Diene acyclic chiral

Chiral auxiliaries may also be used to effect diastereoselective complexations of acyclic dienes. For chiral dienamides (225a-d), the best case employed the sterically demanding (5 )-2-(diphenylhydroxymethyl)pyrrolidine auxiliary. Diene (225d) was complexed with excellent diastereoselectivity but in modest yield (Scheme 64). For substrates (217a-c), diastereomer ratios of the corresponding complexes were poorer (1.5 1 to 4.6 1), likely a result of the increased distance between the auxiliary s chiral center and the diene. 

Inversion of configuration at C-3, which is expected for an anti-disrota-tory reaction path, has been observed, for instance, for the acyclic chiral 1,4-diene 121. 

For the construction of oxygen-functionalized Diels-Alder products, Narasaka and coworkers employed the 3-borylpropenoic acid derivative in place of 3-(3-acet-oxypropenoyl)oxazolidinone, which is a poor dienophile in the chiral titanium-catalyzed reaction (Scheme 1.55, Table 1.24). 3-(3-Borylpropenoyl)oxazolidinones react smoothly with acyclic dienes to give the cycloadducts in high optical purity [43]. The boryl group was converted to an hydroxyl group stereospecifically by oxidation, and the alcohol obtained was used as the key intermediate in a total synthesis of (-i-)-paniculide A [44] (Scheme 1.56). 

Binaphthol-derived titanium complexes [64], prepared from chiral ligands 65 (Figure 3.13), also performed very well in the cycloadditions of conjugated aldehydes with cyclic and acyclic dienes. Judging from the absolute configurations of endo and exo adducts, this catalyst should cover the re-face of carbonyl on its u tz-coordination to s-trans a,/l-unsaturated aldehydes, and hence dienes should approach selectively from the si-face. 

Asymmetric Diels-Alder reactions. Unlike methyl crotonate, which is a weak dienophile, chiral (E)-crotonyl oxazolidinones when activated by a dialkylaluminum chloride (1 equiv.) are highly reactive and diastereoselective dienophiles. For this purpose, the unsaturated imides formed from oxazolidinones (Xp) derived from (S)-phenylalanol show consistently higher diastereoselectivity than those derived from (S)-valinol or (IS, 2R)-norephedrine. The effect of the phenyl group is attributed in part at least to an electronic interaction of the aromatic ring. The reactions of the unsaturated imide 1 shown in equation (I) are typical of reactions of unsaturated N-acyloxazolidinones with cyclic and acyclic dienes. All the Diels-Alder reactions show almost complete endo-selectivity and high diastereoselectivity. Oxazolidinones are useful chiral auxiliaries for intramolecular Diels-Alder... 

In acyclic dienes [1, 5] suprafacial nature of hydrogen shift has also been demonstrated. In acyclic 1, 3 diene with a chiral group at one terminal gave two isomers expected from a suprafacial [1, 5] shift but gave neither of isomers that would result from an antarafacial migration. In this reaction chirality is first transferred from one terminal to the other. 

For diene ligands which are prochiral, complexation results in the formation of a racemic mixture. Resolution of this racemic mixture has been accomplished via either classical methods102, chromatographic separation on chiral stationary phases103 or kinetic resolution104. For certain acyclic or cyclic dienes possessing a pendent chiral center(s)... 

A review of Diels-Alder reactions of fullerenes with acyclic and cyclic dienes has been presented. The addition of substituted pyrimidine o-quinodimethanes (75) to [60]fullerenes yields novel organofullerenes (76) bearing a pyrimidine nucleus covalently attached to the Ceo cage (Scheme 26). The Diels-Alder dimerization of cyclopenta[/]phenanthrene (77) with isobenzindene (78) yields the dimer (79) in 85% yield (Scheme 27). Further evidence has been supplied to support an early reorganization of the r-network in the dimerization of 2-methoxycarbonylbuta-1,3-diene. The Lewis acid-catalysed Diels-Alder reactions of acrylate derivatives of new carbohydrate-based chiral auxiliaries with cyclohexadiene show excellent endo. exo... 

High enantioselectivity was also attained upon additon of the chiral titanium reagent to the reaction of acyclic dienes with oxazolidones of 2-butenoic acid 15a and fumaric acid 19. 

Aliphatic and aromatic nitroso compounds are powerful dienophiles and react with a variety of acyclic, cychc and heterocyclic 1,3-dienes producing cyclic hydroxylamines. The reaction proceeds with a high regioselectivity at room temperature (equation 99 291-293 Asymmetric variation of the reaction with chiral copper-BINAP catalyst has been reported ". The cycloaddition is reversible and some amounts of diene and nitroso components may be observed in reaction products. 

The initial work on the asymmetric [4-1-2] cycloaddition reactions of A -sulfinyl compounds and dienes was performed with chiral titanium catalysts, but low ee s were observed <2002TA2407, 2001TA2937, 2000TL3743>. A great improvement in the enantioselectivity for the reaction of AT-sulfinyl dienophiles 249 or 250 and acyclic diene 251 or 1,3-cyclohexadiene 252 was observed in the processes involving catalysis with Cu(ll) and Zn(ii) complexes of Evans bis(oxazolidinone) (BOX) ligands 253 and 254 <2004JOC7198> (Scheme 34). While the preparation of enantio-merically enriched hetero-Diels-Alder adduct 255 requires a stoichometric amount of chiral Lewis acid complex, a catalytic asymmetric synthesis of 44 is achieved upon the addition of TMSOTf. 

The intermolecular Diels-Alder reaction between the dibromoenone (111) and dienes (112) provides access to bicyclo[5.4.0]undecane systems (113) that are common core structures of many natural products (Scheme 32).118 The alio-threonine-derived O-(/ -biphenyl carbonyl oxy)-/i-phenyloxazaborolidi none catalyses the enan-tioselective Diels-Alder reaction of acyclic enones with dienes.119 The reversal of facial selectivity in the Diels-Alder cycloaddition of a semicyclic diene with a bro-moenone was induced by the presence of the bromo substituent in the dienophile.120 Mixed Lewis acid catalyst (AlBr3/AIMe3) catalyses the Diels-Alder reaction of hindered silyloxydienes with substituted enones to produce highly substituted cyclohexenes.121 Chiral /V-enoyl sultams have been used as chiral auxiliaries in the asymmetric Diels-Alder reactions with cyclopentadiene.122... 

Saigo and co-workers reasoned that, by analogy, high levels of diastereofacial discrimination could be achieved in the Lewis acid-mediated Diels-Alder reaction of dienes with oxazolidinone 108-derived dienophiles. Indeed, excellent regioselectivities (endo. exo) and diastereoselectivities were reached in the Diels-Alder reaction of 109 with cyclic and acyclic dienes using Et2AlCl as the activator (Scheme 24.28).107 The selectivities obtained actually surpassed those reported with cis-1 -amino-2-indanol 1 as the chiral auxiliary (93% de) (see Scheme 24.16).82 The additional bulk... 

In reactions with acyclic dienes, the stereoselection with the (S)-valinol-derived cro-tonate imides is only about 3 1, but diastereoselection is high with N-acyl oxazolidines derived from phenylalanine. The chiral auxiliary is cleaved by transesterification with lithium benzyloxide in 85-95% yield. ... 

Similar diastereoselectivity is observed in reactions of 1 with acyclic dienes. Thus reaction with butadiene catalyzed by ZnCl2 results in the single adduct 3, which was converted into the optically pure alcohol 4 by oxidative removal of the chiral auxiliary group.2... 

As with acyclic dienes, methods have been developed for enantioselective and diastereoselective complexation of prochiral and chiral cyclic dienes. An approach has been developed for the asymmetric catalytic complexation of prochiral eyelohexa-1,3-dienes nsing (1) in the presence of catalytic amounts of l-azabuta-l,3-dienes such as (232) or (233) an enantiomeric excess as high as 86% has been reported. By contrast, attempts to effect diastereoselective complexations using cyclic diene systems eqnipped with chiral auxiliaries have met with limited success. On the other hand, direct complexation of chiral cyclic dienes snch as (234) and (235) proceed with a high degree of diastereoselectivity, where the iron tricarbonyl fragment is directed syn to alcohols or ethers by transient coordination ( heteroatom dehvery ) (Scheme 66). ... 

In contrast to olefins, little is known on catalytic hydroboration of conjugated dienes. Suzuki and Miyaura20 described a 1,4-addition of catecholborane to acyclic 1,3-dienes, catalyzed with tetrakis(triphenylphosphine)pa]ladium(0). An interesting Markovnikov type regioselectivity was observed in the enantioselective dihydroboration of (E)-1-phenyl-1,3-butadiene with catecholborane, catalyzed by chiral rhodium complexes.21 However, the scope of these reactions is not well known, and the choice of catalysts is very limited. 

Cyclic and acyclic enol derivatives 480 can be asymmetrically aziridinated with (A -tosylimino)iodobenzene 481 using a chiral copper catalyst prepared in situ from [Cu(MeCN)4]PF6 and the optically active ligand 479. Collapse of the aminal (i.e., 482) leads to the formation of enantiomerically enriched Q-amino carbonyl compounds 483, although ee s to date are modest <2000EJ0557>. Similarly, dienes can be selectively aziridinated using the chiral Mn-salen complex 484 to give vinyl aziridines 486 in scalemic form (Scheme 124) <2000TL7089>. 

Diels-Alder Reactions. Chiral a,p-unsaturated imides participate in Lewis acid-promoted Diels-Alder cycloaddition reactions to afford products in uniformly excellent endo/exo and endo diastereoselectivities (eq 50 and 51). Unfortunately, this reaction does not extend to certain dienophiles, including methacryloyl imides, p,3-dimethylacryloyl imides, or alkynic imides. Cycloadditions also occur with less reactive acyclic dienes with high diastereoselectivity (eq 52). Of the auxiliaries surveyed, the phenylalanine-derived oxazolidinones provided the highest diastereoselectivities. This methodology has been recently extended to complex intramolecular processes (eq 53). In this case, use of the unsubstituted achiral oxazolidinone favored the undesired diastereomer. 

The dithiocine tetraoxide derived from cyclocondensation of binaphthodithiol with dichloroethylene and oxidation (eq 8) is a chiral version of the bis(phenylsulfonyl)ethylenes. These compounds are useful acetylene equivalents in cycloaddition reactions (see l,2-Bis(phenylsulfonyl)ethylene). Indeed, a chiral acetylene equivalent allows the preparation of optically active hydrocarbons which would be difficult to prepare by classical methods. The dithiocine tetroxide reacts with nonsymmetric dienes to give a single crystalline diastereomeric adduct in most cases. Adducts (1) and (2) were obtained from acyclic and cyclic dienes. 

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