Acetate intermediate

The high cost of coal handling and preparation and treatment of effluents, compounded by continuing low prices for cmde oil and natural gas, has precluded significant exploitation of coal as a feedstock for methanol. A small amount of methanol is made from coal in South Africa for local strategic reasons. Tennessee Eastman operates a 195,000-t/yr methanol plant in Tennessee based on the Texaco coal gasification process to make the methyl acetate intermediate for acetic anhydride production (15). 

Coe et al. reported an efficient modification for the preparation of /-substituted indole analogs for biology screening in good yield. The intermediate P-nitrostyrene 44, prepared from the condensation of 43 with DMFDMA, underwent methanolysis and reduction to provide the aniline acetal intermediate 45. Alkylation of amine 45 was carried out employing standard conditions of reductive alkylation to provide A-alkyl analogs represented by 46. The indole 47 was generated by formation of the oxonium ion (from 46) under acidic conditions, followed by cyclization, accompanied by loss of methanol. 

Schemes 28 and 29 illustrate Curran s synthesis of ( )-hirsutene [( )-1]. Luche reduction58 of 2-methylcyclopentenone (137), followed by acetylation of the resulting allylic alcohol, furnishes allylic acetate 138. Although only one allylic acetate stereoisomer is illustrated in Scheme 28, compound 138 is, of course, produced in racemic form. By way of the powerful Ireland ester enolate Clai-sen rearrangement,59 compound 138 can be transformed to y,S-unsaturated tm-butyldimethylsilyl ester 140 via the silyl ketene acetal intermediate 139. In 140, the silyl ester function and the methyl-substituted ring double bond occupy neighboring regions of space, a circumstance that favors a phenylselenolactonization reac-...

N,O-acetal intermediate 172, y,<5-unsaturated amide 171. It is important to note that there is a correspondence between the stereochemistry at C-41 of the allylic alcohol substrate 173 and at C-37 of the amide product 171. Provided that the configuration of the hydroxyl-bearing carbon in 173 can be established as shown, then the subsequent suprafacial [3,3] sigmatropic rearrangement would ensure the stereospecific introduction of the C-37 side chain during the course of the Eschenmoser-Claisen rearrangement, stereochemistry is transferred from C-41 to C-37. Ketone 174, a potential intermediate for a synthesis of 173, could conceivably be fashioned in short order from epoxide 175. 

Hydrogenolysis of esters to aldehydes or alcohols needs high temperatures and high pressures. Moreover, it leads to the formation of acids, alcohols, and hydrocarbons. In contrast, bimetallic M-Sn alloys (M = Rh, Ru, Ni) supported on sihca are very selective for the hydrogenolysis of ethyl acetate into ethanol [181]. For example while the selectivity to ethanol is 12% with Ru/Si02, it increases up to 90% for a Ru-Sn/Si02 catalyst with a Sn/Ru ratio of 2.5 [182]. In addition, the reaction proceeds at lower temperatures than with the classical catalysts (550 K instead of temperatures higher than 700 K). The first step is the coordination of the ester to the alloy (Scheme 46), and most probably onto the tin atoms. After insertion into the M - H bond, the acetal intermediate decomposes into acetaldehyde and an ethoxide intermediate, which are both transformed into ethanol under H2. 

This procedure illustrates a fundamentally new method for constructing substituted tetrahydrofurans.5-10 This practical method assembles the tetrahydrofuran ring from allylic diol and carbonyl components and in the process forms three ring bonds C(2)-C(3), C(4)-C(5) and 0-C(5). Both aldehydes (eq 1) and ketones (illustrated in the present procedure) can be employed as the carbonyl component. Although it is often convenient to isolate the acetal intermediate, conversion to the 3-acyltstrahydrofuran can also be accomplished in many cases by the direct reaction of the diol and carbonyl components.8 High ds stereoselectivity (at least 20 1) is observed in the preparation of tetrahydrofurans that contain single side chains at carbons 2 and 5 (eq 1). The kinetically controlled product also has the cis relationship of these side chains and the 3-acyl substituent. 

The protocol mentioned above for the preparation of dihydropyrans was also applied to the amidopalladation of allenes 8 and the subsequent RCM of N,0-acetal intermediates 216. This method leads to the expected five-, six- and seven-membered nitrogen-containing heterocycles 217 with good efficiency (Scheme 8.53) [118a],... 

Caution All operations should be conducted in an efficient fume hood. The chloromethyl ether acetate intermediate is potentially toxic. 

Scheme 4 Bismuth(III) triflate-catalyzed synthesis of substituted 3,4-dihydro-2//-1-benzopyrans using an acetal intermediate...

Co(III)] complexes. For example, the coupling of 3-halocholestanes (333) and Michael acceptors affords epimeric mixtures of the 3-homologated steroids (334). The electrochemical nucleophilic acylation of a, 3-unsaturated aldehydes, a,3-unsaturated ketones and a,(3-unsaturated nitriles with acyl anhydrides affords adducts (335) in moderate yields.226a-b Similarly, the reduction of N-methyloxazolinium salts (336) affords die A, O-acetal intermediates (337) which are readily hydrolyzed (Scheme 102).226c... 

The aluminoxy acetal intermediates in ester reduction using DIBALH can be trapped with either TMSOTf93 or TMS imidazole94 to give monosilyl acetals (equation 21). 

Esterification of salicylaldehydes with acetic acid derivatives forms 2-formylphenyl acetate intermediates, which cyclize in high yield to afford 3-substituted coumarins (Scheme 168) <2004SC3129>. 

Oxidation of p-alkoxyphenols. These substrates are oxidized by QH5I(OCOCF3)21 or QH5I(OAc)22 to p-quinones in high yield, probably via hemi-acetal intermediates. CAN can also be used as oxidant but yields are lower. 

The treatment of various chalcones 210 with HTIB or DAIB-TSOH in a methanolic medium leads to the / -keto dimethylacetals 211 (Scheme 59) (02S2490). This rearrangement can also be effected with poly[4-(diacetoxy)-iodo]styrene and has been developed into a one-pot synthesis of isoflavones (e.g., 213) from benzoyl-protected 2/-hydroxychalcones (e.g., 212), via base-catalyzed cyclization of the acetal intermediates. 

In examining the mechanism leading to the nucleophile-mediated conversion of an ester to a ketone, initial addition of a nucleophile to the carbonyl results in formation of a hemi-acetal intermediate. Subsequent collapse of the hemiacetal intermediate liberates a ketone and an alkoxide leaving group. This mechanistic sequence, illustrated in Scheme 7.19... 

Oxonium ions have three bonds to a positively charged oxygen atom. All three bonds can be a bonds as in H30+ or Meerwein ssalt, trimethyloxoniumfluoroborate, a stable (though reactive] compound described in Chapter 21, or one bond can be a it bond as in the acetal intermediate. The term oxonium ion" describes either of these structures. They are like alkylated ethers or O-alkylated carbonyl compounds. 

If the acetal is now hydrolysed, the new stereogenic centre is revealed as an alkylated version of the starting material. It may appear that the alkylation has happened stereo specifically with retention, but what has really happened is that the new stereogenic centre in the acetal intermediate has relayed the stereochemical information through the reaction. 

Nitrocellulose and Cellulose Acetate. Although nitrocellulose and cellulose acetate intermediates have been made and regenerated to form cellulose fibers, neither of these historical processes is still in operation. 

Ketene N, O-acetals are converted to 1,1-enediamines when reacted with amines. For example, compound 45 has been prepared by substitution of the methoxy with amine (equation 16)79. Symmetric 1,1-enediamines are obtained when both the alkoxy and the amino substituents are displaced by the amine employed81. In this way, the reaction between ketene N, O-acetals 50 and piperidine leads to 1,1-dipipefidinoethylene (35) (equation 17)81. Alternatively, 35 can be prepared from the reaction of piperidine with triethyl orthoacetate82,83 or with ethoxyacetylene84, reactions which probably proceed via a ketene TV, 0-acetal intermediate. 

Synthesis of oxathionanes from tu-bromo ketone 108, which is formally a [5+4]-type cyclization, requires Lewis acid-catalyzed cyclic acetal intermediate formation. It was further transformed into the corresponding oxathionanes 109 and 110 using a two-step reductive procedure (Scheme 18, Section 14.10.5.6.1) <20020L3047>. 

Alkylation of umbelliferone with chloromethyl esters of various ahphatic acids affords the corresponding acyloxymethyl ethers (e.g. 27) in good yields [38]. These substrates undergo lipase/esterase-catalyzed hydrolysis in buffer to form umbelliferone (Scheme 1.9) [39]. The mechanism probably involves an unstable hemi-acetal intermediate, which spontaneously reacts to formaldehyde andumbeUiferone... 

The one-step synthesis of isoamyl butyrate from isoamyl alcohol and n-butyral-dehyde, possibly through the formation and subsequent oxidation of an acetal intermediate, could represent a special case in TS-1 catalysis, since the oxidant was molecular oxygen [114]. The authors did not advance any mechanistic hypothesis. -Butyl hydroperoxide, however, produced in situ by the autoxidation of n-butyraldehyde, could have been the true oxidant, by virtue of its dimensional compatibility with the narrow pores of TS-1. 

Olefinic aldehydes have been prepared by bromination of the diethyl-acetal derivatives followed by dehydrobromination (cf. Acetals and Ketals) the unsaturated aldehydes are readily liberated by mild acid treatment of their acetals. Alkoxy aldehydes have also been synthesized through acetal intermediates, which in turn are prepared from sodium alkoxides and bromoacetals. ... 

Acetone was suggested to be formed by oxidation of acetaldehyde followed by coupling of the resulting acetate intermediates [83] ... 

Reduction of aryl ketones using PhaSnH and acetyl chloride has been reported. The stepwise process is indicated in Scheme 18. Depending on the nature of the carbonyl carbon, chloride and/or acetate intermediates are formed, of which the latter is trapped, but the former is further reduced by the tin hydride. 

Bowker M, Cassidy TJ, AUen MD, Li Y (1994) Surface explosions of acetate intermediates on Rh crystals and catalysts. Surf Sci 309 143... 

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