Intramolecular opening

Intramolecular Opening with Oxygen and Nitrogen Nucleophiles... 

Intramolecular opening of 2-azabicyclo[3.3.0]oct-7-ene-3 endo-carboxamide epoxide displays a dependence on the substituent at the nuclear nitrogen [46], The conformational preference for a proximal attack by. the amide function is counterbalanced by an accentuated polarity alternating sequence which enforces the acceptor role of the distal carbon of the epoxide ring when the nuclear nitrogen is part of a carbamate group. 

The second category is that of the electrocyclic reaction, illustrated in Equations 11.6-11.8, characterized by intramolecular opening or closing of a ring by conversion of [Pg.571]

Thermal reaction of the lactenediyne 79 in tetrahydrofuran (THF) in the presence of 1,4-cyclohexadiene undergoes cyclization via the intermediate 80 to give the tricyclic product 81 (Scheme 2). The reaction proceeds by the intramolecular opening of the /3-lactam ring to a larger seven-membered ring <2003EJ01319>. 

The most efficient catalyst for intramolecular opening of epoxides is CSA. The formation of tetrahydrofurans or tetrahydropy-rans is highly dependent on the structure of the hydroxy epoxide. The presence of a saturated chain at the secondary epoxide position leads to the formation of tetrahydrofurans (eq 5) via 5-exo ring closure, whereas an electron-rich double bond at this position gives tetrahydropyrans (eq 6) via 6-endo ring closure. This methodology has also been extended to the synthesis of oxepanes (eq7). ... 

Intramolecular opening of epoxides with a hetero-atom nucleophile promoted by a titanium salt is shown in Table 27. Intramolecular alkylation of an epoxide with a metalated sulfone enhanced by Ti(0-t-Pr)4 has also been reported to give a single cyclization product, as shown in Eq. (289) [650]. 

The reaction was carried out in acetonitrile at 353 K using TBHP as oxidant. Conversions as high as 80 % were obtained. As shown in Scheme 6, it was postulated that the reaction takes place via epoxidation over Ti sites foUowed by acid catalyzed intramolecular opening of the epoxide ring by the 3-hydroxy group. Ti-6 zeolite gave somewhat lower conversions in addition to the preferential formation of furans over pyrans (ratio of ca. 1.5) due to shape selectivity. Ti-MCM-41 and gave furan to pyran ratios of ca. 0.9, comparable to those obtained by the epoxidase conversion of linalool. 

Interestingly, acid-catalyzed intramolecular opening of allylic epoxides by hydroxyl groups proceeds regioselectively via the 7i-stabilized carbenium ion intermediate to form the tetrahydropyran ring. ... 

Intramolecular opening of V-(haloalkyl)azetidin-2-ones via trans amidation affords medium and unstrained ring azalactams. ... 

A further group of acetals has been prepared from manool23 for structure-odour studies. Some 1,5-diepoxides in this group underwent rearrangement based on the intramolecular opening of 8p,9,3-epoxides.24... 

Banfi, L. Guanti, G. Rasparini, M. Intramolecular opening of [l-lactams with amines as a strategy toward enzymatically or photochemically triggered activation of lactenediyne prodrugs. Eur. J. Org. Chem. 2003, 1319-1336. 

Scheme 19.20 Intramolecular opening of penicillin s beta-lactam. This process occurs under acidic conditions and is driven by the energetics associated with relieving the considerable bond-angle strain present in the 4-membered beta-lactam ring.

A simple synthesis of racemic allomuscarine via stereospecific intramolecular opening of the trans-substituted epoxide ring in butyl 4,5-epoxy-2-hydroxyhexanoate was reported by Chmielewski and Guzik (24,25) (Scheme 2). The furane ester intermediate was obtained in an ene reaction between butyl glyoxylate and 1-butene, followed by epoxidation of the... 

The differentially protected F-ring moiety 53 of the altohyrtin group of anticancer macrolides has been prepared using intramolecular opening of epoxide 52 to form the pyranose ring. Compound 52 was obtained by epoxidation of the corresponding unsaturated octitol, which in turn was constructed from small molecules in a multistep asymmetric synthesis. ... 

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