Ortho-alkylation

Acylated Corticoids. The corticoid side-chain of (30) was converted iato the cycHc ortho ester (96) by reaction with a lower alkyl ortho ester RC(OR )2 iu benzene solution ia the presence of i ra-toluenesulfonic acid (88). Acid hydrolysis of the product at room temperature led to the formation of the 17-monoesters (97) ia nearly quantitative yield. The 17-monoesters (97) underwent acyl migration to the 21-monoesters (98) on careful heating with. In this way, prednisolone 17a,21-methylorthovalerate was converted quantitatively iato prednisolone 17-valerate, which is a very active antiinflammatory agent (89). The iatermediate ortho esters also are active. Thus, 17a,21-(l -methoxy)-pentyhdenedioxy-l,4-pregnadiene-liP-ol-3,20-dione [(96), R = CH3, R = C Hg] is at least 70 times more potent than prednisolone (89). The above conversions... 

For another indirect method, in this case for alkylation ortho to an amino group, see Gassman Parton Tetrahedron Lett. 1977, 2055. 

The close proximity of functional groups in 1,2-disubstituted benzenes can sometimes bring about an unexpected reactivity. Attempts to N-alkylate ortho-nitroani-lines under strongly basic reaction conditions, for instance, lead to the formation of N-alkoxybenzimidazoles (Scheme 6.10). The main force driving this reaction is the formation of an imidazole ring, a heteroarene with high resonance energy and thermal stability. 

In other systems, such as in some Friedel-Crafts alkylations, ortho-substitution is desirable. For example, extensive alkylation at both the ortho-and para-positions of phenol with formaldehyde in the presence of an acid catalyst yields highly branched novolac phenolic resin prepolymers [Eq. (26)]. 

Release of hydrocortisone. Initial exploratory in vitro studies designed to ascertain the usefulness of this polymer system have been carried out with a polymer based on 1,2,6-hexanetriol and various alkyl ortho esters. In these studies 2 wt% hydrocortisone was physically mixed into the ointment and the mixture placed into an erosion cell. A pH 7.4 buffer solution was then pumped across the cell at 9.5 ml/h, samples collected using an automatic fraction collector and analyzed for hydrocortisone by HPLC. The same study was also carried out by mixing into the ointment 2 wt% hydrocortisone and 2 wt% adipic acid. Results of these studies are shown in Fig. 32 [49]. The data clearly shows that without the incorporation of an acidic excipient erosion rate of the polymer is so slow that no hydrocortisone is released for two days. However, when an acidic excipient is mixed into the polymer, a fairly constant release is obtained. 

The following reaction parameters can be used to compare the catalyst properties i) conversion of m-cresol, ii) ratio between the selectivity to products of ortho C-alkylation and that of para C-alkylation (ortho/para ratio), and iii) ratio between the selectivity to 3-MA and the selectivity of the products of C-alkylation (0/C ratio). The importance of each of the above mentioned parameters for the reaction of phenol alkylation has been discussed in a previous paper [4]. 

An efficient and stereoselective synthesis of 3,4,6-tri-O-acetyl-ot-D-glucopyranose 1,2-exo-alkyl ortho-acetates (67) has been achieved using DMF dialkyl acetals (68) and tetrabutyl ammonium bromide on acetobromoglucose. The DMF acetal from 1,2 3,4-di-0-isopropylidene bC-D-galactopyranose (69) was also prepared and used to synthesize the mixed sugar orthoester (70). 

Two mechanisms can be envisioned for the Pechmann condensation (1) acid-catalyzed transesterification of the P-keto ester followed by acid-catalyzed cyclodehydration of the resulting aryl acetoacetate (4), or (2) acid-catalyzed Friedel-Crafts-like phenol alkylation ortho to the phenolic hydroxyl group) followed by intramolecular transesterification/cyclization of the resulting o-hydroxycinnamic acid ester (5). 

Another cure mechanism for silanol-terminated polymers is the catalyzed (usually by tin) reaction with alkoxysilyl species exemplified by alkyl ortho- and polysilicates. Schematically, it is... 

Figure 20 Interfacial tensions and solubilization parameters for an anionic system containing alkyl ortho xylene sulfonate, tert-amyl alcohol, and an oil mixture. (From Ref. 63.)...

Tertiary < secondary < n-alkyl Ortho < meta, para... 

Interconversion of Acyl (Meta Director) with Alkyl (Ortho, Para Director)... 

Gross and co-workers reported an interesting direct aldehyde synthesis using alkyl ortho-formates that is particularly suitable for phenols, as alkylformates are unable to act as formylating agents under Friedel-Crafts conditions. Aldehydes of phenol are obtained in good yields when phenols react with orthoformates in the presence of aluminum chloride (Eq 1.12). 

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