Reacetylation

Ahl and Reichstein have pointed out that though it is certain that the structure of emetine includes one, and possibly two, 6 7-dimethoxy-tetrahydroisoquinoline nuclei, the suggestions so far made as to the nature of the rest of the molecule are speculative. They investigated the Hofmann degradation of A-acetylemetine, m.p. 97-9°. This forms a monomethiodide, m.p. 2ia-6°, from which, by the action of silver oxide and potassium hydroxide, followed by eautious tbermal deeomposition and reacetylation,... 

Deghenghi et al studied the reductive alkylation of 16-dehydropregneno-lone acetate in ammonia-tetrahydrofuran using excess lithium and alkyl iodides. Alkylation with methyl iodide followed by reacetylation of the 3-hydroxyl group affords 17a-methylpregnenolone acetate in 20% yield after purification by column chromatography. Ethyl iodide affords the 17a-ethyl analog in 40% yield, but n-propyl iodide affords the 17a-propyl compound in only a 12 % yield. 

Reaction of 17j -acetoxy-3,3-ethylenedioxy-5a-androstan-l-one (1) with methylmagnesium bromide followed by treatment with acid and reacetylation affords the 1-methyl-A -3-ketone (3). The configuration of carbon-1 of the intermediate (2) has not been established/ ... 

Reaction of 3 -acetoxypregn-5-en-20-one (71) with isohexylmagnesium bromide proceeds smoothly, to give the 20-hydroxy compound (72a) after reacetylation. 

When a fourfold excess of methylmagnesium bromide and catalytic amounts of cuprous chloride in tetrahydrofuran are employed, the reaction proceeds within 30 minutes at room temperature. Under these conditions the A -l-methyl-5a-3-ketone (15) affords the 1,1-dimethyl compound (16) in 50% yield after reacetylation. The A -l-methyl-3-methylene steroid (17) is also isolated in 23% yield. 

With l,4,6-triene-3-ones, addition occurs exclusively at the 1,2-double bond to yield la-methyl-A -S-ketones. Grignard reaction and reacetylation of 17)5-acetoxyandrosta-l,4,6-trien-3-one (23) gives the la-methyl-4,6-dien-3-one (24) in a crude yield of 85%. ° ... 

The 1 l)5-hydroxyl group in the A -3-ketone (33) favors attack towards the j -face of the molecule, possibly due to complex formation with the methyl-magnesium halide. Thus, the )5-methyl isomer (34), is obtained as the major product in 22% yield after reacetylation. 

The presence of hydrolyzable long chain branches in PVAc was established by McDowell and Kenyon206 in 1940. They observed a reduction in molecular weight obtained on successively hydrolyzing and reacetylating samples of PVAc. Only branches to the acetate methyl will be lost on hydrolysis of the polymer i.e. on conversion of PVAc to PVA. 

Reacetylation of chitosan under proper conditions leads to products having the same solubility. Experiments showed that the amount of acetic anhydride was the most important factor affecting the N-acetylation degree of the chitosan. The effect of the means of adding materials and the amount of solvent on the reaction could not be ignored [70]. 

Most of the experimental results on chitosans have been obtained with freeze-dried modified chitosans, such as methylpyrroHdinone chitosan, and reacetylated chitosan, as mentioned below. 

Reacetylated chitin gels were used to treat leg and decubitus ulcers in paraplegic subjects [309]. Selected preparative conditions permitted to obtain a self-sustaining gel useful for this use. The treatment periods were 63-182 days and complete healing was obtained. 

To work with a homogeneous series of chitosans with the same distribution of degrees of polymerization but different DAs, a starting sample of chitosan of DA 5.2% was reacetylated. Acetylation was performed with Acetic Anhydride in a hydroalcoholic medium. Characteristics of the various prepared chitosan samples are given in table 1. (Adapted from Montembault et ah, 2005)... 

In the above-mentioned example of the polymer-analogous saponification of poly(vinyl acetate) the reactant and the product differ in their properties, for example, in their solubility however, both compounds have the same average degree of polymerization. The poly(vinyl alcohol) obtained by saponification can, in principle, be esterified back to poly(vinyl acetate) with the original molecular weight the reacetylated polymer then has the same properties as the original material. The viscosity number may be used to check whether in fact any chain scission has occurred during the reaction sequence of saponification and reacetylation (see Example 5-1). 

Poly(vinyl Alcohol) by Transesterification of Poly(vinyl Acetate) Reacetylation of Poly(vinyl Aicohol)... 

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