Acetates, substitution reaction

A combined experimental and computational approach has been undertaken to identify the origin of syn/anti diastereoselectivity in two types of crotylation reactions of aldehydes and ketones (i) multi-component crotylations of simple aldehydes/ketones and (ii) acetal substitution reactions of aldehyde dimethyl acetals with E- and Z-configured crotyltrimethylsilane.175 The stereochemical outcome is nearly identical in the two reactions, and the computational results suggest that this is due to near identical mechanisms an SN1 process involving attack of O-methyl-substituted carboxenium ions by crotylsilane. 

Lewis Acid Promoted Acetal Substitution Reactions. Santelli, M. Pons. J.-M. In Lewis Acids and Selectivity in Organic Synthesis CRC Press Boca Raton, 1996 pp 185-230. 

Reaction sequences involving halogen transfer, followed by non-radical interception of the alkyl iodide or bromide formed can allow for the trapping of products arising from less exothermic or even endothermic atom transfer additions and are exemplified in Scheme 11. Yoon [30] and Curran [31] have demonstrated that the a-halo ethers formed upon addition to vinyl ethers can be trapped with alcohols, leading to formation of acetals. Substitution reactions on the heteroaromatics pyrrole and indole have been carried out through a sequence of steps involving I- or Br-... 

The Use of Swain-Scott Parameters to Determine the Mechanism of Some Acetal Substitution Reactions... 

The origin of enantioselectivity which accompanies the allylic acetate substitution reaction of l,3-diphenylprop-2-enyl acetate with dimethyl malonate, catalysed by palladium complexes having chiral imine-sulfide chelate ligands, has been explored. 

The reactivity of five-membered rings with one heteroatom to electrophilic reagents has been quantitatively compared in a variety of substitution reactions. Table 2 shows the rates of substitution compared to thiophene for formylation by phosgene and iV,AT-dimethylfor-mamide, acetylation by acetic anhydride and tin(IV) chloride, and trifluoroacetylation with trifluoroacetic anhydride (71AHC(13)235). 

Rate data are also available for the solvolysis of l-(2-heteroaryl)ethyl acetates in aqueous ethanol. Side-chain reactions such as this, in which a delocalizable positive charge is developed in the transition state, are frequently regarded as analogous to electrophilic aromatic substitution reactions. In solvolysis the relative order of reactivity is tellurienyl> furyl > selenienyl > thienyl whereas in electrophilic substitutions the reactivity sequence is furan > tellurophene > selenophene > thiophene. This discrepancy has been explained in terms of different charge distributions in the transition states of these two classes of reaction (77AHC(21)119>. 

Reaction between Cy symmetric diols and a,/ -unsaturated aldeliydes yield cliiral ediylenic acetals diat undergo copper-mediated substitution reactions. Widi aiyl or... 

It may not be appropriate to compare the thermal stability characteristics of VC/VAc copolymer to that of a VC homopolymer (PVC). The copolymerization would involve different kinetics and mechanism as compared to homopolymerization resulting structurally in quite different polymers. Hence, copolymerization of VC with VAc cannot be regarded as a substitution of chlorines in PVC by acetate groups. To eliminate the possibility of these differences Naqvi [45] substituted chlorines in PVC by acetate groups, using crown ethers (18-crown-6) to solubilize potassium acetate in organic solvents, and studied the thermal stability of the modified PVC. Following is the mechanism of the substitution reaction ... 

What product would you expect to obtain from a nucleophilic substitution reaction of (S)-2-bromohexane with acetate ion, CH3CO2- Assume that inversion of configuration occurs, and show the stereochemistry of both reactant and product. 

A particularly common cr-substitution reaction in the laboratory is the halogenation of aldehydes and ketones at their a- positions by reaction with Cl2, Br2, or I2 in acidic solution. Bromine in acetic acid solvent is often used. 

Amino groups are often protected as their tert-butoxycarbonvl amide, or Boc, derivatives. The Boc protecting group is introduced by reaction of the amino acid with di-fert-butyl dicarbonate in a nucleophilic acyl substitution reaction and is removed by brief treatment with a strong organic acid such as trifluoro-acetic acid, CF3C02H. 

This is a typical nucleophilic acyl substitution reaction, with p-hvdroxyaniline as the nucleophile and acetate ion as the leaving group. 

Protonation and deprotonation reactions of corroles have already been mentioned (see Introduction). Attempts to achieve electrophilic substitution reactions, at the corrole, e.g. Friedel-Crafts acylation, have been unsuccessful.1 Heating corroles with acetic anhydride yields the corresponding 21-acetyl derivatives l.8a,b... 

This dry ozonation procedure is a general method for hydrox-ylation of tertiary carbon atoms in saturated compounds (Table 1). The substitution reaction occurs with predominant retention of configuration. Thus cis-decalin gives the cis-l-decalol, whereas cis- and frans-l,4-dimethylcyclohexane afford cis- and trans-1,4-dimethylcyclohexanol, respectively. The amount of epimeric alcohol formed in these ozonation reactions is usually less than 1%. The tertiary alcohols may be further oxidized to diols by repeating the ozonation however, the yields in these reactions are poorer. For instance, 1-adamantanol is oxidized to 1,3-adamantane-diol in 43% yield. Secondary alcohols are converted to the corresponding ketone. This method has been employed for the hydroxylation of tertiary positions in saturated acetates and bromides. 

Substitution reactions of 36 with nucleophilic acetate ions, followed by acetylation, furnished two carba-sugar pentaacetates of the a-DL-ido (38 31 % yield) and a-DL-galacto configurations (19 10%). The reactions seemed to involve formation of an intermediary 2,3-cyclic acetoxonium ion. ... 

When 35 was heated in acetic acid containing hydrogen bromide, the tribromide 46 was obtained as a single product in 74% yield. Debromina-tion of 46 with zinc dust in acetic acid furnished the cyclohexene derivative 47, which was converted into compound 48 by osmium tetraoxide hydroxyl-ation and acetylation. The substitution reaction of 48 with acetate ions provided carba-a-DL-glucopyranose pentaacetate (49), which gave the carba-sugar 50 on hydrolysis. ... 

Sulfoximines bearing a chiral sulfur atom have recently emerged as valuable ligands for metal-catalysed asymmetric synthesis.In particular, C2-symmetric bis(sulfoximines), such as those depicted in Scheme 1.51, were applied to the test reaction, achieving enantioselectivities of up to 93% ee. The most selective ligand (R = c-Pent, R = Ph) of the series was also applied to the nucleophilic substitution reaction of l,3-diphenyl-2-propenyl acetate with substituted malonates, such as acetamido-derived diethylmalonate, which provided the corresponding product in 89% yield and 98% ee. 

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