Cyclic dienes structure

Many references exist for cyclic diene structures formed during certain cationic polymerizations (124-127). The exact mechanism involved in the formation of the ladder-like fused ring structure has yet to be determined. 

A rough estimate of the stabilisation of benzene, compared with simple cyclic unsaturated structures, can be obtained by comparing its heat of hydrogenation with those of cyclohexene (7) and cyclohexa-1,3-diene (8) ... 

The synthesis of cis-1,4 polymers was also tried by e use of monomers with an s-cis conformation. The solid-state photopolymerization of pyridone derivatives, which is a six-membered cyclic diene amide and is a tautomer of 2-hydroxypyridine, was attempted [100]. Pyridones make hydrogen-bonded cocrystals with a carboxylic acid in the crystalline state. Because the cyclic structure fixes its s-cis conformation, if the polymerization proceeds, a cis-2,5 polymer would be obtained. Actually, however, the photopolymerization did not occur, contrary to our expectation, but [4-1-4] photodimerization proceeded when the carbon-to-carbon distance for the dimerization was small (less than 4 A) [101]. A closer stacking distance of the 2-pyridone moieties might be required for the topochemical polymerization of cychc diene monomers. 

The heat of hydrogenation of the cyclic diene (8) is very nearly twice that of cyclohexene (7), and the heat of hydrogenation of the three double bonds in a Kekule structure might thus be expected to be of the order of 3 x -120 kJ ( — 28-6 kcal)mol = -360 kJ ( — 85-8 kcal) mol but when real benzene is hydrogenated only —208 kJ (-49-8 kcal) mol are evolved. Real benzene is thus thermodynamically more stable than the hypothetical cyclohexatriene by 151 kJ (36 kcal) mol this compares with only 17 kJ (4 kcal)mol by which a conjugated diene i stabilised, with respect to its analogue in which there is no interaction between the electrons of the double bonds. 

Alder s endo rule applies not only to cyclic dienes like cyclopentadiene and to disubstituted dienophiles like maleic anhydride, but also to open chain dienes and to mo no-substituted dienophiles diphenylbutadiene and acrylic acid, for example, react by way of an endo transition structure 2.113 to give largely (9 1) the adduct 2,114 with all the substituents on the cyclohexene ring cis, and equilibration again leads to the minor isomer 2.115 with the carboxyl group trans to the two phenyl groups. 

The generation of an 1,2-oxazine 4-1 by hetero Diels-Alder reaction is a transformation which opens a versatile array of highly functionalised acyclic and cyclic structures. Thus, pyrrolidine derivatives 4-2, amino alcohols 4-3 and aza sugars 4-4 are easily available from these cycloadducts. Cyclic dienes, e.g. 4-5 are converted into bicyclic adducts 4-6 representing straightforward intermediates for aminocyclitols 4-7 (Fig. 4-1). 

The general structures of cyclic sulfoximines are reported in CHEC-II(1996) <1996CHEC-II(3)319> (see p. 348). Very few examples of sulfoximines were reported in the decade under review. The preparation of cyclic sulfilimides 165 and 173 from (R)-l-/>-tolyl-lA-isothiazol-3-ones 164 and diazoalkanes and cyclic dienes, respectively, have been reported (see Section 4.05.5.11). Furthermore, studies on the alkylation of a-sulfonimidoyl carbanions and on their use as chiral ligands in copper reagents were reported (see compounds 227 and 228, Section 4.05.6.3.4). 

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