Acyclic Acetates

Benzyl Acetals Acyclic acetals are used for the protection of OH groups. The most common ones are derived from formaldehyde. The benzyl acetal of formaldehyde is known as the benzyl-oxymethyl (BOM) protecting group. The hydrogenolysis of the benzylic C-O... 

AUylic oxidation. Cyclohexene is oxidized to cyclohex-2-enyl acetate in 75% yield by the reagent at 70° when catalyzed by potassium bromidefea. 8 hr.). The reaction is slow in the absence of a catalyst. Norbomene is unreactive, but bicyclo[3.2.noclene-2 is oxidized rapidly to about an equal mixture of endo- and cxo-bicycIo[3.2.1]oct-3-ene-2-yl acetates. Acyclic olefins or cyclic olefins prone to allylic rearrangement give a complex mixture of allylic acetates. 

Via 0,0-Acetals Acyclic and cyclic acetals are the most important carbonyl protecting groups of aldehydes and ketones, and also serve as efficient chiral auxiliaries for the synthesis of enantiomerically pure compounds. ... 

It is possible to prepare 1-acetoxy-4-chloro-2-alkenes from conjugated dienes with high selectivity. In the presence of stoichiometric amounts of LiOAc and LiCl, l-acetoxy-4-chloro-2-hutene (358) is obtained from butadiene[307], and cw-l-acetoxy-4-chloro-2-cyclohexene (360) is obtained from 1.3-cyclohexa-diene with 99% selectivity[308]. Neither the 1.4-dichloride nor 1.4-diacetate is formed. Good stereocontrol is also observed with acyclic diene.s[309]. The chloride and acetoxy groups have different reactivities. The Pd-catalyzed selective displacement of the chloride in 358 with diethylamine gives 359 without attacking allylic acetate, and the chloride in 360 is displaced with malonate with retention of the stereochemistry to give 361, while the uncatalyzed reaction affords the inversion product 362. 

Ethylcyanoacetate (227), Rj = CH2C02Et, also reacts with ethylthio-glycolate to afford the corresponding ethyl-2-thiazolylacetate (230), R, -CH2C02Et, R2 = H, after cyclization of the acyclic intennediate in the presence of sodium acetate (542). 

In a similar reaction, bromosuccinic acid and thiobenzamide in ethyl-acetate yielded an acyclic intermediate (229), Ri = Ph, R2 = CH2C02H, which by heating in water cyclizes to the thiazole (230), Rj = Ph and R2 = CH2C02H. (260). 

When side chains of two or more different kinds are attached to a cyclic component, only the senior side chain is named by the conjunctive method. The remaining side chains are named as prefixes. Likewise, when there is a choice of cyclic component, the senior is chosen. Benzene derivatives may be named by the conjunctive method only when two or more identical side chains are present. Trivial names for oxo carboxylic acids may be used for the acyclic component. If the cyclic and acyclic components are joined by a double bond, the locants of this bond are placed as superscripts to a Greek capital delta that is inserted between the two names. The locant for the cyclic component precedes that for the acyclic component, e.g., indene-A - -acetic acid. 

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

Acyclic monothio acetals and ketals can be prepared directly from a carbonyl compound or by transketalization, a reaction that does not involve a free carbonyl group, from a 1,3-dithiane or 1,3-dithiolane. They are cleaved by acidic hydrolysis or Hg(II) salts. 

The anomeric effect is also present in acyclic systems and stabilizes conformations that allow antiperiplanar (ap) alignment of the C—X bond with a lone-pair orbital of the heteroatom. Anomeric effects are prominent in determining the conformation of acetals and a-alkoxyamines, as well as a-haloethers. MO calculations (4-3IG) have found 4kcal/mol as the difference between the two conformations shown below for methoxy-methyl chloride. ... 

Simple acyclic acetals can possess six distinct conformations. 

H2Sil2, CDCI3, —42°, 1-10 min, 100% yield. Aromatic ketals are cleaved faster than the corresponding aliphatic derivatives, and cyclic ketals are cleaved more slowly than the acyclic analogues, such as dimethyl ketals. Substituted ketals such as those derived from butane-2,3-diol, which react only slowly with Mc3SiI, can also be cleaved with H2Sil2. If the reaction is run at 22°, ketals and acetals are reduced to iodides in excellent yield. 

For the construction of oxygen-functionalized Diels-Alder products, Narasaka and coworkers employed the 3-borylpropenoic acid derivative in place of 3-(3-acet-oxypropenoyl)oxazolidinone, which is a poor dienophile in the chiral titanium-catalyzed reaction (Scheme 1.55, Table 1.24). 3-(3-Borylpropenoyl)oxazolidinones react smoothly with acyclic dienes to give the cycloadducts in high optical purity [43]. The boryl group was converted to an hydroxyl group stereospecifically by oxidation, and the alcohol obtained was used as the key intermediate in a total synthesis of (-i-)-paniculide A [44] (Scheme 1.56). 

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