Monocyclic polyenes

Cyclic dienes and polyenes, monocyclic as well as bicyclic, can be metathesized in the same way as cyclic monoenes. As expected, cyclobutene 27), 1,5-cyclooctadiene, and 1,5,9-cyclododecatriene 28) yield the same polyalkenamer, in this case polybutenamer (1,4-polybutadiene), since these reactants consist of the same base units, i.e.—(CH2)2CH==CH— ... 

The achiral triene chain of (a//-rrans-)-3-demethyl-famesic ester as well as its (6-cis-)-isoiner cyclize in the presence of acids to give the decalol derivative with four chirai centres whose relative configuration is well defined (P.A. Stadler, 1957 A. Escherunoser, 1959 W.S. Johnson, 1968, 1976). A monocyclic diene is formed as an intermediate (G. Stork, 1955). With more complicated 1,5-polyenes, such as squalene, oily mixtures of various cycliz-ation products are obtained. The 18,19-glycol of squalene 2,3-oxide, however, cyclized in modest yield with picric acid catalysis to give a complex tetracyclic natural product with nine chiral centres. Picric acid acts as a protic acid of medium strength whose conjugated base is non-nucleophilic. Such acids activate oxygen functions selectively (K.B. Sharpless, 1970). 

The early Escherunoser-Stork results indicated, that stereoselective cyclizations may be achieved, if monocyclic olefins with 1,5-polyene side chains are used as substrates in acid treatment. This assumption has now been justified by many syntheses of polycyclic systems. A typical example synthesis is given with the last reaction. The cyclization of a trideca-3,7-dien-11-ynyl cyclopentenol leads in 70% yield to a 17-acetyl A-norsteroid with correct stereochemistry at all ring junctions. Ozonolysis of ring A and aldol condensation gave dl-progesterone (M.B. Gravestock, 1978 see p. 279f.). 

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

Among planar monocyclic fully conjugated polyenes only those possessing (4n + 2) TT electrons where n is a whole number will have special stability that be aromatic... 

The An + 2) tt electron standard follows from the pattern of orbital energies m monocyclic completely conjugated polyenes The tt energy levels were shown for ben zene earlier m Figure 114 and are repeated m Figure Figure 11 13a and 11 13c... 

Monocyclic conjugated polyenes are referred to as annulenes, and there exists ample experimental evidence to support the conclusions based on application of HMO theory to neutral and charged annulenes. The relationship between stability and structure in cyclic conjugated systems will be explored more fully in Chapter 9. 

The term annulene was coined to refer to the completely conjugated monocyclic polyenes. The synthesis of annulenes has been extended well beyond the first two members of the series [4]annulene (cyclobutadiene) and [6]annulene (benzene). The generality of the Hiickel rule can be tested by considering the properties of members of the annulene series. 

The final entries in Table 7 concern eight-membered monocyclic dienes and polyenes. The unconjugated 1,5-cyclooctadiene was observed to have twist-boat conformation and C2 symmetry. In accordance with what is very often the case in GED studies of cyclic... 

Table 8 presents structures observed for monocyclic dienes and polyenes with rings large enough to accommodate trans C=C double bonds. In a cyclodecadiene molecule strain-free carbon skeletons can only be derived when two double bonds are diametrically placed and have the same configuration (as, cis or trans,trans). Cw,cis-Cyclodeca-1,6-diene (1,6-CDD) may exist in twelve different conformations, and it is therefore noteworthy that it almost exclusively prefers one of these, namely the one indicated in Table 8. This conformer does not have the repulsive transannular HH interactions that destabilize the corresponding saturated molecule in all conceivable conformers.

TABLE 8. Experimentally determined structure parameters for monocyclic dienes and polyenes minimum 10 ring atoms (distances in A, angles in degrees)... 

Linear and branched molecules, as well as some of the monocyclic ones, are identified only by their IUPAC names if their structure is immediately obvious. In the absence of accepted trivial or easy-to-read systematic names, larger polycyclic dienes and polyenes with rather unwieldy IUPAC names have been given numbers (4th column of the Table), which refer to the formula scheme following Table 1. 

As Hiickel formulated it, the An + 2 rule applies only to monocyclic systems. However, as a practical matter it can be used to predict the properties of polycyclic conjugated polyenes, provided the important VB structures involve only the perimeter double bonds, as in the following examples ... 

The it energy of a non-classical conjugated hydrocarbon can be compared directly with that of a classical analogue by the PMO method.14 Consider an even monocyclic polyene. This can be formed by fusion of methyl with an odd AH with one atom less. These components can also be fused to form an acyclic polyene. Comparison gives the aromatic energy of the cyclic system by difference. In this way we find that rings with An + 2 atoms are more stable, and those with An atoms less stable, than analogous acyclic compounds. The same method can be used for the bicyclic systems XVII, XIX, XXI, XXII, XXIII. The procedure is indicated below... 

Next we apply this simple model to the annulenes. These compounds are monocyclic conjugated polyenes with the general molecular formula (CH) with n even. Thus benzene may be considered as [6]-annulene. As n increases, essentially all [ ]-annulenes are non-planar. For instance, cyclooctatetraene, (CH)s, has the well-know tub structure. However, [18]-annulene is nearly planar, as shown below (bond lengths in picometers are displayed in bold italic font). 

For a circular polyene (a planar monocyclic compounds) such as benzene or cyclobutadiene the secular equation is a circulant ... 

In the general case the following expressions for orbital energies of monocyclic polyenes can be obtained ... 

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