Ten membered ring conformational

The absolute configuration of 41 at the stereogenic centers at C-2, C-7, C-8 and C-9 was determined following the same procedure as for compound 40. The tri-/>-bromobenzoate 41c, whose ten-membered ring conformation ( mmx = 67.34 kcal/mol) resembled those of 40, 41A, 40a and 41a showed a typical split CD and this established the chirality of the 7-OBrBz/8-OBrBz as being positive. Therefore, herbarumin II (41) was characterized as (2i , IS, 85, 9i )-2,7,8-trihydroxy-9-propyl-5-nonen-9-olide (41). 

Nine- and Ten-membered Rings.—Conformational energies and interconversion barriers have been calculated for cyclononane. The [3,3,3]-conformation was found to be the most stable. N.m.r. indicated that cyclonona-l,2,6-triene adopts the unsymmetrical twist-boat-chair conformation (19) and undergoes a hindered pseudorotation with an activation energy of 12.9 kcal mol to achieve C2 time-averaged symmetry. ... 

Perhaps the most notable difference between S-N and N-O compounds is the existence of a wide range of cyclic compounds for the former. As indicated by the examples illustrated below, these range from four- to ten-membered ring systems and include cations and anions as well as neutral systems (1.14-1.18) (Sections 5.2-5.4). Interestingly, the most stable systems conform to the well known Htickel (4n -1- 2) r-electron rule. By using a simple electron-counting procedure (each S atom contributes two electrons and each N atom provides one electron to the r-system in these planar rings) it can be seen that stable entities include species with n = 1, 2 and 3. 

In this beautiful synthesis of periplanone B, Still demonstrated a classical aspect and use of total synthesis - the unambiguous establishment of the structure of a natural product. More impressively, he demonstrated the usefulness of the anionic oxy-Cope rearrangement in the construction of ten-membered rings and the feasibility of exploiting conformational preferences of these medium-sized rings to direct the stereochemical course of chemical reactions on such templates. 

The problem of the preferred conformation of cyclodecane has been extensively studied by Dunitz et al. (46). In the crystals of seven simple cyclodecane derivatives (mono- or 1,6-disubstituted cyclodecanes) the same conformation was found for the ten-membered ring (BCB-conformation, Fig. 9). It follows from this that the BCB-conformation is an energetically favourable conformation, possibly the most favourable one. Numerous force field calculations support this interpretation Of all calculated conformations BOB corresponded to the lowest potential energy minimum. Lately this picture has become more complicated, however. A recent force field calculation of Schleyer etal. (21) yielded for a conformation termed TCCC a potential energy lower by 0.6 kcal mole-1 than for BCB. (Fig. 9 T stands for twisted TCCC is a C2h-symmetric crown-conformation which can be derived from rrans-decalin by breaking the central CC-bond and keeping the symmetry.) A force field of... 

This may look like a long stretch for the enol to reach across the ten-membered ring to reach the other ketone, but the conformational drawing in the margin shows just how close they can be. You should compare this conformation with that of a decalin (Chapter 18),... 

Force-field calculations on the epoxidation of (.E -methylcyclodecene, (.E)-3-methylcy-clododecene and ( )-l, 3-dimethylcyclododecene with 11 to 25 conformers within a 3 kcal/mol window for each transition state successfully reproduce the observed diastereoselectivities168 (Table 7). These calculations, however, are, at least for larger than ten-membered-ring systems, too laborious. 

As you now know (p. 1134) aU esters and lartones prefer to be in the conformation shown for the first compound so that they can enjoy stabilization by the anomeric effect. The second ester would also prefer o be like this too (first diagram below) but in this conformation it cannot cyclize at all. Even in the less favourable conformation it uses for reaction, it prefers conjugate to dirert substitution as the latter would give a strained truns-cycloheptene. The first compound can enjoy both the anomeric effert and conjugate addition as a frans-alkene in a ten-membered ring is fine. It is better than fine in the heterocyclic product as the lactone also enjoys the anomeric effect... 

One interesting facet of the germacrane-type sesquiterpenoids is the conformation of the ten-membered ring. This aspect has previously been discussed in terms of transannular electronic effects (anomalous u.v. spectra) and transannular chemical reactions (Cope rearrangement and cyclisations to eudesmane and/or guaiane types). Recently, the power of two spectroscopic techniques has been brought to bear on this problem. The first of these is the use of the Nuclear Overhauser Effect (NOE) and the second is the A"-ray analysis of a suitable derivative. 

A full structure has been reported for the centrosymmetric hydrated 1,6-diphosphacyclodecane derivative (25), in which the ten-membered ring has the normal cyclodecane conformation. The phenyl groups are in trans-pseudo-axial positions and the structure is completed by hydrogen bonds between the hydrate water and phosphoryl groups from two molecules. 

Ten-membered Rings.—rm 5,/ra/w-Bicyclo[8,4,0]tetradeca-2,8-diene (227 R = H) cyclized readily on treatment with acetic acid-perchloric acid to give a mixture of epimeric acetates (228) and tricyclic olefins. Protonation of the double bond occurs from the equatorial direction. Analogous cyclizations of (227 R = Me) and the cyclopropyl derivative (229) were also studied. All three cyclizations are fairly rapid because participation of the transannular double bond is accompanied by the release of medium-ring strain and they are also facilitated by the favourable conformation enforced on the 10-membered ring by the cyclohexane ring. ... 

Synthetic routes to germacrenes are always of interest. This year, McMurry has applied his low-valent titanium based method to bicyclogermacrene (45), from (44), and Schreiber et al. have summarized a short synthesis of germacrene D (48) which relies heavily on the site- and stereo-selective enolization of (46) to (47) using the conformational bias in the ten-membered ring intermediate (46). In a first example of the oxy-Cope (6 - 14 membered)... 

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