Intramolecular eliminative rearrangement

R = H) undergoes a variety of enzyme-catalyzed free-radical intramolecular cyclization reactions, followed by late-stage oxidations, eliminations, rearrangements, and O- and N-alkylations. Working from this generalization as an organizing principle, the majority of known AmaryUidaceae alkaloids can be divided into eight stmctural classes (47). 

Under drastic thermal conditions, the iminosilane-LiF adduct 82 eliminates LiF and the iminosilane intermediate 83 rearranges intramolecularly by C-H bond insertion affording 2,2,4,4-tetra-/i /Z-butyl-l,3-diaza-2,4-disilabicy-clo[3.3.0]octane 84 in 87% yield (Equation 6) <1996JOM203>. 

Reaction of iV-(2-cyanophenyl)benzimidoyl chloride 66 with sterically hindered thioureas gives intermediate l-[(2-cyanophenylimino)phenylmethyl]thioureas 67 (see also Section 14.08.7.4.2), which are prone to intramolecular S-attack and give 2-phenylquinazoline-4(377)-thione 68 as a final elimination/rearrangement product (Scheme 13 <2002MOL96 . 

Thiazolidine rings are normally easy to prepare but this year a new reaction has appeared in which the ring system was produced during a thermally-induced intramolecular eliminative rearrangement. The azido-enones (364) were heated to 140 C leading to a mixture of the products (365) and (366) in equal proportions. The authors have determined that an intramolecular acid-catalysed Schmidt reaction is not involved, and have proposed the triazoline (367) as a reasonable intermediate. They have also shown that in certain cases rearrangement coupled with desulphurization provides a completely regioselective method for conversion of 2-cycloalkenones to the ene lactams (368). 

The spiro-fused cyclopentenone 31 has been prepared by 3-aza-Cope rearrangement, intramolecular Mannich reaction, p-elimination reaction sequence from 30. A mixture of enamine stereoisomers in the initial condensation leads to a mixture of spirocyclic epimers in 31 (Scheme 5). ... 

The reaction of alkenyl mercurials with alkenes forms 7r-allylpalladium intermediates by the rearrangement of Pd via the elimination of H—Pd—Cl and its reverse readdition. Further transformations such as trapping with nucleophiles or elimination form conjugated dienes[379]. The 7r-allylpalladium intermediate 418 formed from 3-butenoic acid reacts intramolecularly with carboxylic acid to yield the 7-vinyl-7-laCtone 4I9[380], The /i,7-titisaturated amide 421 is obtained by the reaction of 4-vinyl-2-azetidinone (420) with an organomercur-ial. Similarly homoallylic alcohols are obtained from vinylic oxetanes[381]. 

Reaction of the carbanion of chloromethyl phenyl sulphoxide 409 with carbonyl compounds yields the corresponding 0-hydroxy adducts 410 in 68-79% yield. Each of these compounds appears to be a single isomer (equation 242). Treatment of adducts 410 with dilute potassium hydroxide in methanol at room temperature gives the epoxy sulphoxides 411 (equation 243). The ease of this intramolecular displacement of chloride ion contrasts with a great difficulty in displacing chloride ion from chloromethyl phenyl sulphoxide by external nucleophiles . When chloromethyl methyl sulphoxide 412 is reacted with unsymmetrical ketones in the presence of potassium tcrt-butoxide in tert-butanol oxiranes are directly formed as a mixture of diastereoisomers (equation 244). a-Sulphinyl epoxides 413 rearrange to a-sulphinyl aldehydes 414 or ketones, which can be transformed by elimination of sulphenic acid into a, 8-unsaturated aldehydes or ketones (equation 245). The lithium salts (410a) of a-chloro-/ -hydroxyalkyl... 

Intramolecular insertion and addition reactions are very rare for alkyl nitrenes. In fact, it is not clear that the nitrenes are formed as discrete species. The migration may be concerted with elimination, as is often the case in the Wolff rearrangement.251... 

The rearrangement of 24 to 19 is an intramolecular process with 1st order kinetics. The rearrangement does not occur by reductive elimination of TMS from 24 and by readdition of free TMS to the [(dtbpm)Pt(O)] fragment in a subsequent C-Si activation step. This possibility can be excluded rigorously by performing the 24 to 19 transformation in fully deuterated Si(CD3)4 or in hexamethydisi-loxane as solvents 24 only yields undeuterated 19 in d -TMS and does not lead to 21 in hexamethyl-disiloxane (which would have to be seen as a dissociative mechanism, as [(dtbpm)Pt(O)] had been previously shown to activate hexamethydisiloxane under the same reaction conditions). 

If the insertion step following oxidative addition occurs on one of the two fragments resulting from oxidative addition, an intramolecular catalytic reaction (C—O — C—C rearrangement) takes place (example 40, Table III). It is interesting to note that two different products—2,6- and 3,6-heptadienoic acids—can be obtained from allyl 3-butenoate. Their ratio can be controlled by adding 1 mole of the appropriate phosphine or phosphite to bis(cyclooctadiene)nickel or similar complex. Bulky ligands favor the 2,6 isomer. It is thus possible to drive the reaction toward two different types of H elimination, namely, from the a or y carbon atoms. 

After elimination of ethoxytrimethylsilane, 7,7-dimethyl-1-norbomene (6) rearranges to 3,3-dimethyl-2-norbomylidene (7), which undergoes a 1,3-CH insertion reaction into the endo-6-H to give tricyclane 8. Nortricyclane formation has been shown to be the fastest intramolecular mode of stabilization of 2-norbomylidene.5,6... 

Under more basic conditions, a-elimination predominates and insertion of the carbene 40 to the solvent gives racemic 22. Non-basic and poorly nucleophilic conditions allow neighboring group participation to form the rearranged substitution product 23 with complete chirality transfer. The participation can be considered as an intramolecular nucleophilic substitution, and does occur only when it is preferable to the external reactions. Under slightly basic conditions with bases in HFIP, participation is allowed, and the weak base can react with the more electrophilic vinylic cation 21 (but not with the iodonium ion 19). A suitably controlled basicity can result in the formation of cycloalkyne 39, which is symmetrical and leads to racemization. These reactivities are illustrated in Scheme 6. 

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

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