Cyclization precursors cyclic dienes

Enyne metathesis can also be used with highly substituted substrates. Catherine Lievre of the Univcrsite de Picardie reports (J. Org. Chem. 2004,69, 3400) that enynes such as 11, readily prepared from carbohydrate precursors, are cyclized by the second generation Grubbs catalyst 2 to the enantiomerically-pure cyclic dienes, exemplified by 12. 

When free radicals are added to 1,5- or 1,6-dienes, the initially formed radical (9) can add intramolecularly to the other bond, leading to a cyclic product (10). When the radical is generated from an precursor that gives vinyl radical 11, however, cyclization leads to 12, which is in equilibrium with cyclopropylcarbinyl radical 13 via a 5-exo-trig reaction. A 6-endo-trig reaction leads to 14, but unless there are perturbing substituent effects, however, cyclopropanation should be the major process. 

The photochemistry of linear conjugated trienes is closely connected to the isomeric 1,3-cyclohexadienes which are both their photochemical precursors and products. Such systems have been dealt with under cyclic 1,3-dienes in the previous section. The photochemical cyclization of m-stilbene to dihydrophenanthrene and ultimately to phenanthrene and hydrogen is formally analogous to reaction (13). This reaction (17) has been observed in a large class of aromatic compounds under a... 

The results of the following experiments give important clues for mechanistic considerations. Isolation of a trienecarboxylate ester as a hypothetical intermediate is not possible in most cases. As an exception, the trienecarboxylate 127 was isolated (64%) as a mixture with the cyclized product 128 (22%) after 4h reaction of 126. Compound 127 was converted to the cyclized product 128 on further exposure to the carbonylation conditions, showing that the allenecarboxylate 127 is a precursor of the cyclized product 128 (Scheme 11-36). Also, the carbonylation of the 1,4-enyne 129 gave only the allene carboxylate 130 (Scheme 11-37). No further carbonylation to give the cyclic ketone 131 or 132 was observed. In addition, the 1,3-diene ester 133 was recovered intact under the same carbonylation conditions without giving the cyclized product 134 (Scheme 11-38). 

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