8-EndOe endo„ cyclization

The 7, i5-unsaturated alcohol 99 is cyclized to 2-vinyl-5-phenyltetrahydro-furan (100) by exo cyclization in aqueous alcohol[124]. On the other hand, the dihydropyran 101 is formed by endo cyclization from a 7, (5-unsaturated alcohol substituted by two methyl groups at the i5-position. The direction of elimination of /3-hydrogen to give either enol ethers or allylic ethers can be controlled by using DMSO as a solvent and utilized in the synthesis of the tetronomycin precursor 102[125], The oxidation of the optically active 3-alkene-l,2-diol 103 affords the 2,5-dihydrofuran 104 in high ee. It should be noted that /3-OH is eliminated rather than /3-H at the end of the reac-tion[126]. 

Phenolic oxygen participates in facile oxypalladation. The intramolecular reaction of 2-hydroxychalcone (105) produces the flavone 106[127]. The ben-zofuran 107 is formed from 2-allyIphenol by exo cyclization with Pd(OAc)2, but benzopyran 108 is obtained by endo cyclization with PdChf S], Normal cyclization takes place to form the furan 109 from 2-(l-phenylethenyl)phe-nol[129]. Benzofuran formation by this method has been utilized in the synthesis of aklavinione (110)[130]. 

The intramolecular reaction oF allcenes with various O and N functional groups offers useful synthetic methods for heterocycles[13,14,166]. The reaction of unsaturated carboxylic acids affords lactones by either exo- or endo-cyclization depending on the positions of the double bond. The reaction of sodium salts of the 3-alkenoic acid 143 and 4-alkenoic acid 144 with Li2PdCl4 affords mostly five-membcrcd lactones in 30-40% yields[167]. Both 5-hexe-noic acid (145) and 4-hexenoic acid (146) are converted to five- or six-mem-bered lactones depending on the solvents and bases[168]. Conjugated 2,4-pentadienoic acid (147) is cyclized with Li2PdCl4 to give 2-pyrone (148) in water[i69]. 

Unlike the intermolecular reaction, the intramolecular aminopalladation proceeds more easily[13,14,166], Methylindole (164) is obtained by the intramolecular exo amination of 2-allylaniline (163). If there is another olefinic bond in the same molecule, the aminopalladation product 165 undergoes intramolecular alkene insertion to give the tricyclic compound 166[178]. 2,2-Dimethyl-l,2-dihydroquinoline (168) is obtained by endo cyclization of 2-(3,3-dimethyiallyl)aniline (167). The oxidative amination proceeds smoothly... 

Radical cyclization of perhydro-l,3-benzoxazines 64, promoted by Bu4SuH in the presence of AIBN gave a mixture of perhydropyrido[2,l-6][1,3]benzoxazin-9-ones 65 and 66 and seven membered tricyclic derivatives 67 and 68, formed in a 6-exo and 7-endo cyclization process, respectively (99TL2421). Cyclization of parent acrylamide 64 (R = R = H) occured with moderate regioselectivity (6-exo/7-endo ratio 65 35) and poor stereoselectivity (65/66 ratio 42 43). The presence of a /3-methyl group in... 

Scheme 8.7 A 6-endo cyclization process in a hydroxy-epoxy ketone.

A 6-endo cyclization by a y-epoxy alcohol can be accomplished through the presence of an appropriately disposed electron-withdrawing group, as found in the total synthesis of (+)-phonomactin. Treatment of the hydroxy-epoxy ketone 30 (R = H) with HC1 afforded the bicyclic compound 31, containing a pyran-4-one ring, in which nucleophilic attack of the y-OH group occurred at the oxirane carbon distal from the unfavorable electronic effect of the carbonyl group (Scheme 8.7) [20a]. 

An easy, silica gel-promoted 6-endo cyclization of y-epoxy alcohol 32 to pyran 33, followed by acid-catalyzed spiroketalization of the keto diol 34, afforded the common tricyclic spiroketal fragment 35 of lituarines A, B, and C (Scheme 8.8) [20b],... 

A11 elegant application of the strategy based on the appropriate use of a 6-exo or 6-endo cyclization of a 5- or y-epoxy alcohol, respectively, to give corresponding adjacent or fused THP rings can be found in the regioselective synthesis of the ABC tris-THP core of thyrsiferol and venustatriol [29b]. 

The cyclization of the homologous epoxide 36 under acidic conditions was also investigated (Table 9.5) [110]. As would be expected, compound 36a reacted by a 6-exo cyclization to give tetrahydropyran 38a (Entry 1). The a, 3-unsaturated hydroxy epoxide 36b gave a 1 3.5 mixture of oxepane 37b and tetrahydropyran 38b (Entry 2). Subjection of 36c and 36d, which both contain more electron-rich 71-systems, to the reaction conditions resulted in preferential 7-endo cyclization to give 37c and 37d, thus confirming the powerful regiodirecting effect of the vinyl moiety (Entries 3 and 4). 

The data from Beckwith s work given in Table 10-9 look very confusing but, as discussed in the four papers by Beckwith s group (see footnote a) in Table 10-9) and in additional comments in Galli s review (1988), they can all be explained on the basis of our present knowledge of aryl radical chemistry, with the exception of the endo cyclization of the 2-(7V-2,-propenylsulfamoyl)-benzenediazonium ion in Scheme 10-81. 

A radical cyclization of a 2-chloroacyl enamine 157 was used to synthesize 2-substituted pyroglutamates 160. Usually, the radical 158 undergoes an initial 5-endo cyclization (path a) and the resulting intermediate 159 attacked electrophiles like methyl acrylate to give the pyroglutamate 160. Unexpectedly, the reaction with methyl methacrylate took another course and a seven-membered... 

Although exo cyclization is usually preferred, there is no strong prohibition of endo cyclization and aryl-controlled regioselectivity can override the exo preference. 

Fig. 10.13. MM2 models of exo and endo cyclization transition structures for 5-hexenyl, 6-heptenyl, and 7-octenyl radicals. Reproduced from Tetrahedron, 41, 3925 (1985), by permission of Elsevier.

Entry 11 involves generation and cyclization of an alkoxymethyl radical from a selenide. The cyclization mode is the anticipated 5-exo with a cis ring juncture. This is a case in which the electronic characteristics of the radical are not particularly favorable (ERG oxygen in the radical), but cyclization nevertheless proceeds readily. The reaction in Entry 12 was used to prepare a precursor of epibatidine. Entry 13 shows a 6-endo cyclization that is favored by steric factors. The 6-endo cyclization is also favored with a tetrahydropyranyloxy substituent in place of the ester, indicating that the electronic effect is not important. Entries 14 to 16 involve acyl radicals generated from selenides. The preferred 6-endo cyclization in Entry 15 is thought to be due to the preference for the less-substituted end of the double bond. Entry 17 is an example of a 5-exo-dig cyclization. 

Entries 18 to 19 pertain to cyclizations of electrophilic radicals generated by oxidations. Entry 18 is the prototype for cyclization of a number of more highly substituted systems. The reaction outcome is consistent with oxidation of the less-substituted enolic position followed by a 6-endo cyclization. The cyclized radical is then oxidized and deprotonated. In Entry 19, the vinyl radical formed by cyclization is reduced by hydrogen abstraction from the solvent ethanol. 

Trost and his group reported a 6-endo cyclization en route to (-)-siccanin (Scheme 20). The minor diastereoisomer of the radical formed during cyclization yielded a THF derivative directly [98,99]. 

The above-mentioned important and impressive applications of titanocene mediated and catalyzed epoxide opening have been achieved by using the already classical 5-exo, 6-exo and 6-endo cyclizations with alkenes or alkynes as radical acceptors. Besides these achievements, the high chemoselectiv-ity of radical generation and slow reduction of the intermediate radicals by Cp2TiCl has resulted in some remarkable novel methodology. 

Employing their catalytic system, the group of Oltra and Cuerva demonstrated that 7-endo cyclizations can be performed in surprisingly high yields. Moreover, 7-endo-cyclizations were used by the same group in elegant catalytic cyclization cascades for the preparation of a number of natural products as shown in Scheme 33 [125]. Barreo et al. reported similar methodology [105]. 

Scheme 33 Oltra s and Cuerva s tandem sequence featuring a 7-endo cyclization...

Finally, Roy and his group reported the first examples of stoichiometric 8-endo cyclizations for the preparation of aromatic ethers [126]. 

Intramolecular radical cyclization of methylenecyclopropanes accompanies ring rearrangement [115]. In the case of (methylenecyclopropyl)propyl radicals, 5-exo-cyclization is observed as illustrated in Scheme 83, but 6-exo- and 1-endo-cyclizations occur with (methyleneeyelopropyl)butyl radicals, depending on the substituent. (Scheme 83)... 

A modified Pictet-Spengler reaction has been applied to the synthesis of thiazolo-quinolines 58 <06T3228>. Condensation of anilines 53 with aryl aldehydes 54 followed by endo cyclization results in the formation of thiazoloquinolines 58 under a variety of traditional Pictet-Spengler protocols such as 2% trifluoroacetic acid in dichloromethane. 

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