Ring closures modes

Which thermal electrocyclic ring closure mode, conrotatory or disrotatory, would the cation in problem 12.1 follow ... 

A particular advantage of the low-mode search is that it can be applied to botli cyclic ajic acyclic molecules without any need for special ring closure treatments. As the low-mod> search proceeds a series of conformations is generated which themselves can act as starting points for normal mode analysis and deformation. In a sense, the approach is a system ati( one, bounded by the number of low-frequency modes that are selected. An extension of th( technique involves searching random mixtures of the low-frequency eigenvectors using Monte Carlo procedure. 

During the discussion of hemiacetal formation in d ribose in the preceding section you may have noticed that aldopentoses have the potential of forming a six membered cyclic hemiacetal via addition of the C 5 hydroxyl to the carbonyl group This mode of ring closure leads to a and p pyranose forms... 

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

Predict which is the more likely mode of ring closure for oxonin ... 

The ring closure of a diene to a cyclobutene can occur with rotation of the two termini in the same conrotatory) or opposite disrotatory) directions. For suitable substituted compounds, these two reaction modes lead to products with different stereochemistry. 

The first mode of ring closure is usually observed for the cyclohexene derivatives 2.115... 

The understanding of the mechanism of cyclopolymerization has been one of the initial driving forces responsible for studies on the factors controlling the mode of ring closure of 5-hexenyl radicals and other simple model compounds.113... 

The preferential 1,5-ring closure of unsubsliluted 5-hexenyl radicals has been attributed to various factors these arc discussed in greater detail in Section 2.3.4. The mode and rate of cyclization is strongly influenced by substituents. The results may be summarized as follows (Scheme 4.13) ... 

As in the photolysis of protonated eucarvone, an acyclic intermediate is proposed in the mechanistic pathway. The protonated dienones 73 and 74 should be thermally stable, since a symmetry-allowed ring closure in the conrotatory mode is precluded in the cyclic system (Woodward and Hoffmann, 1970). Upon irradiation it can undergo a conrotatory ring opening however, to produce the acyclic cations 79 and 80 which in... 

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

The possibility that 34 and 35 were formed via 33 was eliminated and hence it must be concluded that, in contradistinction to the reaction with 2,3-dibromocyclo-pentyl hydroperoxide 38), the Ag02CCF3-induced dioxabicyclization of 3,4-dibromo-cyclopentyl hydroperoxide involves preferential displacement of the ciy-3-bromine. It seems highly probable that this process is assisted by the vicinal bromine, i.e. that the frans-bromonium ion 36 is an intermediate. Failure to observe the analogous mechanism with 2,3-dibromocyclopentyl hydroperoxide presumably reflects the disfavoured nature of the mode of ring closure needed in the corresponding species 37. 

Rate constants at 65°C for ring closure of co-alkenyl radicals in the exo-mode (71) ... 

In addition to nucleophilic reactions, Baldwin s rules also apply to homo-lytic and cationic processes. Table 21 lists rate constants for ring closure of lower -alkenyl radicals (71), in which intramolecular addition to the double bond occurs in the exo-mode (Beckwith, 1981). It is unfortunate that EM-... 

Many of the observations discussed above clearly warrant further investigation. In particular methods for controlling the Z -selectivity of the ring-closure are of high priority. Recent reports [27] discussing the precise mode of action of the ruthenium initiators provide a solid foundation upon which these future studies can be built and success in this area will strengthen even further the already powerful RCM process. 

Electrocyclic processes are also observed with ionic systems, both cationic and anionic. The decisive factor in the stereochemistry is the number of electrons in the delocalized polyenic system. Thus the mode of ring opening or ring closure depends on the n electrons in the open polyene. 

Scheme 11.12 Modes of ring closure of 3-(

---