting in a fashion with carbons C-1, C-9a and C-9 (Figure 9) . 

Chiral carbon framework of the monoterpenoid secologanin, leading to powerful chiral synthons, from readily available meso-forms. Moderate results were obtained with lipases such as porcine liver esterase (PLE), delivering the (15,2/ )-mono-acetate at a yield of 78% with 96% ee (Fig. 10), though preparation of the enzyme seemed to be crucial for the appropriate result. In contrast, pig pancreatic lipase (PPL) was significantly more efficient in forming the (—)-(l/ ,25) enantiomer at a yield of 96% and 98% ee [86, 87]. 

The cyclic diyne (55) has C—C=C angles of 159.3° as determined by an X-ray analysis, which is close to the value found in cyclooctyne by electron diffraction 161 (see above). The carbon framework of the molecule is almost planar with the two CH2—CH2 units in an almost eclipsed arrangement. This and the transannular repulsion between the ii-moieties leads to the large value of 1.57 A for the C3—C4 single bond. All other structural parameters seem to be normal.164)... 

Nonactivated tertiary hydrocarbon sites of enantiopure compounds 210 are oxyfunctionalized enantiospecifically by perfluoro-a.r-2- -butyl-3- -propyloxaziridine 80 under remarkably mild reaction conditions (Equation 8) <19990L281>. The reaction occurs with retention of the configuration at the oxidized stereogenic center and the enantiospecificity is highly independent of both the carbon framework of the substrate as well as functional groups (Table 16). 

A syndiesis of the hypocholesterolemic agent (+)-compactin centers around the Diels-Alder reaction of the chiral diene (282) with die chiral dienophile (281), which provided in a single operation die carbon framework of the target molecule (Scheme 67). However, facial and regiochemical selectivities of the additions were only moderate, giving rise to the formation of two legioisomers (283) and (284) the major product (283) was transformed into (+)-compactin. 

Sometimes unexpected products are formed in dehydration that is, the carbon skeletons of the starting material and product might be different, or the double bond might be in an unexpected location. For example, the dehydration of 3,3-dimethyl-2-butanol yields two alkenes, whose carbon skeletons do not match the carbon framework of the starting material. 

This diastereoselectivity was explained as follows. The reactions catalyzed by MgBt2 or ZnCl2 were true pericyclic reactions via an all-carbon framework of the classical Diels-Alder processes. The phenyl group of benzaldehyde, bound to the Lewis acid, was located in an endo orientation I relative to the diene (Sch. 2). Intermediate 4 from the pericyclic pathway with the cis stereochemistry at the 5- and 6-positions, which was actually isolated, was smoothly converted to the final cis pyrone 3 by treatment with trifluoroacetic acid [10]. 

In the laboratory of J.D. Winkler, the synthesis of the carbon framework of the eleutherobin aglycon was developed using a tandem Diels-Alder reaction and a Grob fragmentation as key steps.The tricyclic fragmentation precursor was subjected to potassium carbonate in DMF at 75 °C to afford the fragmentation product in 68% yield via a dianion intermediate that underwent a spontaneous hemiketalization. 

This c20 pyrophosphate (122), C20H36O7P2, is thought to provide the carbon framework of the diterpene alkaloids such as veatchine (123), atisine (124), and aconitine (125) (Fig. 5). It is not known at what stage the nitrogen is incorporated into the framework established by the skeleton. The potential for terpene rearrangements and the observation that the alkaloids are frequendy found esterified, often by acetic or benzoic acid, as well as free, has led to permutations and combinations producing over 100 such compounds. 

Some of these ideas could be generalized to a classification scheme for substituents in terms of their donor (D) and acceptor (A) properties. Following the usual partition procedure, four groups were specified (1) o acceptor and 7t acceptor, (2) o donor and 71 acceptor, (3) o donor and ti donor, and (4) o acceptor and 7t donor. Substituted benzenes were considered, and each system was partitioned in two subsystems. Subsystem A consisted of the benzene ring and subsystem B of the substituents. The change of electronic charge in the carbon framework of the ring due to substitution was defined as... 

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