Intramolecular reactions ether formation

Various bicyclic and polycyclic compounds are produced by intramolecular reactions] 127]. In the syntheses of the decalin systems 157 [38] and 158 [128], cis ring Junctions are selectively generated. In the formation of 158, allyhc silyl ether remains intact. A bridged bicyclo[3.3. l]nonane ring 159 was constructed... 

The same type of bis-functionalization has been reported for the palladium-catalyzed borylstannylative carbocy-cyclization of 1,6-, 1,5-, 1,7-diynes, bis-propargylamine, and ether.377 It should be noted that even 1,2-dialkylidene cyclobutane can be obtained in reasonable yield. Ito has proposed the related silaborative reaction involving nickel(O) catalysis.378 This reaction has been performed in an intra- and intermolecular fashion. The intramolecular reaction allows the formation of cyclic dienes and the intermolecular process proceeds through a dimerization of alkynes to give acyclic dienes. 

The almost instantaneous intramolecular ether formation by reaction of phenoxy anions, generated from the silyl ethers with a stoichiometric amount of tetra-n-butylammonium fluoride, on mesylate esters has been used to synthesize labile benzo-0-2-isocephams (>90%) [20]. 

For many years, intramolecular reactions such as conformational changes, bond cleavage, bond formation, and valence isomerizations have been observed only when hydrocarbons were reduced with alkali metals in ethereal solvents. In most electrochemical experiments, these reactions were dominated by the electrophilic processes already described. However, progress in experimental techniques [8, 9, 27-29] has made these reactions accessible to electroanalytical investigations, providing new mechanistic insight. 

There are several instances where reactive intermediates present during the synthesis of septanosides have reacted intramolecularly to give ring-contracted or bicyclic products.91 Similar intramolecular reactions (such as the formation of 1,6-anhydro sugars) have precedents in the pyranose literature. Such cases of intramolecular reactivity described in the literature notably involve novel ethers or thioethers as the nucleophilic species in the ring contractions. 

Carbozincation of alkenes is a very facile method for the formation of new C—C bonds. A smooth intramolecular reaction occurs when functionalized 5-hexenyl iodides are treated with diethylzinc in ether, followed by acid work-up (equation 106)416. 

Electrophilic substitutions of alkenyl-, aryl-, and alkynylsilanes with heteroatom-stabilized cationic carbon species generated by the action of a Lewis or Brpnsted acid (acyl cation, oxocarbenium ion, etc.) provide powerful methods for carbon-carbon bond formation. Particularly, intramolecular reactions of alkenylsilanes with oxocarbenium and iminium ions are very valuable for stereoselective construction of cyclic ether and amine units.21-23 For example, the BFj OEt -promoted reaction of (E)- and (Z)-alkenylsilanes bearing an acetal moiety in the alkenyl ligand gives 2,6-disubstituted dihydropyrans in a stereospecific manner (Scheme l).23 Arylsilanes also can be utilized for a similar cyclization.24... 

An orf/io-directed lithiation allows the conversion of 25 to aryl iodide 40. Reductive ether formation of aldehyde 40 with crotyl alcohol yields compound 41. Intramolecular Heck reaction of 41 affords a mixture of the olefins 42 and 43. The undesired alkene 42 can be isomer-ized quantitatively to the desired enol ether 43 with Wilkinson s catalyst. Sharpless dihydroxylation ee 94 %) of the enol ether 43 provides lactol 44, which is oxidized directly to lactone 45. Finally, the pyridone-O-methyl ester is cleaved under acid conditions (45 — 7). 

In the reductive coupling of phenylhexenyldichlorosilane, 75% of the alkenyl groups did not react with radicals, although cyclization was highly favored in the strainless ring. This result means that the lifetime of radicals must be very short and that they can be further reduced to silyl anions prior to intramolecular cyclization. The presence of anionic intermediates is additionally supported by faster reactions in ethereal solvents, first-order kinetics of the monomer, and some model reactions. The formation of silyl... 

The reaction is effective with electron-rich carbonyls such as trimethylsilyl esters and thioesters, as Table 20 indicates. Lactones ate substrates for alkylidenation however, hydroxy ketones are formed as side products, and yields are lower than with alkyl esters. Amides are also effective, but form the ( )-isomer predominantly. This method has been applied to the synthesis of precursors to spiroacetals (499) by Kocienski (equation 115). ° The reaction was found to be compatible with THP-protected hy- oxy groups, aromatic and branched substituents, and alkene functionality, although complex substitution leads to varying rates of reaction for alkylidenation. Kocienski and coworkers found the intramolecular reaction to be problematic. As with the CrCb chemistry, this reaction cannot be used with a disubstituted dibromoalkane to form the tetrasubstituted enol ether. Attempts were made to apply this reaction to alkene formation by reaction with aldehydes and ketones, but unfortunately the (Z) ( )-ratio of the alkenes formed is virtu ly 1 1. ... 

Treatment of cyclopropyl sulfides bearing a hydroxy group in the side chain with ceric ammonium nitrate (CAN) in methanol gives five- and six-membered cyclic ethers.In this reaction, the formation of cyclic ethers is understood by assuming a single electron transfer mechanism, which involves ring opening of the cation intermediate followed by intramolecular nucleophilic addition. 

A second, less used, strategy encompasses the Lewis acid catalyzed intramolecular reaction of a silyl enol ether with a propargyl cation. The latter can be conveniently generated by a cobalt complexed propargyl ether. This complexation strongly helps the carbocation formation. By using cobalt complexation, intramolecular aldol type reactions (for R = OR ) have been accomplished. ... 

In addition to the formation and reactions of carbonyl ylides discussed in the previous section, carbenoids also react intramolecularly with ethereal oxygen atoms to generate oxonium intermediates. When the ether is part of a ring as in substrates 63 a-b, the intramolecular addition of rhodium carbenoids produces bicyclic oxonium intermediates, which generated [5.2.1] oxabicycles 64a-b upon rearrangement by a [2,3]-sigmatropic pathway, Eq. 44 [74]. 

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