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Tetraenes cationic cyclization

Tetraene 4 (Scheme 1.3), when treated with 40 mol % of triflic acid in methylene chloride at -23 °C for 1 h, gives the adducts 5 and 6 in a 1 1 ratio as the main reaction products. The formation of these adducts has been justified [21] by a stepwise mechanism that requires an initial reversible protonation of 4 to produce the allyl cation 7, which then cyclizes to 8 and 9 in a non-reversible process. Deprotonation of 8 and 9 gives 5 and 6, respectively. [Pg.6]

Treatment of polyolefinic ketal 230 with stannic chloride in pentane gave a mixture (30% yield) of about equal amounts of the two racemic D-homoster-oidal tetracyclic isomers 231 (88). In this cyclization, the first cationic intermediate is not chiral and the two faces of the 5,6-double-bond can react with equal facility with the carbonium ion as a consequence, the product obtained (231) is necessarily racemic. The conversion of the open-chain tetraenic acetal 230 having no chiral centers into a tetracyclic system having seven such centers and producing only two (231) out of a possible 64 racemates is a striking tribute to the power of stereoelectronic effects. [Pg.301]

Gassman first observed the intermediacy of allyl cations as dienophiles in connection with efforts to extend the aminium cation-radical catalyzed Diels-Alder reaction to intramolecular cases. It was found that the cyclization of tetraene (26) performed in the presence of 0.2 equiv. of tris(p-bromophe-nyl)aminium hexachloroantimonate [ArsN+ CleSb ] was in fact catalyzed by protic acid. - A better means of catalyzing the cyclization of (26) involves use of 4 mol % trifluoromethanesulfonic acid at -23 C for 6 min, which provides the trans-fused cycloadduct in 88% yield (98% isomeric purity). Similar treatment of (27), however, provided a mixture of cis- and trans-fused products. A subsequent report revealed that allylic alcohols and allylic ethers are useful precursors to the allyl cation dienophiles with... [Pg.520]

The distyrylbenzene derivative (141) is photochemically reactive on irradiation in solution. The solvent of choice is acetonitrile/benzene/water (7 2 1) saturated with ammonia. The reactions encountered with this system are derived from electron transfer initiated by p-dicyanobenzene as the electron accepting sensitizer. This process yields the radical cation (142) of the starting material and also the cyclized radical cation (143). These species are trapped by ammonia to yield the final products (144) and (145) in the yields shown. The naphthyl system [141, R-R = (CH=CH)2] is also reactive and affords the analogous products (146) and (147). A study has examined the photochemically-induced cyclization of tetraenes such as (148) under SET conditions in aqueous acetonitrile solution. A variety of electron accepting sensitizers was used. In the example cited the sensitizer (149) was effective and the cascade cyclization yielded the product (150). [Pg.135]

The synthesis of the macrocycle 30 with n = 4, pentacyclo[12.2.2.2. 2. 2 ]-tetracosa-1,5,9,13-tetraene, was carried out likewise by McMurry et al. [26]. They used the method cited above to cyclize the diketone 29 in an intramolecular reaction. The macrocycle 30, mp >300 C, was isolated in 90% yield which is extremely high. An X-ray crystal structure showed that the distance between two opposite double bonds is 511 pm, i.e. the metal-carbon distance should be about 250 pm for optimal complexation of a metal cation. For this reason the silver ion was chosen. If the tetraalkene 30 is stirred with silver triflate in THF the expected complex 31, stable against light, heat, and air, precipitates as a colorless solid in 66% yield (mp 145 C). [Pg.188]


See other pages where Tetraenes cationic cyclization is mentioned: [Pg.259]    [Pg.532]    [Pg.532]    [Pg.532]    [Pg.532]    [Pg.373]    [Pg.259]    [Pg.259]    [Pg.142]   
See also in sourсe #XX -- [ Pg.532 ]

See also in sourсe #XX -- [ Pg.532 ]




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Cation cyclizations

Cyclization cationic

Tetraenals

Tetraene

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