Epoxides, vinyl ring opening

Entry 8 involves a migration initiated by epoxide ring opening. This reaction involves migration of a vinyl substituent. Entry 9 is a stereospecific migration of the aryl group. The DiBAlH both promotes the rearrangement and reduces the product aldehyde. 

Oxazolines can be obtained by the Lewis acid catalyzed epoxide ring opening of glycidic esters or amides (e.g., 118) with acetonitrile . Oxazolidines are available from the palladium-catalyzed cycloaddition of vinyl epoxides with imines <00H885> or the samarium-promoted reaction of ketimines (e.g., 120) with unfunctionalized... 

In addition, the GMA/EDMA copolymer proved to serve as a basic unit for the fabrication of highly permeable bioreactors in capillary format. Trypsin immobilization after epoxide ring opening with diethylamine and attachment of glutaraldehyde is mentioned as the probably most prominent example [64], The immobilization of trypsin was also carried out using another class of reactive monolithic methacrylate polymer, which is based on 2-vinyl-4,4-dimethylazlactone, acrylamide, and ethylene dimethacrylate [65]. In contrast to GMA/EDMA, trypsin can directly be immobilized onto this kind of monolith, as the 2-vinyl-4,4-dimethylazlactone moieties smoothly react with weak nucleophils even at room temperature. 

Unsaturated aliphatic compounds and heterocyclic compounds may also be metabolized via epoxide intermediates as shown in Figure 4.6 and chapter 5, Figure 14. Note that when the epoxide ring opens, the chlorine atom shifts to the adjacent carbon atom (Fig. 4.6). In the case of the furan ipomeanol and vinyl chloride, the epoxide intermediate is thought to be responsible for the toxicity (see below and chap. 7). Other examples of unsaturated aliphatic compounds, which may be toxic and are metabolized via epoxides, are diethylstilboestrol, allylisopropyl acetamide, which destroys cytochrome P-450, sedormid, and secobarbital. 

When perfluoro-2-methylpent-2-ene reacts with glycidol, the initially formed product is vinyl ether 38, whose epoxide ring opens under the action of mineral acids (8% HBr, 33% HC1). Alcohol 39 formed in the course of ring cleavage is readily cyclized into 1,3-dioxolane 40 in the presence of catalytic amounts of NEt3. 

Allylboration of representative vinylic epoxides with allylboranes afforded a mixture of 1,2- and 1,4-addition products with epoxide ring opening. In contrast, 3,4-epoxycyclopentenes (R = H, Me) unexpectedly resulted in opening the five-membered ring (Equation (147)).613... 

The mechanism of the Wharton transposition is very simiiar to that of the Woiff-Kishner reaction. The epoxyhydrazone is first deprotonated, which triggers the faciie and irreversible epoxide ring-opening. The C-N bond of the resuiting vinyi diazene is broken upon another deprotonation, releasing N2 and a vinyl anion, which in turn affords the desired aiiyiic aicohoi. Aiternativeiy, the formation of a vinyi radical has been proposed. ... 

The radical-induced epoxide ring-opening of a,/3-epoxy-0-thiocarbonyl-imidazolides (23) [equation (3)] has been reported to be a convenient alternative to the Wharton rearrangement (action of hydrazine on epoxides of a,/3-unsaturated ketones) for production of allylic alcohols. /3,y-Disubstituted allylic alcohols with Z-conhguration are the major products formed on addition of alkyl-lithiums to the vinyl epoxide (24) [equation (4)]. ... 

Narjes, F., Bolte, O., Icheln, D., Konig, W.A., and Schaumatm, E. (1993) Synthesis of vinyl-cyclopropanes by intramolecular epoxide ring opening application for an enantioselective synthesis of dictyopterene-A./. Org. Chem.. 58, 626-632. 

While generation of a Mn(V)oxo salen intermediate 8 as the active chiral oxidant is widely accepted, how the subsequent C-C bond forming events occur is the subject of some debate. The observation of frans-epoxide products from cw-olefins, as well as the observation that conjugated olefins work best support a stepwise intermediate in which a conjugated radical or cation intermediate is generated. The radical intermediate 9 is most favored based on better Hammett correlations obtained with o vs. o . " In addition, it was recently demonstrated that ring opening of vinyl cyclopropane substrates produced products that can only be derived from radical intermediates and not cationic intermediates. ... 

Due to the abundance of epoxides, they are ideal precursors for the preparation of P-amino alcohols. In one case, ring-opening of 2-methyl-oxirane (18) with methylamine resulted in l-methylamino-propan-2-ol (19), which was transformed to 1,2-dimethyl-aziridine (20) in 30-35% yield using the Wenker protocol. Interestingly, l-amino-3-buten-2-ol sulfate ester (23) was prepared from l-amino-3-buten-2-ol (22, a product of ammonia ring-opening of vinyl epoxide 21) and chlorosulfonic acid. Treatment of sulfate ester 23 with NaOH then led to aziridine 24. ... 

An alternate way to make block copolymers involving PDMS blocks 124,125) is to have these chains fitted with epoxide functions at chain end, and to react them with a vinylic or dienic polymer carrying terminal COOH functions. Sequential addition of monomers has also been used, the ring opening polymerization of the cyclic trimer (D3) being initiated by the anionic site of a living polymer126). 

Scheme 4.10 gives some examples of application of alkyne carboalumination in synthesis. The reaction in Entry 1 was carried out as part of a synthesis of the immunosuppressant drug FK-506. The vinyl alane was subsequently transmetallated to a cuprate reagent (see Chapter 8). In Entry 2, the vinyl alane was used as a nucleophile for opening an epoxide ring and extending the carbon chain by two atoms. In Entries 3 to 5, the vinyl alane adducts were converted to vinyl iodides. In Entry 6, the vinyl alane was converted to an ate reagent prior to reaction with formaldehyde. 

The key to the success of the vinyl epoxide route would reside in a nucleophilic ring-opening of vinyl epoxide 75. If this ring-opening proceeds regioselectively at C5 via a SN2 pathway, it would have been the most welcome at this stage of our efforts (Scheme 8.21). However, an array of conditions, including Pd(0)-mediated and Lewis acidic conditions, were screened over a period of almost 2 years, but we never observed the desired 1,4-diol 80. Instead, we could at various times see small... 

The regioselective ring-opening of epoxides 34 (R1 = Me, vinyl, Ph etc.) with aminolead compounds R23 PbNEt2, prepared from lithium diethylamide and R23 PbBr, gives good yields of the amino alcohols 3567. 

The 1,3-diene moiety in 227 which included the carbon atoms and CVC was oxidized to the l,4-dihydroxy-2-ene moiety in 238 that was further exploited to functionalise the A-ring as well as for the annulation of the C-ring (Scheme 37). The transformation of 227 into 238 was realized by a diastereoselective epoxidation of 227 to afford a vinyl epoxide (241) that was subjected to the conditions for a Palladium(O)-catalysed allylic substitution with the acetate ion [126]. The mechanism and the stereochemical course of the allylic substitution may be explained as depicted in Scheme 37. Sn2 ring opening of the protonated vinyl epoxide 241 by an anionic Pd complex proceeded with a (3Si) topicity to the r-allyl Pd com-... 

The vinyl silane 247 was unmasked to the ketone 250 by epoxidation, subsequent ring opening of the epoxide with HF in pyridine and concurrent cleavage of the THP and TBS protecting group (Scheme 39). The Stork-Jung annulation was completed through the treatment of the diketone 250 with sodium methoxide to mediate the intramolecular aldol con-... 

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