Nine-membered ring formation

Nine-membered Ring Formation Using Sml2... 

Table 3.6 Apparent energy barriers (kcal/mol) of nine-membered ring formation (P) and 1-hexene liberation via agostic-assisted p-H transfer T) and the differences between the two barriers for each cationic model...

Scheme 34 Formation of the central nine-membered ring and completion of the carbon skeleton in Hirama s improved total synthesis of the marine neurotoxin ciguatoxin CTX3C (183)[90b]...

Following their studies on the trapping of intermediates in the [5 + 2]-cycloaddition with CO, which led to the [5 + 2 + l]-process, the Wender group found that when related reactions were conducted with aryl alkynes under an atmosphere of CO, a novel four-component [5 + 2 + 1 + l]-product was observed.172 A VCP, an alkyne, and 2 equiv. of CO react to give the biaryl compounds shown (Scheme 78) via the formation of a nine-membered ring intermediate. 

The mechanism of this novel process is proposed to involve formation of a nine-membered ring, which after tautomerization undergoes an electrocylic ring closure to form the bicyclo[4.3.0]nonadienone intermediate. Elimination of ROH gives the aromatic product observed (Scheme 79). 

Further research on intramolecular photocyclization of amino enones and amino ketones based on electron transfer has been made by Kraus and Chen [228]. In analogy to the earlier results of Roth and El Raie [229], Kraus and Chen obtain the cyclopropanol derivative 286 as single stereoisomer by direct irradiation of 285. Photolysis of the amino enone 287 does not lead to a three-membered ring product, but only to pyrrolidine 288. The irradiation of the unsaturated keto ester 289 results in the even more unexpected formation of a nine-membered ring product 290. Such remote photocyclizations have rarely been described so far. 

A rationalization of the generally selective formation of the undesired C-16 -C-14 PREF relative stereochemistry on reaction of the chloro-imine-chloroindoline alkene 17a,b with vindoline (3) may be found in a preferred conformation, 25a or 25b, of the cationic intermediate 25, where the C-3 methylene group hinders approach to one face of the nine-mem-bered ring (Fig. 2). A conformationally more flexible nine-membered ring cationic intermediate 29a,b would be expected to be formed from the chloro derivative of a D-secocleavamine 30. From reaction of such a compound with vindoline (3), under the usual protic acid conditions for cou-... 

An interesting formation of a 1,3-dioxan -one from a geminal dichloride and salicylic acid is displayed in Scheme 103. Langer et al. have found that phthaloyl chloride cleanly reacted with salicylic acid. The product was not the expected nine-membered ring but spirotetracycle 219 in 82% yield. The product formation can be explained by assuming an initial equilibration of phthaloyl chloride and 3,3-dichloro-3//-isobenzofuran-l-one <2001EJ01511>. 

In the preparation of cyclic ethers by diene-ene RCM, there is a competition between the formation of cyclic allyl ether with a smaller ring size and of cyclic pentadienyl ether with a larger ring size (Scheme 24). In particular, in the competition between five- and seven-membered ring formation, both dihydrofuran and dihydrooxepine derivatives are formed in comparable amounts, whereas in the competition between seven- and nine-membered rings, a dihydrooxepine forms exclusively <2006JOC3977>. 

The 11-carbon ring of the humulyl carbocation may be retained, as in the formation of humulene (Figure 5.36), or modified to give the caryophyllyl cation containing a nine-membered ring fused to a four-membered ring, as in P-caryophyllene (Figure 5.36). Humulene is found... 

Scheme 3. Formation of nine-membered rings from vinyl allenes and CO.

Suggest a mechanism for this formation of a nine-membered ring. Warning The weak hindered base is not strong enough to form an enolate from the lactone. 

Electrophilic transannular cyclization of nine-membered ring lactam 129 led to formation of protected methyl 6-amino-8-iodo-5-oxooctahydroindolizine-3-carboxylates 130a and 130b in high yields (Equation 4) <20060L2851>. 

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