Bicycles, tandem

Kestrel produced the first carbon fiber bike frame in 1986 and the first all-carbon mountain bike frame in 1988. The Talon SI road bike has a carbon fiber frame weighing only 1.07 kg, using a modular monocoque frame fabricated from three individually molded bladder parts. 

A carbon fiber reinforced nylon resin (Towflex ) has been used to make a fabric for the construction of a mountain bike monocoque frame by compression molding in an A1 mold, satisfying weight, durability and manufacturing requirements. 

A carbon fiber bicycle wheel rim is fabricated by Sun Rims saving 400 g over an A1 rim and with 30% less carbon than that achieved by competitors using carbon fiber braid. A core of foam is rolled into a circle to fit the mold rim periphery and a braid sleeve drawn over the core with the ends overlapped. Extra pieces of carbon fiber prepreg are positioned at high stress points, the reinforcement is impregnated with epoxy resin and the lay-up is then compression molded. The pressures are sufficient for the overlap joint to be uniformly incorporated into the rim thickness. 

Carbon fiber braid has been used by Centro de Materials Compositos Europa to produce 

Trek are renowned for success in the Tour de France with their carbon composite frames and Klein Bikes, owned by Trek, have cfrp chainstays, seat stays and forks [158]. 

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Shibasakihas also extended the use of LLB catalyst to tandem nltro-aldolreacdons providing bicyclic adducts with 65% ee fEq. 3.7S. ... 

The -elimination of a thiyl radical (RS ) terminated a remarkably productive tandem radical bicyclization in Parker s formal total syntheses of ( )-codeine and ( )-morphine (see Scheme 14).29 Subjection of aryl bromide 72 to the conditions indicated generates transient aryl radical 73, an intermediate which engages the substi-... 

Scheme 14. Tandem radical bicyclization-f rag mentation in Parker s synthesis of intermediate 76 en route to codeine (77) and morphine (78).

The key features of Curran s productive and elegant tandem radical cyclization strategy are illustrated in a retrosynthetic analysis for hirsutene (1) (see Scheme 27). The final synthetic event was projected to be an intermolecular transfer of a hydrogen atom from tri-rc-butyltin hydride to the transitory tricyclic vinyl radical 131. The latter can then be traced to bicyclic tertiary radical 132 and thence to monocyclic primary radical 133 through successive hex-5-enyl-like radical cyclizations. It was anticipated that the initial radical 133 could be generated through the abstraction of the iodine atom from... 

Scheme 10.17 illustrates allylation by reaction of radical intermediates with allyl stannanes. The first entry uses a carbohydrate-derived xanthate as the radical source. The addition in this case is highly stereoselective because the shape of the bicyclic ring system provides a steric bias. In Entry 2, a primary phenylthiocar-bonate ester is used as the radical source. In Entry 3, the allyl group is introduced at a rather congested carbon. The reaction is completely stereoselective, presumably because of steric features of the tricyclic system. In Entry 4, a primary selenide serves as the radical source. Entry 5 involves a tandem alkylation-allylation with triethylboron generating the ethyl radical that initiates the reaction. This reaction was done in the presence of a Lewis acid, but lanthanide salts also give good results. 

Inter [4+2]/inter [3+2] The tandem intermolecular [4+2]/intermolecular [3+2] cycloadditions create bicyclic nitroso acetals with up to six stereogenic centers, which can be controlled by the choice of the stereochemistry of each component and the Lewis acids. The nitronate derived from 2-nitrostyrene and 1-trimethylsilyloxycyclohexene reacts with methyl acrylate to give the nitroso acetal in good yield and high diastereoselectivity (Eq. 8.107).154... 

Asymmetric tandem cycloaddition of a chiral carbohydrate nitroalkene with ethyl vinyl ether in the presence of electron-withdrawing alkenes produces a facile assembly of bicyclic systems, which can further be selectively cleaved to give homologated carbohydrates (Eq. 8.110).176... 

Treatment of 467 with more than 2 molar equiv of the corresponding aldehyde gives bicyclic products 468 in good yields. These compounds are useful intermediates in the tandem cycloreversion-cycloaddition process (Equation 58) <2000S1170, 2002S 1885>. 

The concept of tandem generally recalls to mind tandem bicycles in which at least two cyclists ride the same bicycle. If the cyclists want to avoid falling, they must synchronize their movements and the trajectory of the tandem Similarly, tandem MS[28] involves two or more partners, in the specific case the analyzers, which are synchronized. This is particularly important for obtaining structural information and for quantitative analysis. 

An intramolecular palladium-catalyzed tandem cyclization of dienamides 67 in which the amide nucleophile adds twice has been developed (equation 29)60. This reaction constitutes a formal [4+1] cycloaddition and provides a new route to pyrrolizidine and indolizidine alkaloids. Reaction of dienamides 67 in the presence of catalytic amounts of Pd(OAc)2 and CUCI2/O2 as the oxidant afforded bicyclic compounds 68 in good yields. The pyrrolizidine derivative 68 (R = Me, n = 1) was transformed to the alkaloid ( )-heliotridane. 

This catalytic tandem carbene addition-bicyclization of enynes has been very recently applied to the synthesis of fluorinated bicyclo[ .0.l]-amino esters.176... 

Feringa and co-workers described the tandem addition-aldol cyclization protocol leading to the formation of 6,6-, 6,7-, and 6,8-annulated bicyclic systems (Scheme 68).39 Using Cu(n)-29 as catalyst and functionalized organozinc reagents as nucleophiles, the conjugate addition reaction followed by aldol cyclization can offer highly enantioselec-tive annulation products (up to 98% ee). This method can be used in the synthesis of carbocyclic compounds, such as steroids, terpenes, and other natural products. 

Using a protocol for tandem carbonylation and cycloisomerization, Mandai et al.83 were able to synthesize cyclopentene and cyclohexene derivatives in high yield, including fused and 5/>/>0-bicycles (Scheme 25). The cyclohexene Alder-ene products were not isolable methanol addition across the exocyclic double bond (in MeOH/ toluene solvent) and olefin migration (in BuOH/toluene solvent) were observed. The mechanism of methanol addition under the mild reaction conditions is unknown. In contrast to many of the other Pd conditions developed for the Alder-ene reaction, Mandai found phosphine ligands essential additionally, bidentate ligands were more effective than triphenylphosphine. 

When the nucleophile bears an appropriately unsaturated chain, these enantioselective cyclizations can be used to advantage in tandem processes leading to bicyclic systems (Scheme 42) [131,141,142], The greater enantios-electivity observed with the mesylate group in this example may be due to the lower degree of stabilization of the alkene radical cation in the looser ion pair, which leads to more rapid cyclization. 

In addition, deoxoprosphylline [36], pipecolic acids, izidines [37], and the bicyclic alkaloids ( )-isoretronecanol, ( )-trachelanthamidine [38] and 6-epi-poranthellidine [39] were synthesized via tandem hydroformyla-... 

Similarly, tandem hydroformylation/aldol sequences can be applied to the formation of bicyclic and spirocyclic compounds. Thus silyl enol ethers of 3-vinyl and 3-allyl cycloalkanones give ring anellated products (Scheme 33) [86,87]. 

Interesting examples of a tandem Cope-Cope rearrangement are represented by the transformation of Cookson s diester 499, which proceeds thermally to afford its ring-degenerate isomer 500 (330-350 °C, as a melt) (equation 188)246, and by thermal isomerization of bicyclic triene 501 into hexahydro-1-vinylnaphthalene 502 upon heating in chlorobenzene at 220 °C for 20 h (equation 189)247. 

A wide application of Newcomb s method provides a variety of N-heterocyc-lic systems, such as perhydroindoles, pyrrolizidines and aza-brigded bicycles [59, W, 146], The mild reaction conditions are compatible with several funtional groups of the substrate and several trapping agents to functionalize the cyclized product. 2-Substituted pyrrolizidines 132 are accessible by tandem cyclization of iV-allyl-substituted PTOC carbamate 131. In this case the allyl group on the nitrogen serves as an internal trap for the intermediate carbon radical. The Af-methylcyclohept-4-enaminium radical cation, produced from the corres-... 

A series of bicyclo[3.3.0]octanols are accessible by electroreductive tandem cyclization of linear allyl pentenyl ketones 189, as shown by Kariv-Miller et al. [189]. The electrolyses are carried out with an Hg-pool cathode and a Pt-flag anode. As electrolyte, tetrabutylammonium tetrafluororborate is used. The reaction is stereoselective, yielding only two isomers 192 and 193. In a competing reaction, a small amount of the monocyclic alcohol is formed. Since all the monocycles have the 1-allyl and the 2-methyl group in trans geometry it is assumed that this terminates the reaction. The formation of a bicyclic product requires that the first cyclization provides the cis radical anion which leads to cis-ring juncture [190] (Scheme 37). 

The asymmetric tandem cycloaddition of the chiral carbohydrate nitroalkene (35) with ethyl vinyl ether involves the initial formation of the nitronate (36) which reacts exclusively with electron-withdrawing alkenes by 3 -I- 2-cycloaddition to yield chiral bicycles (37) and (38) (Scheme 12). ... 

The Ir(lll) complex also funchoned as a catalyst in a tandem Nazarov cyclization-Michael addition. The reaction of monocyclic a-alkylidene-P-keto-y.b-unsaturated ester with nitroalkene gave bicyclic cyclopentenones which possessed an alkyl side chain, with high yield and diastereoselectivity (Scheme 11.36) [47]. 

A masked allylic boron unit can be revealed through a transition-metal-catalyzed borylation reaction. For example, a one-pot borylation/allylation tandem process based on the borylation of various ketone-containing allylic acetates has been developed. The intramolecular allylboration step is very slow in DMSO, which is the usual solvent for these borylations of allylic acetates (see Eq. 33). The use of a non-coordinating solvent like toluene is more suitable for the overall process provided that an arsine or phosphine ligand is added to stabilize the active Pd(0) species during the borylation reaction. With cyclic ketones such as 136, the intramolecular allylation provides cis-fused bicyclic products in agreement with the involvement of the usual chairlike transition structure, 137 (Eq. 102). 

Harwood and Lilley (87) reported the tandem generation and intramolecular trapping of a stabilized azomethine ylide, derived from the enantiopure template examined in detail in Section 3.2.3. Condensation of 5-hexenal with template 205 under standard conditions led to in situ ylide generation and subsequent cycloaddition of the tethered alkene to furnish 296 as a single enantiomer in 95% yield after purification and this despite the fact that the dipolarophile is unactivated. Hydro-genolytic destruction of the template revealed the bicyclic amino acid 297 in 75% yield (Scheme 3.97). 

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