Terf-Butyl esters, preparation

For subsequent transformations, it was necessary to protect the amino and C-2 carboxyl groups of fra/w-4-hydroxy-L-proline 34. Throughout all of the synthetic work to be described, A-benzoyl amide protection was chosen as it was felt likely that such a functional group would be resistant to most reaction conditions. Initially, a C-2 terf-butyl ester was chosen in an attempt to maximize the stereoselectivity in the planned enamine alkylation reaction however, later experiments revealed that the more straightforward to introduce C-2 methyl ester was equally effective. The preparations for all of the derivatives used are described here. 

The terf-butyl esters are usually prepared by the reaction of an acid with isobutylene in the presence of an acidic catalyst. In the modified procedure instead of isobutylene, fert-butyl alcohol (f-BuOH) is used in the presence of heterogeneous acid catalyst. ... 

Another chromium(VI) oxidant for the preparation of uronic acids is PDC [28]. The oxidation is carried out in an aprotic solvent like dimethylformamide (DMF) or dichloromethane. Acetals and sulfides are stable under these conditions [59]. Although PDC is also used for oxidation of primary alcohols to aldehydes, use of a larger excess and/or a longer reaction time will give the carboxylic acid [60]. PDC can be further activated by addition of acetic anhydride which will shorten the reaction time [61]. If tert-butanol is added to the reaction, the terf-butyl ester can be obtained directly as shown by the conversion of 15 into 16 (O Scheme 5) [62]. Presumably, the intermediate aldehyde forms a hemiacetal with terf-butanol which is then further oxidized to the ester. 

Fractional distillation of the more volatile este CH3COOR in Eq. (5.15) is required to complete the reaction. Transestoification reactions have been used for the preparation of alkoxides of various metals including Zr, Ti, Ta, Nb, Al, La, Fe, Ga, and V. For example, zirconium rerf-butoxide has been prepared from the reaction between zirconium isopropoxide and terf-butyl ester followed by distillation of isopropyl acetate ... 

By adding one equivalent of alcohol to CDI at room temperature with or without base it is possible to isolate the imidazole-iV-carboxylate, which then reacts with a second mole of ROH to yield the carbonate. As in the case of alcoholysis of imidazolides, the reaction can be accelerated so effectively with catalytic amounts of NaOC2H5 or ImNa that it takes place in most cases exothermically, even at room temperature. However, tert-butyl alcohol, even when in excess, affords with CDI and base catalysis at room temperature only the imidazole-N-tert-butylcarboxylate, obviously for steric reasons. At higher temperature the carbonic ester is formed. Mixed carbonates such as ethyl benzyl carbonate or ethyl terf-butyl carbonate can be prepared with two different alcohols added sequentially.C9],[229]... 

For the condensation with the properly derivatized lysine part (112) 3 -terf-butyl-1,5-di-Af-hydroxysuccinimidyl citrate (113) was used (Chart 10). It was prepared from 1,5-dimethyl citrate by reaction with ferf-butyl acetate, alkaline hydrolysis of the methyl ester and coupling with W-hydroxysuccinimide by DCCI (257). 

Peroxy esters 67 were prepared in situ by the reaction of phosphonochloridate and terf-butyl hydroperoxide in diethyl ether. The peroxy ester 67 (R = Ph) is stable for several days at 5 °C in diethyl ether. Most peroxyphosphates 67 with an RO group other than ferf-butylperoxy are unstable even for short periods . This synthetic method was successfully applied for synthesis of ring peroxyphosphates 70 and 71 as colorless oils. They are very unstable and decompose at 25 °C to yield polymeric products and volatile side products . ... 

Similarly to AVal derivatives, ALeu methyl ester was prepared from Leu methyl ester in 90% yield using terf-butyl hypochlorite as the chlorinating reagent and DBU as the base for dehydrochlorination. A number of peptides have been prepared using this approach.124-2X1 The other method reported by Shin et al.1841 involves a one-pot procedure using N-carboxy-Z-a, 3-didehydroleucine anhydride 61 (ALeu-NCA) to give 62 (Scheme 21) 129 841 a ANCA can also be used to prepare Alle derivatives.1841... 

Trimethylsilyl esters are intermediate products in the cleavage of terf-butyl, benzyl, or methyl esters using trimethylsilyl iodideP and also in the preparation of methyl esters using trimethylsilyl chloride. An interesting feature of the sUyl esters is the protection of acids against reduction with hydroborating agents.P d... 

Dihydropyridine (1,4-DHP) derivatives have been widely explored as a consequence of their pharmacological profile and as the most important calcium channel modulators. Nifedipine 2 represents the prototype 1,4-DHP structure found useful in both antianginal and antihypertensive treatment that has been approved for clinical use. The liquid phase-bound )3-keto esters 31(a-c) were prepared by transesterification of methyl or terf-butyl )3-oxo carboxylates 30(a, b) with the ionic liquid phases [HOC mimJlPFg] 29a and [HOC mimJlBF ] 29b under solvent-free microwave irradiations (Fig. 12.49) [33],... 

The reaction is reversible and is catalysed by strong acids 95% sulfuric acid, organic sulfonic acids and ZnCl2146 or boron trifluoride147-149 are especially effective. However, if more than 10% of boron trifluoride is used, polymerization intervenes seriously. For good yields of esters the reaction temperature must be kept as low as possible. The preparation of terf-butyl o-benzoylbenzoate will be detailed as an example 150... 

Vinyl acetate is fairly water soluble and somewhat deficient in hydrolytic stability. The incorporation of a vinyl ester of the higher branched carboxylic acids into a copolymer system improves the resistance of the product to hydrolysis. Thus the latex formed in Preparation 5-10 has been used in formulating exterior paints. Copolymers of vinyl acetate with increasing concentrations of vinyl 2-ethylhexanoate (VEH) were prepared by a procedure similar to that of Preparation 5-10, except that the initiator system consisted of terf-butyl hydroperoxide and sodium formaldehyde sulfoxylate, and the buffer was sodium acetate. As the level of VEH increased, the hydrolytic stability of the copolymer increased significantly [169],... 

Initial attempts by Fujita et al. to effect alkylation at the methyl group of 2-methyl-4-carbo-terf-butoxylcarbonyloxazole (116) entailed blockage of the 5 position with a trimethylsilyl group by preparation of 133 (76). Treatment of 133 with n-butyllithium, rm-butyllithium, and sodium hydride, followed by methyl iodide in each case, yielded the ketone 134, the 5-rm-butyl derivative 135, and the starting ester 116, respectively, with no evidence of deprotonation-alkylation of the methyl group (76). Resort was therefore made to halo-... 

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