Bicyclization mechanism

It should be noted that the observed bicycle motion is one of two a priori possibilties as shown in Scheme 10, which includes the alternative, pivot process. Interestingly, an even number of pivot steps would give rise to the same stereochemistry as the bicycle process, although an odd number of pivot steps affords the reverse stereochemistry relative to the bicycle mechanism. The differentiation between these possibilities resulted from the observation that the endo-aryl reactants of the type in Scheme 11 show a marked enhancement of the 1,4- (2,3) fragmentation at the end of an odd number of bicycle steps. The exo stereoisomers proceed nicely onward to the spiro bicycle photoproduct. This is understood on the basis of van der Waals repulsions of the endo-aryl groups, which leads to enhanced fragmentation due to relief of these repulsions. This, then identifies the aryl groups as endo after an odd number of steps. 

A bicycle mechanic has 10 frames and 16 wheels in the shop. How many complete bicycles can he assemble using these parts ... 

What is a 3.2.1 bicyclic mechanism How is this transformation accomplished - what reagents, what is the mechanism This is not obvious and demands discussion and/or specific examples from the literature. 

Japanese collective protectors were similar to those used by European nations and the United States. An ingenious model capable of purifying air for forty persons derived the power for pumping the bellows from a geared bicycle mechanism. ... 

Atypical Antidepressants. StmcturaHy diverse dmgs such as the tetracyclic mianserin (46) and various bicyclic and tricyclic compounds such as trazodone (47), venlafaxine (48), nefazodone (49), and amfebutamone (50) are atypical antidepressants. The exact mechanism of action is unclear but probably... 

The light-induced rearrangement of 2-phenyl- to 3-phenyl-thiophene may occur by a similar mechanism an equilibrium between the bicyclic intermediate (26) and the cyclopro-penylthioaldehyde (27) has been suggested (Scheme 2). The formation of IV-substituted pyrroles on irradiation of either furans or thiophenes in the presence of a primary amine supports this suggestion (Scheme 3). Irradiation of 2-phenylselenophene yields, in addition to 3-phenylselenophene, the enyne PhC=C—CH=CH2 and selenium. Photolysis of 2-phenyltellurophene furnishes solely the enyne and tellurium (76JOM(108)183). 

These various photoproducts are all valence isomers of the normal benzenoid structure. These alternative bonding patterns are reached from the excited state, but it is difficult to specify a precise mechanism. The presence of the t-butyl groups introduces a steric factor that works in favor of the photochemical valence isomerism. Whereas the t-butyl groups are coplanar with the ring in the aromatic system, the geometry of the bicyclic products results in reduced steric interactions between adjacent t-butyl groups. 

Molecular mechanics calculations with the molecular mechanics force field program were performed to compare thermodynamic stability among araguspongine B (17) (containing two cw-fused perhydropyrido[2,l-Z>] [l,3]oxazine bicycles), araguspongine D (18) (containing two tran -fused perhydropyrido[2,l-Z)][l,3]oxazine bicycles), and araguspongine E (19)... 

The light weight and mechanical efficiency not only allow the bicycle to be powered by a nonathlet-ic human, but it can be walked over extreme terrain, or laden ivith heavy cargo, or picked up and carried. These options make the bicycle more versatile than any other vehicle, and allow a bicycle user door-to-door, on-dcinand transport. 

During the late 1890s, bicycles were the worldwide focus of invention and technical innovation, much as biotech engineering and computers are today. We owe iiiaiiy ot today s industrial maiiutacturing processes, designs for bearings, axles, and gearing mechanisms,... 

During these past few decades, the bicycle itself has vastly improved. The bicycles of the early 1970s usually had mechanical idiosyncrasies. Competition among manufacturers, led by the Japanese companies that entered the U.S. market, resulted in vastly improved quality control. The mountain bike, first made available on a widespread basis m 1983, offered a delightful alternative that mushroomed in popularity, and a decade later, traditional road bicycles had all but disappeared from stores. The mountain bike has become most people s vehicle of choice for city riding as well as recreational trail riding. 

The bicycle s advantages as the world s most mechanically efficient means of transportation are clouded by the limitations of the human engine. To put it in power output terms, the human body can produce sustained power only at modest levels. For most people, 100 watts would be too much, and for an elite athlete, 400 watts is the approximate ceiling. (The athlete may manage a brief burst of 1.1 kilowatts.)... 

The human engine cannot match this power output, yet the mechanical efficiency of the bicycle helps tremendously because a vei y small amount of horsepower can generate great speed. For example, 0.4 horsepower (298 watts) of output can result in 25 niph (40 kph) speeds or better. One set of calculations shows that if a cyclist rode on level ground, with no rolling resistance, and aided by a 25 mph tailwind, it would require only around 0.2 horsepower (150 watts) to sustain a 25 mph pace. 

Geometric considerations in cyclopolymerization are optimal for 1,6-dienes (see 4.4.1.1). Instances of cyclopolymerization involving formation of larger rings have also been reported (see 4.4.1.4), as have examples where sequential intramolecular additions lead to bicyclic structures within the chain (see 4.4.1.2). Various 1,4- and 1,5-dienes are proposed to undergo cyclopolymerization by a mechanism involving two sequential intramolecular additions (see 4.4.1.3). 

Alloys are solid metallic mixtures designed to meet specific needs (see Section 5.15). For example, the frames of racing bicycles can be made of a steel that contains manganese, molybdenum, and carbon to give them the stiffness needed to resist mechanical shock. Titanium frames are used, but not the pure metal. Titanium metal stretches easily, so much so that it becomes deformed under stress. However, when alloyed with metals such as tin and aluminum, titanium maintains its flexibility but keeps its shape. 

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

When dienones 39 and 40 are photolyzed in sulfuric acid they both rearrange to the same product, 2-methyl-5-hydroxybenzaldehyde (41) (Filipescu and Pavlik, 1970). The mechanism for this photorearrangement is consistent with that of the protonated cyclohexadienones already discussed, i.e., disrotatory closure to afford the intermediate bicyclic cations 42 and 43. In this case it is conceivable that the electron-withdrawing effect of the dichloromethyl group forces the subsequent thermal cyclopropyl migration entirely in the direction of the most stable cation 44 to yield the observed product. 

It was shown in the same article that the decarboxylation could also be performed by conventional heating but then copper cyanide was required and a mixture of saturated and imsaturated 2-pyridones 65 and 66 was obtained in a ratio of 1 10 (Fig. 10). A tentative mechanism was suggested for the reagent-free MAOS method where the carbonyl in the 2-pyridone ring is supposed to assist in the decarboxylation yielding an yUde 67 (Fig. 11). The decarboxylated bicyclic 2-pyridone 68 is thereafter obtained after protonation by the solvent. In agreement with the mechanistic suggestion, it was shown that a selective deuteration occurred when deuterated dimethyl sulfoxide (DMSO-de) was used as solvent. 

In the course of studying the bromination reactions of the bicyclic systems we noticed that the reaction temperature has a dramatic influence on the product distribution. Increasing of the temperature gives non-rearranged reaction products (refs. 1,2). For this reason, we submitted 1 to high temperature bromination. To a solution of 1 in decalin at 150 C was added a hot solution of bromine in decalin in one portion. The colour of bromine disappeared immediately. After silica gel chromatography followed by fractional crystallization we isolated four products 2-6 in yields 8, 35, 37, and 9 % respectively. The structure of these compounds has been elucidated on the basis of spectral data by iH NMR and NMR experiments and by comparison with those reported in the literature. Symmetrical endo-c/5-isomer 6 has been observed for the first time. Studies concerning the mechanism of syn-addition show that the syn-adduct can arise either from direct... 

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