Cycloheptane, structure

The present [3 + 4] annulation methodology for the synthesis of seven-membered carbocycles involves a straightforward procedure that also provides the product functionalities (e.g., masked and unmasked ketone carbonyl and trimethylsilyl groups) that can be transformed to hitherto inaccessible or difficult-to-prepare cycloheptane structures. The prior approach,7-9 based on Cope rearrangement of cis-1,2-divinylcyclopropane, bears an intrinsic drawback in that there exist a limited number of methods for stereoselective preparation of the substrate. 

The X-ray structure of 1-dimethylphosphono-l-hydroxycycloheptane (65) was compared only with MM calculations of cycloheptane (216), but it should now be possible to include the substituents with one of the force fields in Table 9. Inorganic perhalophosphazenes, (N=PX2) have been subjected to MM analysis. Compared to hydrocarbons, the P—N—P bending force constant and the two-fold torsional barrier are small (217). 

The enthalpy of fomation of two such species has been measured, namely the cyclopropane and cycloheptane derivatives. The difference between the values for these two species, both as solids, is 238.1 kJmol . Is this difference plausible Consider the difference between the enthalpies of formation of the parent cycloalkanes as solids, 194 kJ mol . The ca 44 kJ mol discrepancy between these two differences seems rather large. However, there are idiosyncracies associated with the enthalpies of formation of compounds with three-membered rings and almost nothing is known at all about the thermochemistry of compounds with seven-membered rings. Rather, we merely note that a seemingly well-defined synthesis of cycloheptyl methyl ketone was shown later to result in a mixture of methyl methylcyclohexyl ketones, and superelectrophilic carbonylation of cycloheptane resulted in the same products as methylcyclohexane, namely esters of 1-methylcyclohexanecarboxylic acid. The difference between the enthalpies of formation of the unsubstituted alicyclic hydrocarbons cycloheptane and methylcyclohexane as solids is 33 kJmol . This alternative structural assignment hereby corrects for most of the above 44 kJ mol discrepancy in the enthalpies of formation of the two oximes. More thermochemical measurements are needed, of oximes and cycloheptanes alike. 

The substituted cycloheptane monoterpenes, also called tropones. Eu-carvone 60, nezukone (4-isopropyl-2,4,6-cycloheptatrienone) 61 and y-thujaplicin 62 (Structure 4.14) most probably arise by an unknown ring expansion of the cyclohexane skeleton. 

Figure 6.38 Data for structure H hydrates of methane with cycloheptane, cyclooctane, and adamantane.

The reaction results of cycloheptane and cyclooctane indicate that the monomolecular pathway is followed, preserving the cyclic structure, without the formation... 

From another viewpoint, the value of the arene-alkene meta cycloaddition arises from its capacity to produce a cycloadduct (66 equation 14) with three new rings and up to six new stereocenters, an impressive feat even when compared with the highly regarded Diels-Alder cycloaddition. Moreover, the cycloadduct can be used in the synthesis of a variety of commonly encountei structural types including cyclopentanes, cycloheptanes, bicyclo[3.2.1]octanes and bicyclo[3.3.0]octanes. While fr uently overlooked in some discussions of reaction classification, the overall processes leading to cycloheptanes, bi-cyclo[3.2.1]octanes and bicyclo[3.3.0]octanes are clearly classifiable as [5C + 2C], [3C + 2C] and [3C + 2C] cycloadditions, respectively. Examples of these types will be given in the following section. 

Benzene annelation significantly increases the barrier to ring inversion processes. Saturated seven-membered rings can invert by pseudorotation (pseudorotation barrier is 2 kcal/mole) which has not been measured by NMR techniques. The AG for the ring inversion process for the benzene-annellated derivatives of cycloheptane (which probably exists in a chair form) was determined to be 10.9 kcal/mole at Tj, = — 57°C (186, 298, 322). Equilibria in tricyclic organic derivatives have been studied more extensively. Several different inversion processes have been reported and depend on structural type. The following are examples of ring inversion processes. 

The cyclopropane system is a tool for inserting a methylene unit into a ring system to form a larger ring structure. Even densely functionalized pyranoids such as 161 [130,131] and cyclohexanes [132] expand into heptanoids and cycloheptanes, respectively. 1,2-C-dibromomethy-lene sugar 163 expands its pyranose ring to give oxepine 164 [133] (O Scheme 39). 

Few of the recently published reference books covering natural products provide the chemist, within a single volume, with most of the detail which is frequently required, viz. names, structure, physical data, together with references to occurrence, structure determination, and synthesis. The revised edition of Karrer, and its first supplement,do provide this detail in their comprehensive coverage of plant products. This strength makes these volumes indispensable, despite the delay in covering the literature, viz. to 1956, and 1957—61 respectively. Newly isolated monoterpenoids now occupy a class of their own in the first supplement, although some cycloheptane monoterpenoids e.g. /8-dolabrin), quinones e.g. /S-thujaplicinol), and tetrahydrocannabinol (the formula is incorrect) are listed separately. 

The most important structural features that influence the conformation and reactivity of cycloalkanes differ depending on whether small (cyclopropane and cyclobutane), common (cyclopentane, cyclohexane, and cycloheptane), medium (cyclooctane through cycloundecane), or large (cyclododecane and up) rings are... 

No details were given so far about the structure of these compounds. One of the most interesting applications of the method could be the synthesis of enantiomerically pure, highly functionalized cycloheptane derivatives, since divinylcyclopropanes should be obtainable by the appropriate rearrangement of other primary products. 

---