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Dienophiles vinyl sulfoxides

From the above results it can be inferred that sulfinyl ethylenes are not good dienophiles because of their low reactivity and poor stereoselectivity. In order to improve both of these factors, different electron-withdrawing groups (able to increase dienophilic reactivity and to restrict conformational movement around the C-S bond, thus improving the stereoselectivity) were incorporated into the double bond. They are presented in the sections below, classified according to the number and kind of the additional activating groups present in the vinyl sulfoxide moiety. [Pg.12]

One of the most interesting points that can be deduced from the studies carried out on the bis-sulfoxides is the much lower reactivity of acyclic dienophiles than that of cyclic examples (compare the results obtained from 80 and 75 or those from 82 and 83). The influence of the conformation around the C-S bond (with higher restrictions in cyclic sulfoxides), on the dienophilic character of the double bond in vinyl sulfoxides, emerge as the most likely causes of the observed differences in reactivity (see later). [Pg.46]

Discrepancies between different researchers derive from the character inter-or intramolecular of the interactions presumably controlling the reactive conformation. Thus, in most of the cases, the population of the different rotamers in the sulfinylated substrate (only governed by intramolecular interactions) is the only factor considered for explaining the observed 7r-facial selectivity. This explanation (static conformational polarization) was formulated by Koizumi and used by many authors to justify the behavior of vinyl sulfoxides acting as dienophiles and dipolarophiles. A second explanation assumes that the interactions of the two reagents in the transition states determine a different reactivity of the rotamers around the C-S bond. This intermolecular factor can become the most important one in the control of the 7r-facial selectivity of the cycloadditions, and therefore the tendency expected from conformational stability criteria was not observed in those cases where the most reactive conformation is not the most populated one. This dynamic conformational polarization has been used just to explain some of the results obtained for sulfinyl quinones and sulfinyl dienes (unexplainable with the above model) but it can be applied to many other cases. [Pg.116]

The last problem associated with the use of the sulfinyl group as a chiral inductor in cycloaddition reactions is related to the endo/exo reactivity. From the available data, the endo directing power of the sulfinyl group is not large. The need to incorporate other functional groups into vinyl sulfoxides, thus increasing their dienophilic reactivity, decrees that the formation of mixtures is difficult to avoid in many cases as a consequence of the competition between the... [Pg.119]

During the past two decades, the asymmetric Diels-Alder reaction has become one of the most powerful tools in asymmetric synthesis as a result of its capacity to create up to four chiral centers in one step, often in a highly stereoselective manner. In the following sections, recent advances in this area using vinyl sulfoxide and vinyl sulfone dienophiles will be considered. It should be noted that, although beyond the scope of this review, many asymmetric Diels-Alder reactions of chiral sulfinyl-1,3-dienes have been reported.111... [Pg.185]

In the past decade Carretero et al. have made significant advances in this area using a variety of activated chiral vinyl sulfoxides containing either one, two, or three additional activating groups.97 The Diels-Alder cycloadditions of several dienophiles based on the 2-sulfmylcyclopentenone skeleton have been investigated... [Pg.186]

The Diels-Alder reactions of a variety of acyclic chiral vinyl sulfoxide dienophiles, bearing additional electron-withdrawing groups at the double bond, have been investigated (Fig. 6). The asymmetric Diels-Alder reaction of enantiopure (5)-benzyl 2-p-tolylsulfinylacrylate (167) with furan, under high pressure, afforded mainly a 2 1 mixture of endo adducts. These were then stereoselectively transformed into (+)-shikimic acid and (+)-5-epi -shikimic acid.101 However, the poor control of the endo/exo selectivity and the low reactivity were the two main problems restricting the general synthetic usefulness of such a-sulfinylacrylate dienophiles. [Pg.186]

Thus, the results of this comparative study clearly indicated that (1) the 7t-facial selectivity of the Diels-Alder reactions of vinyl sulfoxides must be explained by assuming a steric approach control of the diene on the less hindered face of the dienophile (that supporting the lone pair at sulfur), taking into account the relative reactivity of the retainers around the C-S bond, in addition to their populations (2) both the endo and the 7i-facial selectivities of the cycloadditions of vinylsulfox-ides with acyclic dienes are substantially higher than with cyclic ones. [Pg.188]

Carmen Carreno and co-workers have investigated Diels-Alder reactions of a wide variety of benzoquinone-based chiral vinyl sulfoxide dienophiles (Fig. 9). [Pg.189]

Ketene equivalents have found widespread use as partners in Diels-Alder reactions for the construction of cyclic, fused, and bridged unsaturated ketones. However, ketene equivalents based on simple vinyl sulfoxides are poor dienophiles and show low levels of diastereocontrol. Recently, Aggarwal and colleagues reported that (l/ ,3/ )-2-methylene-l,3-dithiolane 1,3-dioxide (187) was a highly reactive and highly selective chiral ketene equivalent.116 Diels-Alder cycloaddition of this... [Pg.192]

Vinyl sulfoxides mostly require a second electron-withdrawing groiq) to increase their dienophilic character. Thus vinyl sulfoxides bearing a carbonyl moiety at Cp undergo carbonyl-directed diene addition followed by a spontaneous elimination of sulfenic acid, as applied to a synthesis of disodium pre-phenate (74) (Scheme 21). ... [Pg.324]

The chirality of a sulfoxide moiety may be also exploited for asymmetric [4 + 2] additions of 1,3-dienes to vinyl sulfoxides. However, the latter are poor dienophiles and thus require an additional activating group. [Pg.369]

Vinyl sulfoxides are useful Diels-Alder dienophiles. Thermolysis of the adducts gives rise to alkenes via sulfoxide elimination and so over the two steps the vinyl sulfoxides are the synthetic equivalents of alkynes, e.g. phenyl vinyl sulfoxide reacts with anthracene and 1,4-diphenylbuta-1,3-diene with concomitant sulfoxide elimination under the Diels-Alder conditions to give adducts (70) and (71 Scheme 15). 3-Phenylsulfinyl-a,3-unsaturated-carbonyl compounds have also been used as Diels-Alder dienophiles in chemistry developed for a synthesis of disodium prephenate and epiprephenate. With the electron rich diene (72), vinyl sulfoxides (73) and (75) gave rise to products (74 equation 33) and (76 equation 34) after hydrolysis. For the prephenate synthesis the phenylsulfinylmethylenelactone (77) was used, giving cyclohexadienone (78 Scheme 16) after sulfoxide elimination and hydrolysis. ... [Pg.1023]

Vinyl sulfoxides have been extensively employed as dienophiles in Diels-Alder [2 + 4] cycloadditions. For instance, the vinyl sulfoxide (51) adds to cyclopentadiene (52) to give the adducts (53) and (54) (Scheme 25). In the cycloaddition, the endo, syn-form (54) (80% yield) is favoured in comparison with the endo, a /-isomer (53). [Pg.76]

The auxiliary-controlled, noncatalyzed [4 + 2] cycloaddition of homodienophiles has been thoroughly studied and the results are often compared with Lewis acid catalyzed reactions (see Section 1.6.1,1.1.2.2.2.). A very interesting version of dienophiles bearing a chiral auxiliary are chiral vinylic sulfoxides in which the chirality results from the dissymmetric sulfur atom. The first example studied is the Diels-Alder reaction of ( + )-(/ )-4-methylphenyl vinyl sulfoxide (1) with cyclopentadiene (2), in which four possible diastereomers 3-6 are obtained in an endojexo ratio of (5 + 6)/(3 + 4) 64 36. The diastereomers are further separated the (2 /J)-isomers (3 and 5) being favored. Desulfuration produces pure ( + )- or (-)-dehydronorcamphor. [Pg.550]

The poor reactivity of simple enantiomerically pure vinyl sulfoxides has forced the development of dienophiles in which an additional electron-withdrawing group is present either connected to the C —C double bond (e.g., 9a) or to the double bond and additionally to the sulfur atom (e.g., 9e). A typical procedure for the preparation of these sulfoxides is shown below. [Pg.551]

The use of phenyl vinyl sulfone or phenyl vinyl sulfoxide as the dienophile allows for aromatization by elimination of the sulfur substituent (Equation (152)) <89LA1275>. [Pg.202]

Nitroethene and vinyl sulfoxides have also been employed as ketene equivalents. Nitroethene is an excellent dienophile and oxidation of the initial nitro-adduct gives the corresponding ketone. However, the thermal instability of nitroethene limits its appUcation to cycloadditions with reactive dienes. An attractive feature of vinyl sulfoxides as ketene equivalents is that they can be obtained in optically active form because of the chirality of the sulfoxide group, thus allowing enan-tioselective Diels-Alder reactions. Cycloaddition of p-tolyl vinyl sulfoxide with cyclopentadiene requires heat and gives a mixture of all four (two exo and two... [Pg.167]

An interesting application of the chiral cyclopalladated complexes is palladium-promoted asymmetric Diels-Alder reactions of l-phenyl-3,4-dimethylphosphole (Scheme 4). In the original report on the Diels-Alder reaction of the phosphole reported by Nelson and co-workers, the dichloropalladium species was employed as a promoter.f In 1994, Leung showed that a chiral cyclopalladated complex was capable of promoting the Diels-Alder reaction. In this reaction, the chiral palladacycle worked as a chiral auxihary and showed almost perfect diastereoselectivity. It has been revealed that simultaneous precoordination of a diene and a dienophile to the palladium center is essential for the reaction. Thus, both dienes and dienophiles should possess Lewis basic functionahty. The diene l-phenyl-3,4-dimethylphosphole has been the only substrate examined so far, while a variety of dienophiles, such as vinylphosphine, vinyl-sulfoxide, vinylsulfide, acrylamide, vinylarsine, vinylpyridine, vinyl-pyrrole, or methylenequinuchdinone, have successfully been applied to this asymmetric Diels-Alder reaction. [Pg.108]

The synthesis of nonracemic a,P-unsaturated sulfoxides bearing an electron-withdrawing group at the a-position has received attention [28-30] because these types of substrates have proved useful as chiral dienophiles (see Section 5.5) [28,29]. These methods suffered from the disadvantage that several steps were involved. Maignan and coworkers [31] have recently described a one-pot synthesis of nonracemic a-acyl- and a-hydroxyalkyl-a,P-unsaturated sulfoxides from 2-ethoxyethyl p-tolyl sulfoxide (16), readily prepared from (/ )-p-tolyl vinyl sulfoxide by addition of sodium ethoxide in ethanol. Thus, treatment of (16) with LDA in THF, followed by the addition of ethyl acetate, gave enantiomerically pure a-acyl-a,P-unsaturated sulfoxide (18) in 69% yield via the intermediate (17) (Scheme 5.6). [Pg.162]

Early reports on the use of racemic vinyl sulfoxides as dienophiles were chiefly concerned with their use as acetylene equivalents, since the primary cycloadducts may extrude sulfenic acid [137-139]. For example, heating phenyl vinyl sulfoxide and anthracene in chlorobenzene afforded dibenzobarrelene (159) in 83% yield (Scheme 5.53) [137]. [Pg.192]

The first report on the use of enantiomerically pure sulfinyl dienophiles in Diels-Alder cycloadditions was by Maignan and Raphael [156], who utilized (+)-(R)-p-tolyl vinyl sulfoxide (26a) as an optically active dienophile (Scheme 5.58). A mixture of endo (177a) and (177b) and exo (178a) and (178b) diastereoisomers were formed as a result of poor selectivity. [Pg.196]

Subsequent work has shown that simple, unactivated enantiomerically pure vinyl sulfoxides such as p-tolyl vinyl sulfoxide, without further substitution on the double bond, are not effective dienophiles for inducing diastereoselectivity in asymmetric Diels-Alder reactions. In the majority of cases, further substitution of the double bond by electron-withdrawing groups is necessary to achieve high stereoselectivity under mild conditions. However, a recent report by Ronan and Kagan [157,158] has shown that (5)-p-tolyl vinyl sulfoxide (26b) can be efficiently activated toward Diels-Alder cycloaddition by transformation into a sulfoxonium... [Pg.196]

Vinyl sulfoxides substituted at the p-position by electron-withdrawing groups have been used extensively as enantiomerically pme dienophiles in asymmetric Diels-Alder cycloadditions. Maignan and colleagues utilized methyl (Z)-(/ )-3-p-tolylsulfinyl propenoate (184), which reacted with cyclopentadiene under mild conditions high stereoselectivity and diastereoselectivity were observed [159] in formation of the endo and exo cycloadducts (185a) and (185b) (Scheme 5.60). [Pg.197]

The first report of the use of optically active a,P-unsaturated sulfoxides as dipolarophiles was by Koizumi et al. in 1982 [196] and preceded the first report of their use as optically active dienophiles, despite the fact that the vast majority of subsequent work in this area has been in the use of dienophiles in asymmetric Diels-Alder reactions. The 1,3-dipolar cycloaddition between (i )-(+)-p-tolyl vinyl sulfoxide and acyclic nitrones such as (261) gave the major cycloadducts such as (262) (90% de) in 57% overall yield (Scheme 5.85). [Pg.212]

Dienophiles such as nitroethene 56, a-chloroacrylonitrile 57, and vinyl sulfoxide 58 can be used as ketene equivalent in the synthesis of organic compounds to improve the yield of the products. The cycloadducts of cyclopentadiene derivatives obtained from these dienophiles are used in the synthesis of prostaglandins. [Pg.61]


See other pages where Dienophiles vinyl sulfoxides is mentioned: [Pg.6]    [Pg.8]    [Pg.11]    [Pg.12]    [Pg.14]    [Pg.38]    [Pg.47]    [Pg.117]    [Pg.118]    [Pg.120]    [Pg.120]    [Pg.185]    [Pg.190]    [Pg.205]    [Pg.1023]    [Pg.859]    [Pg.238]    [Pg.166]    [Pg.315]    [Pg.1011]    [Pg.1023]   
See also in sourсe #XX -- [ Pg.863 ]




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Dienophil

Dienophile

Dienophiles

Sulfoxides vinyl, as dienophiles

Sulfoxides, vinyl

Vinyl sulfoxide

Vinyl sulfoxides sulfoxide

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