Diene stereoisomerism

P -Bonding is obviously also the initiating step in the complex photoisomerization sequence of the stereoisomeric 1,5-dien-3-ones (162) and (163) in ethanol. After low conversions of the starting dienones, an isomer containing an analogous chromophoric system [(164) and (165), respectively] was found to build up temporarily in each case. On longer photolysis times, both compound pairs (162)/(164) and (163)/(165), are consumed, and the mixtures of the four diastereomers (166)-(169) were isolated from both runs. According to separate irradiation experiments with each of these products, (166) and (167) on one hand, and (168) and (169) on the other, are... 

Compound 6 is a pivotal intermediate in Schreiber s synthesis. It was hoped that the conspicuous and strained bridgehead cyclobutene substructure in 6 would undergo a conrotatory electrocyclic ring opening upon thermolysis to give an isomeric 1,3-diene (8, Scheme 1). In the event, when a solution of cyclobutene 6 in toluene is confined to a sealed tube and heated to 180°C for 12 h, a stereoisomeric mixture of 1,3-dienes 7 and 8 is produced in an excellent yield of 95% (7 8 ca. 5 1). Finally, irradiation of the 5 1 mixture of cis-7 and trans-8, or of each independently, establishes a photostationary state in which the desired trans isomer 8 predominates (8 7 ca. 10 1). 

A further hetero-Diels-Alder reaction with inverse electron demand between o-QM 3 as the dienophile and either of the two diastereomers of spiro dimer 9 as the diene provided the spiro trimers 31 and 32 (Fig. 6.25). The absolute configuration was derived from NMR experiments. It was moreover shown that only two of the four possible stereoisomeric trimers were formed in the hetero-Diels-Alder reaction the attack of the o-QM 3 occurred only from the side syn to the spiro ring oxygen.28... 

Mycomycin. TV. Stereoisomeric 3,5-Diene Fatty Acid Esters. J. Amer. 

Stereochemistry Coordination Polymerization. Stereoisomerism is possible in the polymerization of alkenes and 1,3-dienes. Polymerization of a monosubstituted ethylene, such as propylene, yields polymers in which every other carbon in the polymer chain is a chiral center. The substituent on each chiral center can have either of two configurations. Two ordered polymer structures are possible — isotactic (XII and syndiotactic (XIII) — where the substituent R groups on... 

Tetrazine (171) and its derivatives are electron-deficient cycloaddends, which undergo [4 + 2] cycloadditions with inverse electron demand. When bicyclopropylidene (3) was added to a dichloromethane solution of 171, its red color disappeared within 1.5 h at room temperature. The white crystalline product isolated in 86% yield turned out to be a mixture of at least two stereoisomeric compounds 174, trimers of the 8,9-diazadispiro[2.0.2.4]deca-7,9-diene (173) evidently formed via the normal [4 + 2]-cycloadduct 172 after nitrogen extrusion (Scheme 25) [13b]. 

Subsequently, other publications have appeared involving the DA reactions of porphyrins with other dienes. For instance, the reaction of meso-tetrakis(pentafluorophenyl)porphyrin Id with the diene generated from pyrrole-fused 3-sulfolene gave rise to the isoindole-fused chlorin derivative 8 accompanied by a mixture of stereoisomeric bacteriochlorins 9 (Scheme 2) <98CC2355>. 

The concept of using diazaquinones for diene system protection was accomplished in the case of some vitamin D derivatives, as shown in Scheme 37 (86JOC4819). Acetyl vitamin D2152 was protected with phtha-lazinedione 83a, providing a mixture of both possible stereoisomeric adducts from which 153 was isolated. Then usual ozonolysis procedure afforded aldehyde 154, which after the Wittig coupling and hydrogenation provided 155. The most difficult part of the sequence proved to be the deprotection of this compound, but this transformation was accomplished... 

Double bonds present along a polymer chain are stereoisomeric centers, which may have a cis or trans configuration. Polymers of 1,3-dienes with 1,4 additions of the monomeric units contain double bonds along the chains and may contain up to two stereoisomeric tetrahedral centers. Stereoregular polymers can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndio-tactic if two stereoisomeric tetrahedral centers are present. In the latter case erythro and threo structures are defined depending on the relative configurations of two chiral carbon atoms.1... 

The mechanistic basis of iridium-complex-catalyzed enantioselective hydrogenation is less secure than in the rhodium case. It is well known that square-planar iridium complexes exhibit a stronger affinity for dihydrogen than their rhodium counterparts. In earlier studies, Crabtree et al. investigated the addition of H2 to their complex and observed two stereoisomeric intermediate dihydrides in the hydrogenation of the coordinated cycloocta-1,5-diene. The observations were in contrast to the course of H2 addition to Ms-phosphine iridium complexes [69]. 

Scheme 1.72. Stereoisomerization observed in the Zr-promoted bicyclization of dienes.

Diradical intermediates are found more commonly in thermal 2 + 2 cycloadditions. The best substrates for such additions are fluoroalkenes and allenes. Thus P.D. Bartlett and his coworkers have demonstrated that the cycloaddition of l,l-dichloro-2,2 difluroethylene to stereoisomeric hexa-2, 4 dienes yields a mixture of isomeric products in a non stereospecific way. 

Dienes reacted considerably faster than the (Z)-isomers, and this feature was utilized to separate the (Z)-isomer from the stereoisomeric mixture. When 6-phenyl-1,3-hexadiene (EIZ = 50/50) was treated with 0.6 molar amounts of PPh3 and CF3S03H at 0 °C for 6 h, the ( )-phosphonium salt (53%) was formed. The unreacted (2)-1,3-diene could be separated by ether extraction in 41% yield. This procedure provides easy access to (Z)-1,3-dienes.22... 

If it enters in 1,2 or 3,4 fashion, chiral centers are introduced and relative stereoisomerism must be considered. Actually, as it turns out, things are not so complicated as they might be because of a strong preference for one mode of addition. Complications ate quite sufficient, however, as they include a head-to-head head-to-tail option not normally open to vinyl monomers but available to unsymmetrically substituted dienes adding in 1,4-fashion. In fact, little progress can be made in structural determination except by use of C-13 nmr at superconducting frequencies. 

For an unsymmetrical dienophile, there are two possible stereochemical orientations with respect to the diene. The two possible orientations are called endo and exo, as illustrated in Fig. 6.3. In the endo transition state, the reference substituent on the dienophile is oriented toward the % orbitals of the diene. In the exo transition state, the substituent is oriented away from the % system. For many substituted butadiene derivatives, the two transition states lead to two different stereoisomeric products. The endo mode of addition is usually preferred when an electron-attracting substituent such as a carbonyl group is present on the dienophile. The empirical statement which describes this preference is called the Alder rule. Frequently, a mixture of both stereoisomers is formed, and sometimes the exo product predominates, but the Alder rule is a useful initial guide to prediction of the stereochemistry of a Diels-Alder reaction. The endo product is often the more sterically congested. The preference for the endo transition state... 

The all-trans-all-isotactic and all-trans-all-syndiotactic structures for the 1,4-polymerization of 1,3-pentadiene are shown in Fig. 8-6. In naming polymers with both types of stereoisomerism, that due to cis-trans isomerism is named first unless it is indicated after the prefix poly. Thus, the all-trans-all-isotactic polymer is named as transisotactic l,4-poly(l,3-penta-diene) or isotactic poly( -3-methylbut-l-ene-l,4-diyl). 

Stereoisomerism in 1,4-poly-1,3-dienes results in significant differences in the properties of the cis and trans polymers. The trans isomer crystallizes to a greater extent as a result of higher molecular symmetry and has higher Tm and Tg values (Table 8-1). These trends are... 

Infrared spectroscopy has been used for quantitatively measuring the amounts of 1,2-, 3,4-, cis-1,4-, and trans-1,4-polymers in the polymerization of 1,3-dienes its use for analysis of isotactic and syndiotactic polymer structures is very limited [Coleman et al., 1978 Tosi and Ciampelli, 1973]. Nuclear magnetic resonance spectroscopy is the most powerful tool for detecting both types of stereoisomerism in polymers. High-resolution proton NMR and especially 13C NMR allow one to obtain considerable detail about the sequence distribution of stereoisomeric units within the polymer chain [Bovey, 1972, 1982 Bovey and Mirau, 1996 Tonelli, 1989 Zambelli and Gatti, 1978],... 

When 2-aza-3-silyloxy-l,3-diene 336 was heated with aliphatic or aromatic aldehydes in toluene, stereoisomeric tetrahydro-l,3-oxazin-4-one derivatives 200 and 337 were obtained without Lewis acid catalysis (Equation 34). The cycloaddition proved to be highly diastereoselective in favor of the OTr/o-adducts, leading to the >-isomers 200 as the main products <1999TL7079, 2002S2043>. 

In CHEC-II(1996) <1996CHEG-II(7)229>, intermolecular Diels-Alder reactions were conducted with furan to give nonaromatic species. This example explores the reaction of the diene 259 with dihydrofuran. The reaction proceeds with 100% conversion to give a mixture of stereoisomeric products, one of which, 260, is obtained in pure form (Equation 91) <2001HCA513, 2005EJ01142>. 

SCHEME 39. A probable mechanism for stereoisomerization of 2-pallada-l,3-dienes... 

Trifluoroethene has been reported to undergo cycloadditions with conjugated dienes to give both [2 + 2] and [4 + 2] cycloadducts.25 Although mixtures of regio- and stereoisomeric cyclobutanes are obtained in these cases it still constitutes a synthetically useful reaction. 

Diene synthesis.6 /3-Acetoxy carboxylic acids undergo loss of CH,COOH and C02 when refluxed in T1IF or DMSO in the presence of triethylamine (1 equivalent) and catalytic amounts of Pd(0). This fragmentation is highly stereoselective the (F.)-alkene is formed predominately, irrespective of the stereochemistry of the substrate. The method is particularly useful for stereocontrolled synthesis of 1,3-dicnes from stereoisomeric mixtures. 

The photoconversion of spiro[benzofuran-2,r-cyclohexa-3, 5 -diene]-2, 3(2//)-diones to dibenzofurans is recorded (81JCS(Pi)870). It was proposed that excitation of the dienone chromophore of the grisedienone (401) causes either concerted or radical ring opening to pairs of stereoisomeric dienylketenes. One member of each pair can undergo a thermal intramolecular va + tts cyclization to yield a /3-lactone subsequent loss of C02 would yield the products. 

Seven stereoisomeric and regioisomeric vinyloxetans (384 to 390) were obtained on photoaddition of acetaldehyde to ( )- or (Z)-penta-l,3-diene.336 Other instances of [2 -I- 2] photoaddition of ketones to conjugated dienes have been described.337 Iminooxetans are similarly prepared by addition of carbonyl-containing compounds to ketenimines.338... 

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