05,6 protection, acetal protecting

Method B. In a 500 ml. round-bottomed flask, provided with a reflux condenser protected by a cotton wool (or calcium chloride) drying tube, place 59 g. of succinic acid and 102 g. (94-5 ml.) of redistilled acetic anhydride. Reflux the mixture gently on a water bath with occasional shaking until a clear solution is obtamed ca. 1 hour), and then for a further hour to ensure the completeness of the reaction. Remove the complete assembly from the water bath, allow it to cool (observe the formation of crystals), and finally cool in ice. Collect the succinic anhydride as in Method A. The yield is 45 g., m.p. 119-120°. 

The solid appears to be a mixture of the complexes CH,COOH.BF, and 2CH COOH.BF,. The latter appears to be a liquid and is alone soluble in ethylene dichloride the former is a solid. The solid moiioocetic acid complex is obtained by saturating an ethylene dichloride solution of acetic acid with boron trifluoride, filtering and washing the precipitate with the solvent it is hygroscopic and should be protected from moisture. It may be used as required 0-75 mol is employed with 0-26 mol of ketone and 0 6 mol of anhydride. 

We can either protect the two hydroxyl groups in A as a cychc acetal or use four mols of PhMgBr and waste two of them. 

Selective reduction of a benzene ring (W. Grimme, 1970) or a C C double bond (J.E. Cole, 1962) in the presence of protected carbonyl groups (acetals or enol ethers) has been achieved by Birch reduction. Selective reduction of the C—C double bond of an a,ft-unsaturated ketone in the presence of a benzene ring is also possible in aprotic solution, because the benzene ring is redueed only very slowly in the absence of a proton donor (D. Caine, 1976). 

C—C double bonds may be protected against electrophiles by epoxidation and subsequent removal of the oxygen atom by treatment with zinc and sodium iodide in acetic acid (J.A. Edwards, 1972 W. Kndll, 1975). Halogenation has often been used for protection, too. The C—C double bond is here also easily regenerated with zinc (see p. 138, D.H.R. Barton, 1976). 

Six protective groups for alcohols, which may be removed successively and selectively, have been listed by E.J. Corey (1972B). A hypothetical hexahydroxy compound with hydroxy groups 1 to 6 protected as (1) acetate, (2) 2,2,2-trichloroethyl carbonate, (3) benzyl ether, (4) dimethyl-t-butylsilyl ether, (5) 2-tetrahydropyranyl ether, and (6) methyl ether may be unmasked in that order by the reagents (1) KjCO, or NH, in CHjOH, (2) Zn in CHjOH or AcOH, (3) over Pd, (4) F", (5) wet acetic acid, and (6) BBrj. The groups may also be exposed to the same reagents in the order A 5, 2, 1, 3, 6. The (4-methoxyphenyl)methyl group (=MPM = p-methoxybenzyl, PMB) can be oxidized to a benzaldehyde derivative and thereby be removed at room temperature under neutral conditions (Y- Oikawa, 1982 R. Johansson, 1984 T. Fukuyama, 1985). 

We shall describe a specific synthetic example for each protective group given above. Regiosdective proteaion is generally only possible if there are hydroxyl groups of different sterical hindrance (prim < sec < tert equatorial < axial). Acetylation has usually been effected with acetic anhydride. The acetylation of less reactive hydroxyl groups is catalyzed by DMAP (see p.l44f.). Acetates are stable toward oxidation with chromium trioxide in pyridine and have been used, for example, for protection of steroids (H.J.E. Loewenthal, 1959), carbohydrates (M.L. Wolfrom, 1963 J.M. Williams, 1967), and nucleosides (A.M. Micbelson, 1963). The most common deacetylation procedures are ammonolysis with NH in CH OH and methanolysis with KjCO, or sodium methoxide. 

Trichloro- and 2,2,2-tribromoethoxycarbonyl (Tceoc and Tbeoc) protecting groups are introduced with the commercially available 2,2,2-trihaloethyl chloroformates. These derivatives are stable towards CrOj and acids, but can smoothly be cleaved by reduction with zinc in acetic acid at 20 °C to yield 1,1-dihaloethene and CO. Several examples in lipid (F.R. Pfeiffer, 1968, 1970) and nucleotide syntheses (A.F. Cook, 1968) have been described. 

The blocking and deblocking of carboxyl groups occurs by reactions similar to those described for hydroxyl and amino groups. The most important protected derivatives are /-butyl, benzyl, and methyl esters. These may be cleaved in this order by trifluoroacetic acid, hydrogenolysis, and strong acid or base (J.F.W. McOmie, 1973). 2,2,2-Trihaloethyl esters are cleaved electro-lytically (M.F. Semmelhack, 1972) or by zinc in acetic acid like the Tbeoc- and Tceoc-protected hydroxyl and amino groups. 

The most commonly used protected derivatives of aldehydes and ketones are 1,3-dioxolanes and 1,3-oxathiolanes. They are obtained from the carbonyl compounds and 1,2-ethanediol or 2-mercaptoethanol, respectively, in aprotic solvents and in the presence of catalysts, e.g. BF, (L.F. Fieser, 1954 G.E. Wilson, Jr., 1968), and water scavengers, e.g. orthoesters (P. Doyle. 1965). Acid-catalyzed exchange dioxolanation with dioxolanes of low boiling ketones, e.g. acetone, which are distilled during the reaction, can also be applied (H. J. Dauben, Jr., 1954). Selective monoketalization of diketones is often used with good success (C. Mercier, 1973). Even from diketones with two keto groups of very similar reactivity monoketals may be obtained by repeated acid-catalyzed equilibration (W.S. Johnson, 1962 A.G. Hortmann, 1969). Most aldehydes are easily converted into acetals. The ketalization of ketones is more difficult for sterical reasons and often requires long reaction times at elevated temperatures. a, -Unsaturated ketones react more slowly than saturated ketones. 2-Mercaptoethanol is more reactive than 1,2-ethanediol (J. Romo, 1951 C. Djerassi, 1952 G.E. Wilson, Jr., 1968). 

Since (A) does not contain any other functional group in addition to the formyl group, one may predict that suitable reaction conditions could be found for all conversions into (A). Many other alternative target molecules can, of course, be formulated. The reduction of (H), for example, may require introduction of a protecting group, e.g. acetal formation. The industrial synthesis of (A) is based upon the oxidation of (E) since 3-methylbutanol (isoamyl alcohol) is a cheap distillation product from alcoholic fermentation ( fusel oils ). The second step of our simple antithetic analysis — systematic disconnection — will now be exemplified with all target molecules of the scheme above. For the sake of brevity we shall omit the syn-thons and indicate only the reagents and reaction conditions. 

Hydroxylysine (328) was synthesized by chemoselective reaction of (Z)-4-acet-oxy-2-butenyl methyl carbonate (325) with two different nucleophiles first with At,(9-Boc-protected hydroxylamine (326) under neutral conditions and then with methyl (diphenylmethyleneamino)acetate (327) in the presence of BSA[202]. The primary allylic amine 331 is prepared by the highly selective monoallylation of 4,4 -dimethoxybenzhydrylamine (329). Deprotection of the allylated secondary amine 330 with 80% formic acid affords the primary ally-lamine 331. The reaction was applied to the total synthesis of gabaculine 332(203]. 

Direct nitration of aniline and other arylamines fails because oxidation leads to the formation of dark colored tars As a solution to this problem it is standaid practice to first protect the ammo group by acylation with either acetyl chloride or acetic anhydride... 

Alternatively the benzyloxycarbonyl protecting group may be removed by treat ment with hydrogen bromide m acetic acid... 

Step 2 The Boc protecting group is removed by treatment with hydrochloric acid m dilute acetic acid After the resin has been washed the C terminal ammo acid IS ready for coupling... 

Removal of the ketal protecting groups, followed by oxidation with sodium dichromate ia acetic acid gave... 

Pos twe-Tone Photoresists. The ester, carbonate, and ketal acidolysis reactions which form the basis of most positive tone CA resists are thought to proceed under specific acid catalysis (62). In this mechanism, illustrated in Figure 22 for the hydrolysis of tert-huty acetate (type A l) (63), the first step involves a rapid equihbrium where the proton is transferred between the photogenerated acid and the acid-labile protecting group ... 

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