Cyclization of dienes

Scheme 4 Mechanism of the zirconocene-catalyzed reductive cyclization of dienes...

Scheme 10 Cyclization of dienes using a cationic zirconocene 58 and phenylsilane...

Scheme 14 Cp 2Ln-R catalyzed reductive cyclizations of dienes in the presence of silanes...

Scheme 16 Origin of the diastereoselectivity in the cyclization of dienes with 70...

A new approach to piperidines via cyclization of dienes, such as 158, employs a phosphorus hydride mediated radical addition/cyclization reaction <06JOC3656>. This reaction proceeds with complete regioselectivity to create the 6-exo-trig product 159, although as an inseparable mixture of two of the four possible diastereomers. 

Free-radical chain reactions have been reviewed60. The cyclization of dienes by the action of free radicals is illustrated for the case of the 1,6-heptadiene derivative 90 (E = CC Me) in equation 56. Treatment with tosyl radicals, produced from tosyl chloride and a catalytic amount of dibenzoyl peroxide, generates the radicals 91, which cyclize to 92. The latter reacts with tosyl chloride to form 93 and tosyl radicals are regenerated. The product is obtained in 85% yield as a 6 1 mixture of cis- and fraws-isomers61. 

Scheme 38 Heteroatom variants of the Zr-promoted cyclization of dienes, enynes, and related compounds. 

Table 6.14 Reductive cyclization of dienes catalyzed by Me2SiCp CpLnCH(SiMe3)2 [124].

The tandem zirconocene-induced co-cyclization of dienes or enynes/insertion of allyl carbenoid/addition of electrophile is a powerful method for assembling organic structures. Two illustrations of its application are the synthesis of the dollabelane natural product acetoxyodontoschismenol 99 [57,62,63] and the one-pot construction of linear terpenoids 100 (Scheme 3.25) [59,64],... 

The addition of hypophosphites to alkenes under Et3B initiation is also reported [27]. Piettre described recently the addition of diethylthiophosphite to alkenes leading to the formation of thiophosphonates (Scheme 9, Eq. 9a) [28]. Interestingly, this reaction can be used for cyclization of dienes and ring opening of strained alkenes such as a-pinene (Eq. 9b). Parson prepared an... 

Reduction of aromatic rings 5-15 Cyclization of dienes or diynes 5-18 Cyclization of unsaturated Grignard... 

TABLE 4.6. Zr-Catalyzed Enantioselective Cyclization of Dienes Containing Allylamine Moieties Induced by n-BuMgCl... 

Furthermore, intramolecular bis-silylative cyclization of dienes 28 with face selectivity gives a diastereomeric mixture of 29 and 30 (equation 16). The size of the substituents on the nonterminal silicon influences the ratio of 29/30, which follows the order Me < Et < Ph < /-Hu < / -Hr (cf. entry 17 of Table l)27. 

Polar cyclization of diene hydroperoxide 304 has been successfully effected by Hg(NC>3 )2 to produce a 2 1 mixture of 1,2-dioxolane 305 and 1,2-dioxane 306. In contrast, a radical cyclization with (Bu CX)(. ())2 and O2 or /V-iodosuccinirnidc, respectively, is more selective giving only one 1,2-dioxane425. 

Methods based on the cyclization of diene derivatives have been described. Thus, chalcose (152) has been synthesized by a cyclocondensation reaction of the silyloxy diene 166 with acetaldehyde (Scheme 50).241... 

The 14-electron species denoted here as Cp2M (M = Ti, Zr), generated from Group 4 metallocenes, mediate useful oxidative cyclizations of dienes 231 [103,104], enynes 233 [105] and diynes 235 [105,106] via the metallacycles 232, 234 and 236. These metallacycles are useful intermediates, which are hydrogenolysed, carbonylated, halogenated, attacked by electrophiles and converted to functionalized cyclic compounds such as 238 and 239 [107],... 

Cyclization of dienes.3 Cp2Zr also promotes cyclization of 1,6-dienes to trans-1,2-disubstituted cyclopentanes. Thus 1,6-heptadiene (1) on treatment with Cp2Zr followed by bromination affords the trans-dibromide 2. In contrast, use of a related reagent, Cp ZrCl (Cp = pentamethylcyclopentadienyl), effects cyclization to the isomeric ds-dibromide (equation I). Electrophiles other than bromine can... 

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