Diene racemic

Optically active tricyclo[7.3.1.0. 0" ]trideca-5,10-diene racemizes at 40°C with log k = 15.6 — 28 900/2.3RT (18°C range). The molecule is predisposed to a boatlike 3,3-shift, and undergoes the reaction almost a billion times faster than 1,5-hexadiene itself undergoes the boat-like 3,3-shift (see Chapter 7, Section 4.1) (Scheme 14.2). 

Q 6. The following diene racemizes with a half-hfe of 24 h at 50 °C. Explain. 

PROBLEM 20.49 Figure 20.55 shows the products of the Cope rearrangement of i ( -3,4-dimethyl-l,5-hexadiene from hoth the chairlike (four-center) and boatUke (six-center) transition states. Work out the products expected from the other possible form of the diene, racemic 3,4-dimethyl-l,5-hexadiene. Predict what products will actually be formed. 

Structure 3.2a was assigned to deoxyschizandrin on basis of its UV (identical to the UV of 3.1b) and NMR spectra, as well as its incapability of racemization. Optically active 6,7-cw-substituted 1,2,3,4-dibenzocycloocta-1,3-dienes racemize readily (93). Indeed, synthetic dimethyl trans-2, 3, A, ",2", 3"-hexamethoxy-1,2,3,4-dibenzocycloocta-l,3-diene-6,7-dicarboxylate (45a) (91) proved to be a key intermediate in the synthesis of deoxyschizandrin (3.2a) (Chart 14) (93). 

Following diene racemizes with half life of 24 hrs at 50°C. Give the mechanism. 

Chiral diene—iron tricarbonyl complexes were acylated using aluminum chloride to give acylated diene—iron complexes with high enantiomeric purity (>96% ee). For example, /ra/ j -piperjdene—iron tricarbonyl reacted with acyl haUdes under Friedel-Crafts conditions to give l-acyl-l,3-pentadiene—iron tricarbonyl complex without any racemization. These complexes can be converted to a variety of enantiomericaHy pure tertiary alcohols (180). 

A key transformation in Corey s prostaglandin synthesis is a Diels-Alder reaction between a 5-(alkoxymethyl)-l,3-cyclopenta-diene and a ketene equivalent such as 2-chloroacrylonitrile (16). As we have already witnessed in Scheme 3, it is possible to bring about a smooth [4+2] cycloaddition reaction between 5-substituted cyclopentadiene 15 and 2-chloroacrylonitrile (16) to give racemic 14 as a mixture of epimeric chloronitriles. Under these conditions, the diastereomeric chloronitriles are both produced in racemic form because one enantiotopic face of dienophile 16 will participate in a Diels-Alder reaction with the same facility as the other enantiotopic face. In subsequent work, Corey s group demonstrated that racemic hydroxy acid 11, derived in three steps from racemic 14 (see Scheme 3), could be resolved in a classical fashion with (+)-ephe-... 

Scheme 8 presents the sequence of reactions that led to the synthesis of the B-ring of vitamin B12 by the Eschenmoser group. An important virtue of the Diels-Alder reaction is that it is a stereospecific process wherein relative stereochemical relationships present in the diene and/or the dienophile are preserved throughout the course of the reaction.8 Thus, when the doubly activated dienophile 12 (Scheme 8) is exposed to butadiene 11 in the presence of stannic chloride, a stereospecific reaction takes place to give compound 27 in racemic form. As expected, the trans relationship between... 

Cu(i(-catalyzed kinetic resolutions of racemic, cyclic 1,3-diene monoepoxides through the use of dialkylzinc [123] or trialkylaluminium reagents [124] have re-... 

A cyclobutene ROCM sequence was also used in a synthesis of racemic sporochnol (410), a naturally occurring feeding deterrent toward herbivorous fish (Scheme 80) [170]. Exposing cyclobutene 406 (0.01 M in boiling 1,2-dichloroethane) in the presence of ethylene to second-generation catalyst C (8 mol%) led to 1,5-diene 407 in 73% yield, along with 9% of the homodimer derived from 407 by involving the less hindered double bond. Site-selective hy-... 

The cycloaddition of chiral, racemic and non-racemic alkoxybutadienes 109 with phenyltriazolinedione led to aza compounds [110] in high yield, with good facial selectivity (diastereomeric excess 87-92%) (Equation 2.31). The cycloadditions of the same dienes with N-phenylmaleimide require Lewis acid catalysis. 

The diolefin 1064 gives rise to the isoxazoline 1065, which cannot eliminate tri-methylsilanol 4 [122]. Cychzation of the co-nitroolefin 1066 with trimethylchloro-silane (TCS) 14/triethylamine at -35 °C then HCl-induced removal of trimethyl-silanol 4 leads, in 85% yield, to the dimer 1067, which is converted in two more steps into racemic pyrenophorin 1068 [112] (Scheme 7.39). Further cyclizations of co-nitroolefins [109] to monomeric or dimeric isoxazolines have been described. Conjugated dienes such as butadiene afford a mixture of the mono or bis adducts [115-117]. 

Irradiation of cycloocta-2,4-dien-l-one (55) in pentane gives a racemic photodimer, anti-tricyclofSAO.O Jhexadeca- , 11 -diene-3,16-dione (60) in 10% yield along with polymeric materials 34). Efficient and enantioselective photodimerization of 58 was achieved by irradiation of the 2 1 inclusion complex 59 formed between 2 a and 5813). When a solution of 2a and an equimolar amount of 58 in ether-hexane (1 1) was kept at room temperature for 12 h, 59 was obtained as colorless needles of mp 105 to 108 °C. Irradiation of 59 in the solid state for 48 h gave (—)-60 of 78 % ee in 55 % yield. 

It has been reported that irradiation of bicyclo[5.1.0]octa-3,5-dien-2-one (63) in methanol leads to a mixture of racemic tricyclo[4,2.0.03,5]oct-7-en-2-one (65), 37, and cyclohepta-l,3,5-triene35). Control of the reaction was also tried in expectation... 

Fig. 14A-C Chromatography of the racemic monoepoxy derivatives (I—III) of Z3,Z6,Z9-18 on chiral HPLC columns A Chiralpak AD B Chiralpak AS C Chiralcel OJ-R. The solvent system for the former two normal-phase columns is 0.1% 2-propanol in n-hexane (0.45 ml/min), and that of the third column is 15% water in MeOH (0.45 ml/min). Homo-conjugated dienes, epo3,Z6,Z9-18 H (I) and Z3,Z6,epo9-18 H (III), were detected by UV (215 nm), and Z3,epo6,Z9-18 H (II) was detected by RID. The earlier eluting isomers have a 3S,4R, 6S,7R, or 9R,10S configuration...

The tandem-Knoevenagel-ene reaction is a powerful tool to synthesize five-and six-membered carbocycles.2 5 The process is exemplified by the diastereoselective synthesis of 4a. Compound 4a has been obtained In both enantiomeric forms and as a racemate according to the procedure described here. The sequence includes the Knoevenagel reaction of citronellal, 1, and dimethyl malonate, 2, followed by the intramolecular ene cyclization of the chiral 1,7-diene 3 to yield the trans 1,2-disubstituted products 4a and 4b. Whereas the thermal cyclization of 3 at 160°C provides 4a and 4 b in a ratio of only 89.7 10.3, the Lewis acid... 

The initially expected (75) cis-hydrometallation or olefin-insertion step with fumarate (R = C02Me) yields the threo isomer 8, which then undergoes the k2 step with retention to give racemic 1,2-dideuterosuccinate. Such retention is necessary to give the usually observed (7, p. 407) overall cis addition of H2 to olefinic bonds, but this study provided the first direct experimental proof, the difficulty being the scarcity of stable metal alkyl-hydride intermediates. The Cp2MoH2 complex also catalyzes hydrogenation of 1,3- or 1,4-dienes to monoenes (197). 

Selenium dioxide is also an oxygen donor to alkenes. In this case, however, the initial reaction of the double bond is with the selenium center followed by two pericyclic steps. After hydrolysis of the organo-selenium intermediate, the result is a hydroxylation at the allylic carbon position65. Thus, limonene (2) yields racemic p-mentha-l,8(9)-dien-4-ol66. The high toxicity of selenium intermediates and prevalence of many rearrangements has limited the widespread use of the reagent in synthesis. 

HSiMe2OTBDPS),43h and benzhydryldimethylsilane (HSiMe2CHPh2).43 As in the racemic variant, efficient cycli-zation generally requires preorganization of the 1,6-diene through homoallylic geminal disubstitution (Scheme 14). 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

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