Azulenes 3 + 2 annulations

The allenylsilanes are excellent nucleophiles and they can react with a variety of electrophilic species in annulation processes that provide access to diverse products. Allenylsilane 112 (Eq. 13.36) reacts with tropylium fluoroborate 111 to provide azu-lene 113 [35]. The reaction is slow and it is necessary to use an acid scavenger so as to inhibit protiodesilylation by the fluoroboric acid that is generated during the course of the annulation. The excess tropylium salt abstracts a hydride from the reaction intermediate leading to the azulene. There are relatively few direct methods for the synthesis of azulenes. 

Highly strained substrates can be transformed into even more strained isomers by ODPM rearrangement. This has been shown by Murata et al. for the synthesis of a valence isomer of azulene [45]. Albeit the photochemical reaction yielded only 20-25% of the bicyclo[1.1.0]butane derivative 15, the synthesis of the precursor cyclobutene (14) is straight-forward from the bicyclo[3.3.0]octenone 13 (Sch. 18). This substrate has obviously a diverse reactivity pattern when directly excited, however, triplet sensitization reduces these competitive pathways because alkene excitation is excluded. Also benzo-annulated azulene valence isomers were generated by this approach [46]. 

Similar [3 + 2] annulations of allenylsilanes 86 and propargylsilanes 87 with a, /i-unsaturated ketones, ketones, 7V-acyliminium ions, nitrosonium ions and tropylium cations giving the corresponding cyclopentenes (cyclohexenes), dihydrofurans, 3-pyrrolines, isoxazoles and azulene derivatives, respectively, have been reported (equations 60 and 6i)142.t54,i68-t72>... 

Seven procedures describe preparation of important synthesis intermediates. A two-step procedure gives 2-(HYDROXYMETHYL)ALLYLTRIMETH-YLSILANE, a versatile bifunctional reagent. As the acetate, it can be converted to a trimethylenemethane-palladium complex (in situ) which undergoes [3 + 2] annulation reactions with electron-deficient alkenes. A preparation of halide-free METHYLLITHIUM is included because the presence of lithium halide in the reagent sometimes complicates the analysis and use of methyllithium. Commercial samples invariably contain a full molar equivalent of bromide or iodide. AZULENE is a fundamental compound in organic chemistry the preparation... 

The donor-acceptor-substituted calicene 8 undergoes an interesting cycloaddition with 6,6-bis(dimethylamino)fulvene (9) to give the fulvene-annulated azulene 10 besides the substitution product 11. ... 

Allenylsilanes combine with electron-deficient alkenes or alkynes regio- and stereo-selectively to afford hi ly substituted and functionalized cyclopentenes (Scheme 10). The [3 -i- 2] annulation reaction has been used for heteroannulation approaches to five-membered oxygen and nitrogen heterocycles. One particularly useful application of the method is that readily available tropylium salts can function as allenophiles in a general [3 + 2] annulation route to substituted azulenes (equation 3). ... 

From azulenes by annulation onto the hve-membered ring 2.3.5.7 From 1-haloazulenes. The use of functionalized azulenes, especially halo and amino derivatives (Sections 2.3.5.1-2.3.5.5), is regarded to be the most universal approach to polycyclic systems based on azulene (82JOU132). Many of the precursors for their part are readily accessible from active troponoids (04EJO899). 

Crombie, A.L., Kane, J.L., Shea, K.M. and Danheiser, R.L. (2004) Ring expansion-annulation strategy for the synthesis of substituted azulenes and oligoazulenes. 2. Synthesis of azulenyl halides, sulfonates, and azulenylmetal compounds and their application in transition-metal-mediated coupling reactions. J. Org. Chem., 69, 8652-67. 

The Danheiser annulation has also been reported to be useful for the preparation of substituted azulene products." For example, treatment of silylallene 58 with tropylium cation 59 at 23 °C produces intermediate cyclopentene dihydroazulene 60, which is not isolated but rather undergoes in situ dehydrogenation with a second equivalent of tropylium cation 59 to provide azulene 61. The use of the tert-butyldimethylsilyl group as opposed to the trimethylsilyl group is necessary since the trimethylsilyl group tends to undergo premature desilylation rather than cyclization to the azulene. Substitution at the 3-position of the allene was also found to significantly... 

Tf2NPh was successfully employed in the synthesis of substituted azulenes. Unstable 1-hydroxyazulenes were formed by a ring expansion-annulation strategy and trapped in situ by Tf2NPh as triflate esters (eq 17), which were utilized in Suzuki coupling reactions. 

The azulene synthesis proceeds best with 1,3-dialkyl (t-butyldimethylsilyl)allenes. (Trimethylsilyl)allenes desilylate to generate propargyl-substituted cycloheptatrienes as significant byproducts. As observed in the other [3 + 2] annulations discussed already, allenylsilanes lacking C-1 alkyl substituents do not participate in the reaction. 

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