2- Hexenal cyclization

Catalysts were prepared by impregnation of Pt inside the pore structure of carbon fibers. Care was taken to eliminate the active metal from the external surface of the support. A very high dispersion of Pt was measured. Four reactions were carried out in a fixed-bed reactor competitive hydrogenation of cyclohexene and 1-hexene, cyclization of 1-hexene, n-heptane conversion and dehydrogenation of cyclohexanol. Three types of carbon fibers with a different pore size and Pt-adsorption capacity along with a Pt on activated carbon commercial catalyst were tested. The data indicate a significant effect of the pore size dimension on the selectivity in each system. The ability to tailor the pore structure to achieve results drastically different from those obtained with established supports is demonstrated with heptane conversion. Pt on open pore carbon fibers show higher activity with the same selectivity as compared with Pt on activated carbon catalysts. 

The reactions of heptane conversion and 1-hexene cyclization indicate a selectivity pattern based on the size of the product molecules. Negligible conversion to toluene was measured in the small pores carbon fibers, while other carbon fibers gave results similar to those of the activated carbon. 

Reactions of 3- and 4-piperidone-derived enamines with a dienester gave intermediates which could be dehydrogenated to tetrahydroquinolines and tetrahydroisoquinolines (678). The methyl vinyl ketone annelation of pyrrolines was extended to an erythrinan synthesis (679). Perhydrophenan-threnones were obtained from 1-acetylcyclohexene and pyrrolidinocyclo-hexene (680) or alternatively from Birch reduction and cyclization of a 2-pyridyl ethyl ketone intermediate, which was formed by alkylation of an enamine with a 2-vinylpyridine (681). 

The second aromatization reaction is the dehydrocyclization of paraffins to aromatics. For example, if n-hexane represents this reaction, the first step would be to dehydrogenate the hexane molecule over the platinum surface, giving 1-hexene (2- or 3-hexenes are also possible isomers, but cyclization to a cyclohexane ring may occur through a different mechanism). Cyclohexane then dehydrogenates to benzene. 

Sequential radical cyclizations are also featured in an efficient and clever synthesis of the cedrane framework 83 (see Scheme 15).30 Compound 81, the product of a regioselective Diels-Alder reaction between isoprene (79) and nitroethylene (80), participates in a nitroaldol reaction (Henry reaction) with 5-methyl-4-hexenal in the presence of a basic resin to give 82. Because the nitro group in... 

Ethyl l-cyano-2-methylcyclohexanecarboxylate has been prepared by catalytically hydrogenating the Diels-Alder adduct from butadiene and ethyl 2-cyano-2-butenoate3 and by the procedure described in this preparation.4 8 This procedure illustrates a general method for the preparation of alicyclic compounds by the cyclization of <5-ethylenic carbon radicals l.6 Whereas the primary 5-hexen-l-yl radical 1... 

The intramolecular formal [3+3] cycloaddition reaction of l- [l-phenyl-2-(4-oxobut-2-enyloxy)ethyl]amino cyclo-hexen-3-one at 150°C in the presence of piperidinium acetate afforded /ra 3--l,4a-77-l-phenyl-l,2,4,4a,7,8,9,10-octahydro[l,4]oxazino[4,3- ]quinolin-7-one <2002JA10435>. At 85°C, the 6-(l-piperidnyl)-l,2,4,4a,5,6,7,8,9,10-dec-ahydro derivative formed, which could be converted into the l,2,4,4a,7,8,9,10-octahydro derivative by heating at 150 °C. Cyclization of iV-[(2-butyl-2-oxoethoxy)acetyl]-3,4-dimethoxyphenylethylamine on the action of TFA gave llb-butyl-1,3,4,6,7,1 lb-hexahydro[l,4]oxazino[3,4- ]isoquinolin-4-one <1997JOC2080>. 

Alternatively, Ballini devised a new strategy to synthesize tri-alkylated pyrroles from 2,5-dialkylfurans and nitroalkanes <00SL391>. This method involves initial oxidation of 2,5-dimethylfuran with magnesium monoperoxyphthalate to cA-3-hexen-2,5-dione (6). Conjugate addition of the nitronate anion derived from the nitro compound 7 to 6 followed by chemoselective hydrogenation of the C-C double bond of the resulting enones 8 (obtained by elimination of nitrous acid from the Michael adduct) completes the conversion to the alkylated y-diketones 9. Final cyclization to pyrroles 10 featured improved Paal-Knorr reaction conditions involving reaction of the diketones with primary amines in a bed of basic alumina in the absence of solvent. 

Wallis185 examined reactions of dimethyl 2-hexen-4-ynedioate with thioureas and thio-amides and observed addition at C-5 via the sulfur atom of these nucleophiles the adducts often cyclize spontaneously to iminothiazolidinones (equation 80). 

A comparison of the cyclization rates of alkanes and alkenes may help to distinguish between associative and dissociative ring closure mechanisms, just as in the case of Cg dehydrocyclization of hexane and hexenes. 

A stereoselective cyclization of (Z)-(2R,3R,4R)-6-cyclohexyloxy-1,3,4-tribenzyloxy-5-hexen-2-ol promoted by mercuric trifluoroacetate or PhSeCl is successfully achieved (38). 2-Deoxy-0(-hexopyranoside derivative is obtained almost exclusively by the treatment of 22 with mercuric trifluoroacetate followed by reductive work up, while a predominant formation of the 3 3no 6r is achieved by the reaction of 22 with PhSeCl, and the successive deselenylation. 

The intramolecular carbolithiation of 6-lithio-l-hexene (9) was studied after lithiation of 6-chloro-l-hexene (8) in the presence of a catalytic amount of DTBB (5%). At —78 °C the corresponding organolithium compound 9 is stable, giving the expected products 10 by reaction with different electrophiles. However, when the lithiation step was carried out at — 30 °C a cyclization reaction took place, so that a new organolithium intermediate 11 was formed, which reacted with the same electrophiles to give cyclic products 12 (Scheme 5). ... 

Hydrocarbon bond saturation and cyclization also play roles in water solubility. Figure 6.7 shows that, among the six-carbon hydrocarbons, the various forms of hexane, C6H14, have the lowest solubility, and the hexenes and cyclohexane with the formula C6H12 have three times the solubility. Fewer hydrogen atoms consistently lead to higher solubilities, and benzene has one hundred times the water solubility of normal and iso-hexanes. 

The simple 1,2-dihydropyridine (63) cyclizes to the 2-azabicyclo[2.2.0]hexene (64) derivative upon irradiation (79JA6677). This reaction is the basis of a general synthetic route to dihydropyridines that are difficult to prepare by other means (Scheme 3). Dihydropyridines with more complicated substitution patterns, i.e. (66), undergo a photochemical rearrangement to valence isomers (68) and (69) (71T2957). The 1-azahexatriene (67) is believed to be an intermediate in this reaction (equation 21). 

As with any intermediate, a transient radical can be implicated from products formed in a reaction specific to the radical of interest. Experimentally, this is the basis of so-called mechanistic probe studies. An application of this method might employ, for example, 6-bromo-l-hexene as a probe for a radical intermediate as shown in Figure 4.3. If the 5-hexenyl radical is formed as a transient with an adequate lifetime, then cyclization of this radical to the cyclopentyhnethyl radical could eventually give the cyclic product, and detection of the cyclic product provides evidence that a radical was formed. The mechanistic probe approach is deceptively simple, however. To be useful, one must exclude other possibilities for formation of the rearranged product and demonstrate that the transient was formed in the reaction of interest and not in a side reaction. The latter is especially difficult to demonstrate, and, unfortunately, some mechanistic probe studies that seemingly provided proof of radical intermediates were later found to be complicated by radical-forming side reactions. 

Growe and co-workers independently reported a procedure for the cyclization/hydrosilylation of enones and enals similar to that reported by Buchwald." As an example, reaction of a 1 1 mixture of 3,3-dimethyl-5-hexenal and triethoxysilane catalyzed by Gp2Ti(PMe3)2 (20 mol%) in pentane at room temperature for 3 h gave m-l,l,3-trimethyl-4-triethoxysilyloxycyclopentane in 88% yield as a single diastereomer (Equation (38)). 

The lactone intermediate is prepared in another industrial process by cyclization of homofarnesic acid in the presence of SnCU as a catalyst [184]. Pure diastereomers are obtained by acid cyclization of trans- and cw-4-methyl-6-(2,6,6-trimethyl-cyclohex-l(2)enyl)-3-hexen-l-ol, prepared from 2-methyl-4-(2,6,6-trimethylcyclo-hex-l(2)-enyl)-2-butenal [185]. 

In several examples the reductive halide-hydrogen exchange has been studied on a preparative scale. For example, the indirect electroreduction of 2-chloropyridine in DMF using anthracene as mediator gives pyridine in 83-86 % yield 2 . For the dehalogenation of 1-chlorohexane (80% yield), naphthalene is applied as redox catalyst. Similarly, 6-chloro-hexene yields 1-hexene (60%) and methylcyclopentane (25%), which is the product of the radical cyclization . The indirect electrochemical reduction of p- and y-bromocarboxylic esters forms coupling and elimination products besides the dehalogenated products... 

As shown in Figure 42 for the Norrish II reactions of a simple ketone, 2-nonanone, not only do the shapes of the products differ from those of the reactant, but so do their molecular volumes [265]. Interestingly, the volume of the fragmentation products, 1-hexene and 2-hydroxypropene (which ketonizes to acetone), are closer in volume to 2-nonanone than is either of the cyclization products. They are also capable of occupying more efficiently the shape allocated by a stiff solvent matrix to a molecule of 2-nonanone in its extended conformation the cross-sectional diameter of either of the cyclobutanols is much larger than that of extended 2-nonanone or the fragmentation products when spaced end-on. Both of these considerations should favor fragmentation processes if isomorphous substitution for the precursor ketone in the reaction cavity is an important requirement for efficient conversion to photoproducts. 

Few examples of stereoselectivity in 6-exo cyclizations to piperidine derivatives have been reported. The aminocarbonylation reactions of systems with an allylic hydroxy substituent that are quite stereoselective in cyclization to pyrrolidine systems (equation 106) are nonselective in cyclizations to piperidine systems.237 Cyclizations with mercury(II) acetate also proceed with low selectivity (59-69% cis isomer).237 A series of aminoalditols have been synthesized by aminomercuration of oxygen-substituted 6-(/V-benzylamino)hexenes. The stereochemistry of these cyclizations (equation 112)247 does not appear... 

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