Polyene sequences cyclization

This reaction scheme also accounts for the selective suppression of the two families of aromatic compounds formed in the pyrolysis of PVC induced by metal oxides. In fact, metal oxides (or better, the metal chlorides) are able to accelerate the rate of crosslinking of the linear polyene sequences and this process would compete with the parallel intramolecular cyclization of the polyenes. In the meantime, the amount of crosslinked polyene chains produced would be appreciably higher with respect to that formed in the pyrolysis of pine PVC. In fact, a reasonable increase of the residual char formed in the pyrolysis of PVC has also been observed. 

Larger polyene sequences may enter into intra- and intermolecular cyclization reactions, so that initially PVC degradation is not necessarily accompanied by chain cleavage [599],... 

As HCl is released from PVC the discolouration increases with a colour shift towards red. This is attributed to the build up of polyene sequences, which can be studied by UV spectroscopy (Figure 12). It appears that the average polyene length is fairly short, reaching only 6-10 double bonds in conjugation. During dehydrochlorination there is also an increase in MW due to crosslinking, and some cyclization occurs. 

Intramolecular cyclization reactions of polyene sequences account for the formation of benzene and other aromatic products and also provide a route to the carbonaceous residue formed at high temperatures. 

Acyl radicals can be generated and they cyclize in the usual manner. A polyene-cyclization reaction generated four rings, initiating the sequence by treatment of a phenylseleno ester with Bu3SnH/AIBN to form the acyl radical, which added to the first alkene unit. The newly formed carbon radical added to the next alkene, and so on. Acyl radicals generated firom Ts(R)NCOSePh derivatives cyclize to form lactams. ... 

Surprisingly, additions of electrophiles, radicals, and nucleophiles to methylene cyclopropane have rarely been executed under synthetic conditions. A sequence leading to the complement inhibitor K-76 published recently" very smartly takes advantage of a methylenecyclopropane participation in the polyene cyclization shown in equation 197. Hydroxylation of the vinylcyclopropane double bond and hydrogenolysis of the three membered ring are subsequent essential reactions. 

Three major reaction sequences or strategies dominate steroid syntheses (1) Diels-Alder reactions, (2) all kinds of aldol and Michael addition reactions, and (3) biomimetic syntheses corresponding to polyene cyclizations. We shall give a few examples of each of these approaches, starting with the polyene cyclizations (Akhrem and Titov, 1970 Anand et al., 1970 Blickenstaff et al., 1974). 

For nongroup-selective examples, one of the first reports of an enantioselective intramolecular Mizoroki-Heck reaction was a polyene cyclization (Scheme 12.22) [23b], The trienyl triflate 5 underwent two intramolecular cyclization reactions to give the tricycle 6 in high yield and 45% ee. A cascade intramolecular Mizoroki-Heck-hydride capture sequence was used in the synthesis of retinoid derivatives from aryl iodide 100 to give benzofuran 101 in 80-81% ee [49]. Poor enantioselectivity was observed when neutral reaction conditions were employed. 

A reaction called the Johnson polyene cyclization (based on the Stork-Eschenmoser hypothesis) converts triene A into the polycyclic molecule B. When first discovered, an initially formed carbocation at one end of the polyene reacted with a nearby alkene to form a ring containing a new cation. This reacted with another nearby alkene, etc. The reaction was plagued by low yields and formation of polymeric material and decomposition products. This transformation required many years to perfect and two improvements were the use of a cyclopentenol unit on the left to initiate the sequence, which became a cyclopentene unit, and an alkyne unit on the right to end the sequence by generating a methyl ketone. Briefly discuss why these two improvements helped the problems inherent to this reaction. 

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