Intramolecular transesterification

NHCs have also been shown to promote the reaction of benzoins and methyl acrylate to produce y-butyrolactones (Scheme 44) [143], In the absence of dimeth-ylimidazolium iodide, the reaction does not proceed. The mechanism is still under investigation, although the authors propose that the transformation may proceed via a tandem transesterification/intramolecular Michael addition LXVin or Michael... 

Glycols may undergo intramolecular cyclization or cycHcaHy condense with other molecules to form a number of ring stmctures. Transesterification of carbonates with ethylene glycol produces ethylene carbonate [96-49-1] (eq. 4). Numerous materials catalyze carbonate transesterifications. 

For the dihydroxylation of the achiral enyne 11, Corey and co-workers have used the AD-mix-a, and without isolation of the intermediate diol 12, obtained the hydroxylactone 13 which is formed through an intramolecular transesterification ... 

Lipases also catalyze the intramolecular transesterification (lactonization) of hydroxy esters. Macrolactonization of a racemic hydroxy ester in the presence of PSL provided the corresponding (R)-lactone (Figure 6.22). This compound is the naturally occurring enantiomer of the pheromone produced by the merchant grain beetle [70]. Chemical macrolactonizations require high dilution to minimize... 

When (62) was treated with ethyl vinyl ether the cycloadduct (63) is afforded. If (62) is reacted with electron donating dienophiles such as allyl alcohols, transesterification and intramolecular cycloaddition occurs in the presence of a catalytic amount of distannoxane catalyst to give cis-fused polycyclic systems such as (64) <96T733>. 

The activating capacity of boronate groups can be combined with the ability for facile transesterification at boron to permit intramolecular reactions between vinyl-boronates and 2,4-dienols. 

Tandem transesterification and diastereoselective intramolecular 1,3-dipolar cycloaddition of a-methoxycarbonylnitrones with chiral allyl alcohols give polycyclic compounds in one step with high stereoselectivity (Scheme 8.14).76 Transition state Ain Scheme 8.14 is more favorable than B because B has severe steric interaction (allylic 1,3-strain).77... 

An additional example of a cycloamylose-induced rate acceleration which may be reasonably attributed to a conformational effect is the facilitation of the transfer of the trimethylacetyl group from the phenolic oxygen of 9 to the aliphatic oxygen of the adjacent hydroxymethyl group to form 10. This intramolecular transesterification is remarkably enhanced relative to a comparable intermolecular reaction,6 and occurs, at pH 7.0 and 25.5°, with a rate constant of 0.0352 sec-1 (Griffiths and Bender, 1972). An even larger rate enhancement is achieved upon inclusion of this material within the cyclohexaamylose cavity—fc2 = 0.16 sec-1. This fivefold acceleration cannot be satisfactorily explained either by a microsolvent effect which would be expected to depress the rate of the reaction or, at this pH, by covalent... 

The intramolecular transesterification occurs approximately 106 times faster than the intermolecular transfer of the trimethylacetyl group to surrounding water molecules (Griffiths and Bender, 1972). 

The isoquinolinium ylide 241 reacts with allyl alcohol in a [3+2] cycloaddition to give the tetracyclic product 242 (Equation 32) the primary cycloaddition product spontaneously undergoes an intramolecular transesterification to give the isolated furanone. Similarly, reaction of such ylides with vinylene carbonate gives the tetracycles 243 (Equation 33) <1988BCJ2513>. 

Hynninen and coworkers <99JCS(PT1)2403> used a similar approach to prepare phytochlorin-C6o diad 38 (Scheme 11). The protocol employed the pyrolysis of the natural chlorophyll a molecule 35, followed by transesterification and demetallation to furnish derivative 36. Subsequent oxidation of 36 with OsCU and NaI04 has allowed the synthesis of the formyl derivative 37, which was further used as precursor of the azomethinic ylide intermediate in the 1,3-DC reaction with Cm leading to the formation of diad 38. Photochemical studies revealed that this diad underwent a fast intramolecular photoinduced electron transfer in polar solvents such a benzonitrile <99JACS9378>. 

Compound 10 has also been used to quantify double Lewis acid activation by two cobalt (HI) ions [37]. In 12, the RNA analogue 2-hydroxypropyl-phenyl phosphate (HPPP) is coordinated to the dinu-clear cobalt site. It is well known that in this substrate the hydroxypropyl group is an efficient intramolecular nucleophile. Release of phenol by intramolecular cyclization is much faster than the reaction by nucleophilic attack of bridging oxide, as observed in 11. At pH >8, transesterification rate is linearly dependent on hydroxide concentration since OH" acts as an intermolecular base for the deprotonation of the hydroxypropyl group. The second order rate constant for the hydroxide-dependent cleavage is 4 x 105 times larger than the second-order rate constant for the hydroxide-dependent spontaneous transesterification of hy-droxypropyl-phenyl phosphate. 

The understanding of the SSP process is based on the mechanism of polyester synthesis. Polycondensation in the molten (melt) state (MPPC) is a chemical equilibrium reaction governed by classical kinetic and thermodynamic parameters. Rapid removal of volatile side products as well as the influence of temperature, time and catalysts are of essential importance. In the later stages of polycondensation, the increase in the degree of polymerization (DP) is restricted by the diffusion of volatile reaction products. Additionally, competing reactions such as inter- and intramolecular esterification and transesterification put a limit to the DP (Figure 5.1). 

Deprotonation of acidic C6 by DBU gives a carbanion, which undergoes a Michael reaction to CIO. The new carbanion at CIO can deprotonate C3 to give a new carbanion, and this can undergo an aldol reaction to C12. Now our two new C-C bonds have been formed. We still have to break C2-C6 and two C-0 bonds. The alkoxide at 013 can deprotonate MeOH, which can then add to C2. Fragmentation of the C2-C6 bond follows to give a C6 enolate. The C6 enolate then deprotonates 013, and intramolecular transesterification occurs to form the 013-C7 bond and to break the C7-09 bond. MeO then comes back and promotes El elimination across the C3-C12 bond to break the Cl2-013 bond and give the product. The intramolecular transesterification explains why C7 becomes an acid and C2 remains an ester in the product. 

Curtius degradation of the acyl azide and subsequent transesterification with tert-butanol and hydrolysis resulted in the A/-BOC amino acid 156. Compound 156 was readily cyclized with thionyl chloride, resulting in anhydride 157 diendo, R = H, Me diexo, R = H) in one step. In the cyclization step, acid chlorides were formed first and thereafter an intramolecular cyclization took place with loss of hydrogen chloride (93BSB227,93T1985). 

Enamine 106 (derived from Meldrum s acid), in a process of mono-decarboxylating transesterification and subsequent intramolecular alkylation, is cyclized to form enamino ester 107 (90H(31)1251). The direct route by flash vacuum thermolysis does not work in the case of 6/7 bicyclic 107. Methylene compound 109 originates (analogously to bicyclic 40b) from... 

Scheme 4 Intramolecular and intermolecular transesterification side reactions...

The formation of vinylcarbamates is restricted to secondary amines and also to terminal alkynes, which is in line with the formation of a metal vinylidene intermediate. It is noteworthy that even starting from secondary amines, the presence of a hydroxy group in propargylic alcohols drove the reaction towards the formation of fi-keto carbamates, resulting from initial Markovnikov addition of the carbamate anion to the triple bond followed by intramolecular transesterification [10]. The proposed general catalytic cycle which applies for the formation of vinylic carbamates is shown in Scheme 10.2. 

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