Patai S., Rappoport Z. 2000 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 2 / 0739 884

This difference in reaction rates can be attributed to different energies of corresponding transition states. According to the Woodward –Hoffmann rules57, the Cope rearrangement is a sigmatropic [3,3]-shift (see Section IV.C.1) while the butadienylcyclopropane rearrangement can be considered as an sigmatropic antarafacial [1,5]-shift with inversion at the migrating carbon atom.

The thermolysis of the bicyclodiene 109 at 225 °C gives rise to equilibrium mixture of cyclooctatriene and its transformation products (see below)54. More recently the influence of a methoxy group on the thermal behavior of the bicyclo[5.1.0]octa-2,4-diene system was studied56. Heating of 8-endo-methoxydiene 115 in cyclooctane at 95 °C gaves rise to methoxy substituted diene 117 and not to the product 116 of butadienylcyclopropane rearrangement (equation 41). The thermolysis of the 8-exo-isomer 118 has taken place as an equilibrium reaction to give 6-endo-methoxy diene 119 (equation 42)56. These two reaction partners were separated by TLC.

Recently, molecular orbital calculations (MP2/6-31GŁ//RHF/6-31GŁ level) which cover a series of bicyclic systems from the stable bicyclic compound 109 to the unknown 6,8- dioxabicyclo[5.1.0]octa-2,4-diene (2,3-epoxyoxepin, 120), as well as the two intermediate 8-oxa- (121) and 6-oxa- derivatives (122), were carried out58. These structures are interesting because the bicycle 120 is suggested as a transient intermediate in the metabolic oxidation of benzene leading to the muconaldehyde, which is responsible for the hematotoxicity of benzene.

It was found that the treatment of compound 148 with t-BuOK in THF gave the starting material (39%) and tert-butyl ethers 149 and 150 (in 43% combined yield) in a ratio 3 : 2 (equation 47)63.

Another paper64, which also contains a literature survey about the problem discussed, describes the rearrangements of C7H6 systems which can be generated by thermolysis of phenyldiazomethane. By using spectral methods and chemical reactions, the formation of bicyclo[4.1.0]hepta-2,4,6-triene (139), cycloheptatrienylidene (137a) and bicyclo[3.2.0] hepta-1,3,6-triene (135) was excluded, and evidence for the formation of intermediate cycloheptatetraene (137b) (see equation 45) was furnished.

N O

H N

• •

CONH2

•

(136)

(45)

H

••

• •

(149)

(47)

•

•

• •

CH

Me

OBu-t

(150)

B. Cyclic Polyenes

1. Cycloheptatrienes

For cycloheptatriene and a series of its derivatives various thermal unimolecular processes, namely conformational ring inversions, valence tautomerism, [1,5]-hydrogen and [1,5]-carbon shifts, are known. An example of such multiple transformations was described65 which can provide a facile approach to new polycyclic structures by a one-step effective synthesis (yields up to 83%) of the two unique ketones 156 and 157. The thermolysis of the neat ether 151 at 200 °C for 24 h gives initially the isomeric allyl vinyl

ethers 152–154 by successive [1,5]-hydride shifts, and subsequent Claisen rearrangement (see Section IV.E.1) provides the allyl cycloheptadienone 155 which easily undergoes an intramolecular Diels – Alder reaction to afford a 50 : 50 mixture of the two isomeric ketones 156 and 157 (equation 48)65.

synthesis of various bridged polycyclic systems66, viz. ketones 160 and 161. Tropone reacts with butenyl magnesium bromide ( 78 °C, 75%) to form a mixture of 2-(3- butenyl)dihydrotropones 158 and 159, the pyrolysis of which (200 – 210 °C, neat or in heptane solution) leads to 60% total yield of the isomeric homoprotoadamantenones 160 and 161 and the tricyclic ketone 162 in a ratio of 58 : 18 : 24, respectively (equation 49)66.

Similar results were obtained for the synthesis of azapolycycles having relatively rigid skeletal frameworks. Thus, refluxing of N-substituted N-allylamines 163 under conditions of high dilution in xylene gave the cycloadducts 164 in 67% yield along with the products of hydrogen shift 165 and 166 (equation 50)67.

A novel thermal rearrangement with loss of sulfur dioxide leading to the stilbene or styrene derivatives 169 and 170 in highly stereospecific manner was carried out by heating (in dioxane, DMSO, dioxane –water or THF) the sulfene-tropone adducts ( -sultones) 167 or 168 (equations 51 and 52)68.

It is worthwhile mentioning here that the thermal hydrogen shifts can occur not only in the cyclic seven-membered substrates but also in open-chain systems. For example, the antarafacial thermal [1,7]-sigmatropic hydrogen shift in epimeric cis-isotachysterol analogues 171 and 172 which give 173 and 174 was reported (equations 53 and 54)69. This work was carried out in order to investigate the reactions of parent previtamin D3 to afford vitamin D3. However, it should be noted that an analogous example was already known for a long time (see e.g. Chapter 7 in Reference 5).

(49)

O O

+

O

2. Cyclooctatrienes

The reversible rearrangement of 1,3,5-cyclooctatriene 175 into bicyclo[4.2.0]octa-2,4- diene 176 was first postulated70 and then corroborated71 by Cope and coworkers almost 50 years ago. These authors71 and their followers72 have shown that isomers 175 and 176 can be separated and both undergo interconversion to the same equilibrium mixture of 85% 175 and 15% 176 by a short-term heating at 80 –100 °C (equation 55). This equilibrium system was later investigated intensively, including the finding of the conditions of the photochemical transformations72. However, from the synthetic standpoint the system consisting of the fused cyclooctatriene and cyclopropane rings is apparently the most interesting. An initial short communication about the behavior of such systems was published in 196173. The norcaradiene vinylogue, cis-bicyclo[6.1.0]nona-2,4,6-triene 177, as well as some of its derivatives were obtained by addition of carbenes to the cyclooctatetraene. It was shown that the bicyclic triene 177 is thermally labile and rearranges easily already at 90 °C without any catalyst to the indene derivative 178 (equation 56)73. The dihalocarbene adducts 179 in which the halogen atoms show no solvolytic activity rearrange almost quantitatively at 80 – 90 °C to afford the indene derivatives 181. The location of the halogen atoms in the products 181 allows one to exclude the possibility that the bicyclic compounds 179 rearrange via the cyclononatetraene intermediate 182. Instead, the tricyclic structures 180 are assumed to be the most probable intermediates in this process (equation 56)73.