Endo-reaction product

To justify the experimental result we could follow the stepwise mechanism discussed in the previous section. The phthalimide system is a good electron acceptor in the excited state and the alkylsilane can be an electron donor, so a photoinduced electron transfer process could lead to the charged separated diradical 13. Transfer of the electrofugal TMS group to the oxyanion carbonyl oxygen would form azo-methyne ylide 14 that can be trapped by the mefliyl acrylate to yield the endo-reaction product 12 (Scheme 16.9). 

Endo-exo product mixtures were isolated using the following procedure. A solution of cyclopentadiene (concentration 2-10" M in water and 0.4 M in oiganic solvents) and the dienophile (concentration 1-5 mM) in the appropriate solvent, eventually containing a 0.01 M concentration of catalyst, was stirred at 25 C until the UV-absorption of the dienophile had disappeared. The reaction mixture (diluted with water in the case of the organic solvents) was extracted with ether. The ether layer was washed with water and dried over sodium sulfate. After the evaporation of the ether the... 

The Diels-Alder reaction provides us with a tool to probe its local reaction environment in the form of its endo-exo product ratio. Actually, even a solvent polarity parameter has been based on endo-exo ratios of Diels-Alder reactions of methyl acrylate with cyclopentadiene (see also section 1.2.3). Analogously we have determined the endo-exo ratio of the reaction between 5.1c and 5.2 in surfactant solution and in a mimber of different organic and acpieous media. These ratios are obtained from the H-NMR of the product mixtures, as has been described in Chapter 2. The results are summarised in Table 5.3, and clearly point towards a water-like environment for the Diels-Alder reaction in the presence of micelles, which is in line with literature observations. 

Table 5.3. Endo-exo product ratios of the Diels-Alder reaction of 5.1c with 5.2 in surfactant solution compared to water and organic solvents.

More detailed stereochemical studies on the Diels-Alder reaction between cyclopen-tadiene and 2-phenylsulphinylacrylic acid 560 revealed that the formation of endo-syn products 561 is strongly favoured (75-80%) over that of the endo-anti forms661 (equation 359). 

In the presence of Eu(fod)3 the endo-oycloadduct is the predominant reaction product in the presence of SnCl4 the abnormal product is predominant... 

The micellar effect on the endo/exo diastereoselectivity of the reaction has also been investigated. The endo/exo ratio of the reaction of cyclopentadiene with methyl acrylate is affected little (compared to water) by the use of SDS and CTAB [73b], while a large enhancement was observed in SDS solution when n-butyl acrylate was the dienophile used [74]. The ratio of endo/exo products in the reaction of 1 with 113c is not affected by CTAB, SDS and C12E7 [72a]. 

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... 

Diels-Alder reaction is one of the most fundamental reactions for organic synthesis. Its synthetic utility is unquestioned. The stereochemistry of the reactions has attracted much attention. The retention of stereochemistry in the diene and the dienophile, the predominant formation of endo-attack products in the reactions of cyclic dienes, and highly controlled regioselectivity in the reactions of substimted dienes and... 

The stereoselectivity of some Diels-Alder reactions was also strongly affected in water.26 At low concentrations, in which both components were completely dissolved, the reaction of cyclopentadiene with butenone gave a 21.4 1 ratio of endo/exo products when they were stirred at 0.15 M concentration in water, compared to only a 3.85 1 ratio in excess cyclopentadiene and an 8.5 1 ratio with ethanol as the solvent. Aqueous detergent solution had no effect on the product ratio. The stereochemical changes were explained by the need to minimize the transition-state surface area in water solution, thus favoring the more compact endo stereochemistry. The results are also consistent with the effect of polar media on the ratio.27... 

The reaction of dienes bearing an A-dienyl lactam moiety with activated olefins was examined by Smith.94 The lactams were excellent enophiles and provided exclusively the ortho regioisomer with good selectivity for the endo (Z) product (Eq. 12.33). 

A mixture of exo- and endo-isomers of 5-methylbicylo[2.2.1]hept-2-ene is hydrogenated with the aid of five equivalents of triethylsilane and 13.1 equivalents of trifluoroacetic acid to produce a 45% yield of < <7o-2-methylbicylo[2.2.1] heptane (Eq. 71). The same product is formed in 37% yield after only five minutes. The remainder of the reaction products is a mixture of three isomeric secondary exo-methylbicylo[2.2.1]heptyl trifluoroacetates that remains inert to the reaction conditions. Use of triethylsilane-l-d gives the endo-2-methylbicylo-[2.2.1]heptane product with an exo-deuterium at the tertiary carbon position shared with the methyl group. This result reflects the nature of the internal carbocation rearrangements that precede capture by the silane.230... 

In order to complete the list of carbon oxides C =0 (n = 1-9) it should be mentioned that, as it was already discussed for C5O, also the higher members C7O, C80, and C9O have not been yet isolated in a matrix, but they could be generated by Endo et al.134,135 in the gas phase by an electric discharge in a mixture of C3O2 and argon. The reaction products were analyzed by rotational spectra, from which the linear cumulenic structure and the bond lengths were derived. 

If the tether consists of three atoms (-RCH—X-( ) - (n= 1)), the stereoselectivity of the process is very high, and trans isomer 188a prevails among the reaction products, regardless of the composition of the tether, due to the endo approach of the tether to the Z isomer of the nitronate. An increase in the length of the tether to four atoms (n = 2) leads to a sharp decrease in selectivity of cycloaddition, and cis isomer (188b) prevails among the reaction products. Evidently, the latter isomer is formed as a result of the exo approach of the tether to the nitronate. 

The complex [Cp2Zr(OTf)2(thf)] is a catalyst for the Diels—Alder reactions of 105 compared to the corresponding thermal reactions [82,83] (Scheme 8.45). The isomer ratio of the reaction products (endo/exo or regioisomers) is higher in catalyzed than in thermal reactions. However, because the zir-conocenium triflate is also a catalyst for the polymerization of 1,3-dienes, the Diels—Alder reaction is sometimes completely suppressed in the case of less reactive dienophile-diene combinations. 

In contrast with the reaction of 4-yn-l-ols, both 6-endo-dig and 5-exo-dig cyclization modes were observed in the Pdl2/KI-catalyzed oxidative car-bonylation of2-(l-alkynylbenzyl)alcohols (Eq. 43) [120]. The preferential formation of the lH-isochromene or the 1,3-dihydroisobenzofuran derivative turned out to be dependent on the substitution pattern of the substrate. In particular, lH-isochromenes were obtained as the main reaction products when the triple bond was substituted with an alkyl group and with a primary alcoholic group, while the isobenzofurans were preferentially formed with a tertiary alcoholic group and when the triple bond was terminal or conjugated with a phenyl group. 

TABLE 6. Influence of ultrasonic ))) irradiation on the yield and [endo]/[exo] product ratio of the reaction of cyclopentadiene with methyl vinyl ketone in various organic solvents84... 

In a similar approach, Garner et al. (78) made use of silicon-based tethers between ylide and dipolarophile during their program of research into the application of azomethine ylides in the total asymmetric synthesis of complex natural products. In order to form advanced synthetic intermediates of type 248 during the asymmetric synthesis of bioxalomycins (249), an intramolecular azomethine ylide reaction from aziridine ylide precursors was deemed the best strategy (Scheme 3.84). Under photochemically induced ylide formation and subsequent cycloaddition, the desired endo-re products 250 were formed exclusively. However, due to unacceptably low synthetic yields, this approach was abandoned in favor of a longer tether (Scheme 3.85). 

Similar 1,2-migration/cyclization is observed in the reaction of 5-hexynyl tosylate 92a with zincates (equation 44)61. In addition to the exo-cyclization leading to (alkyli-dene)cyclopentane 93a, endo-cyclization occurs competitively to give cyclohexene 94a, which is a major product in the reaction with PthZnl.i. In the reaction of the phenyl-substituted derivative 92b, two endo-cyclization products 94b and 94 (1 1) were obtained as well as exo-cyclization product 93b. A unified mechanism in which the -electrons of the alkynylzincate 95 participate in exo- and endo-cyclization (Scheme 8) has been proposed to rationalize these observations. The intermediacy of cyclohexyne 96 was verified... 

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