Epoxy esters, rearrangement

The complementary ABO ester (2,7,8-trioxabicyclo[3.2.1]octyl ester) is prepared from the epoxy ester by rearrangement with Cp2ZrCl2/AgC104. The OBO ester is more easily cleaved by Brpnsted acids than is the ABO ester, but the ABO ester is cleaved more easily by Lewis acids, thus forming an orthogonal set. The ABO ester can be cleaved with PPTS (MeOH, H2O, 22°, 2 h LiOH) the OBO ester is cleaved at 0° in 2 min. ... 

The rearrangement of isopent-3-enyl epoxy esters with Cp2ZrCl2/AgC104 yields ABO esters (2,7,8-trioxabicyclo[3.2.1]octane Asymmetric Bicyclo-Octane esters), which are base-stable protecting groups for carboxylic acids [57,79,80] (Scheme 8.40). 

The full paper on the synthesis of onikulactone and mitsugashiwalactone (Vol. 7, p. 24) has been published.Whitesell reports two further useful sequences (cf. Vol. 7, p. 26) from accessible bicyclo[3,3,0]octanes which may lead to iridoids (123 X=H2, Y = H) may be converted into (124) via (123 X = H2, Y = C02Me), the product of ester enolate Claisen rearrangement of the derived allylic alcohol and oxidative decarboxylation/ whereas (123 X = 0, Y = H) readily leads to (125), a known derivative of antirride (126) via an alkylation-dehydration-epoxi-dation-rearrangement sequence. Aucubigenin (121 X = OH, R = H), which is stable at —20°C and readily obtained by enzymic hydrolysis of aucubin (121 X = OH, R = j8-Glu), is converted by mild acid into (127) ° with no dialdehyde detected sodium borohydride reduction of aucubigenin yields the non-naturally occurring isoeucommiol (128 X=H,OH) probably via the aldehyde (128 X = O). ... 

A farther example of the same type of rearrangement is provided by the observation of Komfeld and oo-workers,888 drawn durim lla-ir synthesis of lysergic acid, that treatment of the 0,y-epoxy ester derivative shown in Eq. (509) with the base saroosine (tbe ethyl ester of A -methylglydue) led to a lactone instead of the desired nucleophilic addition product. 

Photochemically induced ring cleavage reactions have also been reported in a,/i-epoxydiazomethyl ketones" and in a,/f-epoxy esters (glycidic esters).100 Two separate photoreactions are involved, however, in the rearrangement of phenyl epoxycinnamate (118) to the flavone (119), as shown in Scheme 7.101... 

The formation of the six-membered ether ring via epoxy ester-ortho ester cyclic ether rearrangement supports the hypothesis that epoxy ester-ortho ester cyclic ether rearrangement may be involved in the biosynthesis of ladder-type marine polyether toxins. This reaction represents a biomimetic preparation of medium ring cyclic ethers. 

Rearrangement of the epoxy ester 317 affords a 1 1 mixture of the tetrahydropyran 318 and a steroidal tetra-hydrofuran by-product. This methodology offers a new hypothesis regarding the biosynthesis of polyether natural... 

Remotely unsaturated epoxy acids undergo fission rearrangement when treated with BF3 Et20. Hence, cis and trans ketocyclopropane esters are produced from the unsaturated epoxy ester methyl vernolate (Eq. 46) [88]. 

Fission-rearrangement of epoxides. Following his suggestion that epoxy-acids may be important intermediates in biosynthesis,19 Gunstone with Conacher 20 found that when the unsaturated epoxy ester methyl vemolate (1) is treated with boron trifluoride etherate in cold benzene solution it is converted into a number of products, two of which cannot be separated from each other but, together, account for a yield of 34%. On the basis of chemical properties, infrared, NMR, and mass spectra they are believed to be the cis and trans forms of the ketocyclopropane ester (2). 

The Ci5 phosphonium salt (10) was converted with n-butyllithium to the anionic form and then reacted with the epoxy ester (492) to give methyl 13,14-epoxyretinoate (493), which is unstable. In neutral methanol at room temperature, (493) had a half-life of 7 min, and underwent rearrangement to give the 14-hydroxy compound (30) (Davalian and Heathcock, 1979b). Compound (30) was also obtained from Cjg ketone (136) in a Darzens reaction. The spectral data of... 

Compound (185) underwent rearrangement in the presence of acetic acid to give (186), and in the presence of methanolic potassium hydroxide gave 4-hy-droxyretinoic acid (187) (Rosenberger, 1982) (see Section V). The 7,8-ep-oxy-7,8-dihydroretinoids (498), (499), (500), and (501) were synthesized by coupling the (3-ionone epoxide (494) to the anion of the all-trans C7 phosphonate (495). The epoxy esters (496) and (497) were obtained after purification by HPLC. 

A high yielding rearrangement of epoxy-esters, under Lewis acid conditions, to give bicyclic ort/io-esters is reported (Scheme 64). ... 

Epoxi ester 1 effeetively rearranges to a 1 1 mixture of acetates 2 and 3 under acidic conditions (0.5 M H2SO4 in THF/H2O 9/1). It is believed that the reaction occurs through the formation of an orthoester intermediate 4 tiiat is subsequently hydrolyzed in the acidic medium (Scheme 25.1). This route provides a convenient method for the synthesis of polyhydroxylated compounds with control of the stereochemistry during the epoxide ring-opening step. 

The epoxy ester-orthoester rearrangement occurs in acidic medium through the intermediacy of a dioxycarbenium ion, which forms an orthoester by intramolecular quenching. The mechanism is an example of neighboring group participation of... 

The acid rearrangement of the epoxy ester moiety of 12 (Fig. 25.1) is the key step in the synthesis of orthoesterol B, a marine natural product with antiviral activity. Based on the meehanism proposed for the formation of 4 formulate the strueture of flie orflioester fliat should be obtained in the rearrangement of 12. Interestingly, when diastereomer 13 was treated under the same eonditions, it remained unaltered for months. Could you explain why ... 

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