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Addition reactions continued intramolecular

There were two more stereocenters to set. It was expected that cuprates would add to the open face of the strained cyclobutene. The control of the other stereocenter was more problematic. One solution was to prepare an a-sulfonyl lactone. To this end, the ketone was converted to the secondary carbonate. As hoped, conjugate addition was followed by intramolecular acylation, but the reaction continued to full acyl transfer, to give 10. Fortunately, desilylation of 10 proceeded with concomitant lactonization. Desulfonylation then gave 2, which could be brought to high by recrystallization. [Pg.56]

Both intermolecular and intramolecular additions of carbon radicals to alkenes and alkynes continue to be a widely investigated method for carbon-carbon bond formation and has been the subject of a number of review articles. In particular, the inter- and intra-molecular additions of vinyl, heteroatomic and metal-centred radicals to alkynes have been reported and also the factors which influence the addition reactions of carbon radicals to unsaturated carbon-carbon bonds. The stereochemical outcome of such additions continues to attract interest. The generation and use of alkoxy radicals in both asymmetric cyclizations and skeletal rearrangements has been reviewed and the use of fi ee radical reactions in the stereoselective synthesis of a-amino acid derivatives has appeared in two reports." The stereochemical features and synthetic potential of the [1,2]-Wittig rearrangement has also been reviewed. In addition, a review of some recent applications of free radical chain reactions in organic and polymer synthesis has appeared. The effect of solvent upon the reactions of neutral fi ee radicals has also recently been reviewed. ... [Pg.100]

Isoxazoles and isoxazolidines fused with sugar systems are found to show biological activity. In a continuation of our studies on the utilization of a variety of protocols on the sugar-derived chirons we embarked onto the 1,3-dipolar cycloaddition reactions. Accordingly, intramolecular oxime olefin cyclo-addition (lOOC) reactions on the chiron (91 X=NOH) gave an isoxazolidine-fused saccharide (92) (Scheme 22.21). [Pg.174]

This chapter and Chapter 10 continue our cataloging of the standard reactions of organic chemistry. To the SnI, Sn2, El, and E2 reactions we now add a variety of alkene addition reactions. Although there are several different mechanisms for additions, many take place through a three-step sequence of protonation, addition, and deprotonation. The following new problems allow you to practice the basics of addition reactions and to extend yourself to some more complex matters. Even simple additions become complicated when they occur in intramolecular fashion, for example. These problems also allow you to explore the influence of resonance and inductive effects, and to use the regiochemistry and stereochemistry of addition to help work out the probable mechanisms of reactions. [Pg.404]

The addition reaction of organolithium compounds easily proceeds especially to intramolecular carbon-carbon double bonds (3.17) or to conjugated double bonds. The latter addition reactions proceed continuously. These reactions are utilized in butadiene and SBR polymerization and the organolithium compound is mostly used... [Pg.39]

Trost and his co-workers continue to develop new synthetic methods, such as the cyclization of (144) to (145), " which are based on the chemistry of organo-palladium compounds. The 1,3-dipolar addition of a nitrone to an olefinic double bond yields an oxazoline, but if the reaction is intramolecular it can provide an excellent method for the synthesis of carbocycles. For example, the conversion of (146) into (147) is a key step in a new Synthesis of a-bisabolol (148). ... [Pg.264]

It is interesting to note that the oxa-analogous Michael addition was reported for the first time in 1878 by Loydl et al. [19] in their work on the synthesis of artificial malic acid, which was five years ahead of the discovery of the actual Michael reaction described first by Komnenos [20], Claisen [21], and later Michael in 1887 [22] as one of the most important methods for C—C bond formation. In continuation of the early work on the oxa-Michael addition [23], the inter- and intramolecular additions of alkoxides to enantiopure Michael acceptors has been investigated, leading to the diastereo- and enantioselective synthesis of the corresponding Michael adducts [24]. The intramolecular reaction has often been used as a key step in natural product synthesis, for example as by Nicolaou et al. in the synthesis of Brevetoxin B in 1989 [25]. The addition of oxygen nucleophiles to nitro-alkenes was described by Barrett et al. [26], Kamimura et al. [27], and Brade and Vasella [28]. [Pg.10]

Less basic malonic ester anions may be employed for the twofold alkylation of dibromides. Cyclic 1,1-dicarboxylic esters are formed, if the reaction is executed in an appropriate manner. In the synthesis of cyclobutane diester A the undesired open-chain tetraester B was always a side product (J.A. Cason, 1949), the malonic ester and its monoalkylation product were always only partially ionized. Alkylation was therefore slow and intermolecular reactions of mono-alkyl intermediates with excess malonic ester prevailed. If the malonic ester was dissolved in ethanol containing sodium ethoxide, and 1,3-dibromopropane as well as more sodium ethoxide were added slowly to the solution, 63% of A and only 7% of B were isolated. The latter operations kept the malonic ester and its monoalkylated product in the ionic form, and the dibromide concentration low, so that the intramolecular reaction was favored against intermolecular reactions. The continuous addition of base during the reaction kept the ethoxide concentration low, which helped to prevent decomposition of the bromide by this nucleophile. [Pg.23]

Yates and coworkers have examined the mechanism for photohydration of o-OH-8. The addition of strong acid causes an increase in the rate of quenching of the photochemically excited state of o-OH-8, and in the rate of hydration of o-OH-8 to form l-(o-hydroxyphenyl)ethanol. This provides evidence that quenching by acid is due to protonation of the singlet excited state o-OH-8 to form the quinone methide 9, which then undergoes rapid addition of water.22 Fig. 1 shows that the quantum yields for the photochemical hydration of p-hydroxystyrene (closed circles) and o-hydroxystyrene (open circles) are similar for reactions in acidic solution, but the quantum yield for hydration of o-hydroxystyrene levels off to a pH-independent value at around pH 3, where the yield for hydration of p-hydroxystyrene continues to decrease.25 The quantum yield for the photochemical reaction of o-hydroxystyrene remains pH-independent until pH pAa of 10 for the phenol oxygen, and the photochemical efficiency of the reaction then decreases, as the concentration of the phenol decreases at pH > pAa = 10.25 These data provide strong evidence that the o-hydroxyl substituent of substrate participates directly in the protonation of the alkene double bond of o-OH-8 (kiso, Scheme 7), in a process that has been named excited state intramolecular proton transfer (ESIPT).26... [Pg.45]

Employing a staked plate microreactor (channel dimensions = 100 pm, total volume = 2 ml), Acke and Stevens (2007) reported the continuous flow synthesis of a series of chromenones via a multicomponent route consisting of a sequential Strecker reaction-intramolecular nucleophilic addition and tautomerization, as depicted in Scheme 2. [Pg.106]


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See also in sourсe #XX -- [ Pg.959 , Pg.960 , Pg.961 , Pg.962 , Pg.963 , Pg.965 ]




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Addition reactions (continued

Addition—Continual

Continuous reactions

Intramolecular addition

Intramolecular reactions addition

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