Exo-enes

The anti selectivity increases as the disubstituted side of the double bond becomes more crowded (Scheme 8). This is illustrated with the trisubstituted alkenes 16, 17 and 13. Alkene 16 shows the normal cis effect selectivity where only 10% of the anti ene adduct is formed. However, as the size of the cis alkyl substituent increases from methyl in 16, to isopropyl in 17 and ferf-butyl in 13, the anti selectivity increases from 10% to 42% and to >97%, respectively. The same trend is also noted in substrate 19. A substantial deviation from cis effect selectivity is observed by replacing one methyl group in 18 with a ferf-butyl group in 19. The totally unreactive methylene hydrogens in 18 (cis effect) become reactive in 19, producing the exo ene adduct in 38% yield. 

Thermal ene reaction of -pinene with methyl pyruvate provides a 1 1 mixture of endo and exo ene adducts. This reaction can also be carried out in quantitative yield at 40 kbar for 17 h at 25 Contrary to a published report 8-phenylmenthyl pyruvate does not undergo a Lewis acid catalyzed ene reaction with 1-hexene. However, reaction of rra/u-2-phenylcyclohexyl pyruvate with 1-hexene and 2 equiv. of TiCU for 15 min at 0 C affords a 15 1 mixture in 85% yield of ene adduct (74) in 86% dia-stereomeric excess and tetrahydrofuran (75) derived from (74) by protonation of the double bond and cy-clization. 

Furan and maleic anhydride undergo the Diels-Alder reaction to form the tricycHc 1 1 adduct, 7-oxabicyclo [2.2.1]hept-5-ene-2,3-dicarboxyHc anhydride (4) in exceUent yield. Other strong dienophiles also add to furan (88). Although both endo and exo isomers are formed initially, the former rapidly isomerize to the latter in solution, even at room temperature. The existence of a charge-transfer complex in the system has been demonstrated (89,90). 

Aqueous ring-opening metathesis polymerization (ROMP) was first described in 1989 (90) and it has been appHed to maleic anhydride (91). Furan [110-00-9] reacts in a Diels-Alder reaction with maleic anhydride to give exo-7-oxabicyclo[2.2.1]hept-5-ene-2,3—dicarboxylate anhydride [6118-51 -0] (24). The condensed product is treated with a soluble mthenium(Ill) [7440-18-8] catalyst in water to give upon acidification the polymer (25). Several apphcations for this new copolymer have been suggested (91). 

Germacyclopentadie ne, 1 - exo-fl uoro-1 -methyl-2,3,4,5-tetraphenyl-iron tricarbonyl complex structure, 1, 617 Germacyclopentane, 1-phenyl-bromination, 1, 607 Germacyclopentanes, 1, 605-609 chemical properties, 1, 607-608 synthesis, 1, 605-607 Germacyclopentenes synthesis, 1, 610-612 Germacyclopent-3-enes properties, 1, 612 reactions... 

In a rather different approach optically active chromium complexes of 2,3-dihydro-1 H-in-denone are used as chiral enolate precursors. These chiral complexes react with 3-buten-2-one in benzene using l,5-diazabicyclo[4.3.0]non-5-ene as the base. The diastereomeric ratio of the product is the same irrespectively of whether the exo- or the Noisomer of the chromium... 

Dimsyl anion 88 is known to add to styrene, and to 1,1-diphenylethylene in the presence of a base, forming 3-arylpropyl methyl sulfoxides121. Treatment of ( )-3,3-dimethyl thiacyclo-oct-4-ene-l-oxide 89 with n-BuLi gave exo-4,4-dimethyl-2-thiacyclo-[3.3.0]octane 2-oxide 90, a bicyclic addition product of the internal double bond. A similar cyclization was observed in the reaction of 91 with n-BuLi122. 

Recently, Block and coworkers220 reported a striking difference between exo and endo-sulfoxide 160. While the former remained unchanged even after refluxing in toluene for 20 h, the endo-sulfoxide 160, rearranged at room temperature, presumably via a [2,3]-sigmatropic shift (Section II.B), to 4-ethyl-2-oxa-3-thiabicyclo[3,3,0]oct-7-ene (161), a rare example of an isolable sultene185 (equation 66). 

The cycloaddition of glyoxylic acid with cyclopentadiene in water at pH 6 and 60 °C is slow and occurs with low yield and low diastereoselectivity [18] (Scheme 6.17). Proton (pH = 0.9) [18], copper salts [27] and Bi(OTf)3 [28] accelerate the reaction and increase the diastereoselectivity. The lactones 28 and 29 originate from endo and exo cycloadducts 27, respectively. The proposed rearrangement is depicted in Scheme 6.17 for the major endo adduct 30. A competitive ene reaction that originates 28 and 29 cannot be excluded [28]. 

The endo pyramidization results in the high exo reactivity and, in fact, this pyramidi-zation was confmned by Wipff and Morokuma [37], The orbital distortion also implies large negative electrostatic potential in the exo face. Very recently, Abbasoglu and Yilmaz [38] calculated a derivative of norbomene, i.e., endo tricyclo[3.2.1.0 ] oct-6-ene and confirmed the exo extension of the HOMO accompanied by the endo pyramidization, the large negative electrostatic potential on the exo face, and the preferential exo addition of Br. ... 

Recently a reverse perturbation effect of a cyclopropyl group on facial selectivities was described in two bicyclic systems, bicyclo[2.2.2]octane 59 and norbomane (bicyclo[2.2.1]heptane) 60 [124]. Bicyclo[2.2.2]octene 59a, annulated with an exo-cyclopropyl group, i.e., exo-tricyclo[3.2.2.0 ]non-6-ene, and 7-methylenenor-bomane 60a, annulated with an exo-cyclopropyl group, i.e., 8-... 

Cycloaddition of bicyclo[2.2.1]hept-2-ene-2,3-dicarboxylic anhydride 81 with cyclopentadiene was also studied by Bartlett et al., who found exclusive top addition, the top-endo/top-exo ratio being 3 2 [147]. The endolexo ratio is significantly different from that of 80 (60-70 1). The observed top selectivity in norbornadiene (80) and norbomene (81) derivatives is consistent with the inherent top reactivity of norbomanone 25 and norbomene 57. Orbital unsymmetrization of the dienophile... 

It should be noted, however, that despite many examples of the acceleration of Diels-Alder reactions by the use of aqueous media, Elguero59 reported that the Diels-Alder reaction between cyclopentadi-ene and methyl (and benzyl) 2-acetamidoacrylates proceeded better in toluene than in water both in yield and in exo/endo selectivity. Additionally, ultrasonic irradiation did not improve the yield. 

Grobe15 has described the pyrolysis of 1 -methyl-1 -vinyl- and 1,1 -diviny 1-1-silacyclobutanes 166 which led to the formation of methylvinylsilene and divinylsilene, respectively. Under the experimental conditions used, it was suggested that the silenes rearrange to exo-methylene- 1-silacyclo-propanes 167 which extrude methylsilylene or vinylsilylene, respectively. In support of this proposal, when the reactions were carried out in the presence of 2,3-dimethylbutadiene, the anticipated silylenes were trapped as their respective l-silacyclopent-3-enes 168. 

Debande and Huybrechts lnt. J. Chem. Kinetics, 6 (545), 1974] have studied the gas phase Diels-Alder additions of propylene (P) to cyclohexa-1, 3-diene (Chd) to give the exo-and endo-isomers of 5-methylbicyclo (2,2,2) oct-2-ene (MBO). The initial reaction rates were both found to be of the mixed second-order type with... 

Recently, a new multicomponent condensation strategy for the stereocontrolled synthesis of polysubstituted tetrahydropyran derivatives was re-published by the Marko group, employing an ene reaction combined with an intramolecular Sakurai cyclization (IMSC) (Scheme 1.14) [14]. The initial step is an Et2AlCl-promoted ene reaction between allylsilane 1-50 and an aldehyde to afford the (Z)-homoallylic alcohol 1-51, with good control of the geometry of the double bond. Subsequent Lewis acid-media ted condensation of 1-51 with another equivalent of an aldehyde provided the polysubstituted exo-methylene tetrahydropyran 1-53 stereoselectively and... 

Recently, enhanced endo selectivity has been reported in the Diels-Alder reaction of (E)-l-acetoxybuta-1,3-dienes with methyl (J-nitroacrylate. The selectivity is compared with that of the reaction using 1-methoxybuta-l,3-dienes and l-trimethylsilyloxybuta-l,3-di-enes.16 The degree of electron richness of a diene is an important consideration in endo.exo selectivity issues. In particular, electron-rich dienes favor the formation of cvo-nitrocycload-ducts (Eq. 8.9). 

Synthesis of block copolymers of norbornene derivatives, with different side groups, has been reported via ROMP [101]. Initially, exo-N-bulyl-7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboximide was polymerized in acetone at room temperature with a ruthenium initiator (Scheme 40). The conversion of the reaction was quantitative. Subsequent addition of norbornene derivative carrying a ruthenium complex led to the formation of block copolymers in 85% yield. Due to the presence of ruthenium SEC experiments could not be performed. Therefore, it was not possible to determine the molecular weight... 

Analogues endo and exo cycloadducts have been observed in the reactions of cyclopentadiene (6) with 21 or methoxybicyclo[3.1.0]hex-l-ene (23) (Scheme 5) [11]-... 

A mixture of exo- and endo-isomers of 5-methylbicylo[2.2.1]hept-2-ene is hydrogenated with the aid of five equivalents of triethylsilane and 13.1 equivalents of trifluoroacetic acid to produce a 45% yield of < <7o-2-methylbicylo[2.2.1] heptane (Eq. 71). The same product is formed in 37% yield after only five minutes. The remainder of the reaction products is a mixture of three isomeric secondary exo-methylbicylo[2.2.1]heptyl trifluoroacetates that remains inert to the reaction conditions. Use of triethylsilane-l-d gives the endo-2-methylbicylo-[2.2.1]heptane product with an exo-deuterium at the tertiary carbon position shared with the methyl group. This result reflects the nature of the internal carbocation rearrangements that precede capture by the silane.230... 

R,R)-2-exo-3-endo-bis(diphenylphosphino)-bicyclo[2.2.1]hept-5-ene naphthyl no reaction... 

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