Cyclization of conjugated

The cyclizations of conjugated nitrile ylides forming substituted oxazoles and thiazoles were computed up to the MP4/6-31H-G level [OOJOC47]. Relative to 23, oxazole-4-carboxylic acid24 is stabilized by about -38.1 kcal/mol (Scheme 18). 

Figure 30.7 Photochemical cyclizations of conjugated dienes and trienes. The two processes occur with different stereochemistry because of their different orbital symmetries.

The cyclization of conjugated polyenes and the inverse reaction were those processes which provided superb materials 142> leading to the Woodward-Hoffmann rule 51>. 

Ring expansion of cyclic ketones 8-24 Treatment of cyclic boranes with CO 8-29 Cyclization of conjugated dienes and trienes... 

A simple route to furo[3,2-6]pyridine is based upon the one-step generation of the pyridine ring by gas-phase cyclization of conjugated iminyl radicals <80JCS(Pi)2200). O-Alkyloximes have proved to be suitable precursors (Scheme 18) (81S464). 

Electrocyclic reactions involve the cyclization of conjugated polyenes. For example, 1,3,5-hexatriene cyclizes to 1,3-cyclohexadiene on heating. Electrocyclic reactions can occur by either conrotatory or disrotatory paths, depending on the symmetry of the terminal lobes of the tt system. Conrotatory cyclization requires that both lobes rot lte in the same direction, whereas disrotatory cyclization requires that the lobes rotate in oj )posite directions. The reaction course in a specific case can be found by looking at the symmetry of the highest occupied molecular orbital (HOMO). 

As was mentioned above, the classical Bergman cyclization of conjugated enediynes proceeds at elevated temperatures (160-200 °C), while a similar transformation within the structural framework of enediyne antibiotics occurs readily at 37 Obvious considerations prompted the suggestion that the proximity of the terminal sp centers, as estimated by the critical cd distance, is the main factor governing the ease of the cyclization. This simple consideration led to the design of several monocyclic compounds containing the enediyne moiety, as shown in Scheme 4.92. ... 

Electrocyclic reactions involve the cyclization of conjugated poly-... 

Carbon-carbon bond formation by intramolecular cyclization of conjugated iminium ions is also possible, as illustrated by the transformation of amine oxide (36) to (37), a key intermediate in the synthesis of the pyridocarbazole alkaloid ellipticine (38 Scheme 7). ... 

At the end of this section, it may be said that even though most five- and six-membered heterocyclic rings can now be readily prepared, the synthesis of seven-membered rings is not straightforward. 1,7-Cyclizations of conjugated 1,5-dipoles and related reactions may be a subject for more work in the future. 

Cycloaromatization. These catalysts effect isomerization/cyclization of conjugated... 

In cyclization of conjugated dienes, typically butadiene, coordination of two molecules of butadiene gives rise to the bis-jr-allyl complexe 12. The distance between the terminals of two molecules of butadiene becomes closer by 7r-coordination to Pd(0), and the oxidative cyclization is thought to generate either the l-pallada-2,5-divinylcyclopentane 13 or l-pallada-3,7-cyclononadiene 14. 

Synthesis and Cyclizations of Conjugated Systems Derived from Organoboranes... 

Hong S, Kawaoka AM, Marks TJ. Intramolecular hydroami-nation/cyclization of conjugated aminodienes catalyzed by organolanthanide complexes.Scope, diastereo and enantiose-lectivity, and reaction mechanism. J. Am. Chem. Soc. 2003 125(51) 15878-15892. 

Basak, A., Das, S., Mallick, D. and Jemmis, D. (2009) Which one is preferred Myers-Saito cyclization of ene-yne-allene or Garratt-Braverman cyclization of conjugated bisallenic sulfone A theoretical and experimental study. Journal of the American Chemical Society, 131(43), 15695-15704. 

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