Polyenes bicyclizations

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— ... 

Scheme 10.1 gives some representative examples of laboratory syntheses involving polyene cyclization. The cyclization in Entry 1 is done in anhydrous formic acid and involves the formation of a symmetric tertiary allylic carbocation. The cyclization forms a six-membered ring by attack at the terminal carbon of the vinyl group. The bicyclic cation is captured as the formate ester. Entry 2 also involves initiation by a symmetric allylic cation. In this case, the triene unit cyclizes to a tricyclic ring system. Entry 3 results in the formation of the steroidal skeleton with termination by capture of the alkynyl group and formation of a ketone. The cyclization in Entry 4 is initiated by epoxide opening. 

The largest contribution and variety in the family of polyenes is to be found in the group of bicyclic and polycyclic compounds. For this chapter we selected those compounds which represent the most important prototypes of different kinds of interaction, namely cyclopropyl-conjugation, spiroconjugation, hyperconjugation and homoconjugation. 

There are many bicyclic dienes and polyenes. If for no other reason than to show that seemingly homologous series often show profound complications, in Table 3 we present the enthalpies of formation of the bicyclo[2.2.n]alka-2,5-dienes, bicyclo[2.2.n]alk-2-enes and bicyclo[2.2.n]alkanes, species 68, 69 and 70, respectively, wherein we limit our attention to the cases of n = 0, 1 and 2. It is seen that the enthalpies of formation of the bicycloalkadiene, bicycloalkene and bicycloalkane always become more negative in that... 

The non-planar polyene nature of azepines renders them susceptible to a variety of intra-and inter-molecular pericyclic processes. The azepine-benzeneimine valence isomerization has been discussed in Section 5.16.2.4, and the ring contractions of azepines to benzenoid compounds in the presence of electrophiles is covered in Section 5.16.3.3. In this section the thermal and photochemical ring contractions of azepines to bicyclic systems, their dimerizations and their isomerizations via sigmatropic hydrogen shifts are discussed. Noteworthy is a recent comprehensive review which compares and contrasts the many and varied valence isomerizations, dimerizations and cycloadditions of heteroepins (conjugated seven-membered heterocycles) containing one, two and three heteroatoms (81H(15)1569). 

Brominative cycHzadon of polyenes. Treatment of ncrolidol (2) with 1 results in formation (in low yield) of a- and /5-synderol (3 and 4) and the bicyclic ethers 3/3-bromo-8-epicaparrapi oxide (5) and the C -cpimcr. 

Conjugated linear trienes were reported to undergo cyclization to form seven-membered ring systems.112 Cyclic polyenes, in turn, isomerize to yield bicyclic compounds. In the transformation of nonatriene (10), the role of the base was demonstrated to catalyze the formation of the conjugated triene [Eq. (4.23)], which... 

Another example is the cyclization of the racemic allylic alcohol 232 at -80°C which furnished the racemic tetracyclic bis-olefin 233 in 70% yield (89, 90). Ozonolysis of 233 gave the bicyclic triketone aldehyde 234 which underwent under acidic conditions a double intramolecular aldol cyclodehydration to produce racemic 16,17-dehydroprogesterone 235. This represents the first synthesis of a steroid via the now so-called "biomimetic" polyene cyclization method. 

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... 

Polyene Heck-type cyclization,3 Overman s group has reported bicyclization of trienyl triflates to spirobicyclic systems in the presence of Pd(OAc)2/P(C6H5)3 (1 4) and 2 equiv. of N(C2H5)3. Under these conditions 1 cyclizes to 2 in 72% yield. This cyclization is particularly facile when catalyzed by Pd(OAc)2 and (R,R)- or (S,S)-DIOP (4, 273) in a 1 1 ratio. In this case, the tricyclic dienone (2) is obtained in... 

The cycloaddition to linear polyenes can afford, in principle, different regioisomers, see 25 and 26, but often the diazenes add to the 1,3-diene moiety which is less hindered or less substituted at the terminal carbon atoms. The cycloaddition to cyclic or bicyclic polyenes, however, can proceed by the normal [4 + 2] as well as the [6 + 2] or [8 + 2] cycloaddition mode. 

This electrocyclization leading to a 1,6-fused cyclohexadiene also takes place with polyenes. A photochemical example from the vitamin A field is exemplified by the conrotatory phot clization of the (7Z)-isomer (IW) of retinal (185) to the bicyclic derivative (187). The photocyclization procedure has also been used in the aromatic series. - Thus the photocyclization-oxidation reaction of l-phenyl-4-(2 -thienyl)-1,3-butadiene (188) gave 4-phenylbenzo[b]thiophene (189). Similarly, the 3 -thienyl analogue (190) afforded 7-phenylbenzo[b]thiophene (191), the reaction exhibiting high selectivity for cyclization to the logically more reactive thiophene nucleus. ... 

Bicyclization of polyenes provides access to fused ring systems. When the cyclization leads to a cyclohexane ring, the reaction usually proceeds via a chairlike transition state. Thus, the allylic cation-promoted cyclization of the butenylcyclo-hexenol below presumably proceeds via formation of the allylic carbocation followed by cyclization. Equatorial interception of the resultant cation by the solvent furnishes the bicyclic formate ester. ... 

Several years ago, we found that the isomerization of n-butylammonium ( Z,Z)-muconate produces the corresponding EE-isomer in a high yield in the crystalline state under photoirradiation [41]. This solid-state photoisomerization was re e e< t0 he a one-way reaction and no EZ-isomer was formed during the reaction, unsaturated compounds such as olefins, polyenes, and azo compounds generally undergo reversible one-bonded photoisomerization to form a mixture of omers. Previously, we pointed out the possibility that the isomerization of the but thni(i er Va ves the s°hd state f°ll°ws the bicycle-pedal reaction mechanism, et al [4 eta s °ffhe molecular dynamics ofthe reaction had not been clarified. Saltiel react an< fl have independently discussed volume-conserving... 

---