Protective groups double bonds

The synthesis of lipidated peptides is implicated by the base-lability of the thioester and the acid sensitivity of the prenyl group double bonds. Thus, new protecting groups are required which can be removed under extremely mild, preferably neutral, conditions. 

The dibromide itself is usually prepared from the same alkene and so the reaction is not particularly useful for the synthesis of alkenes. It is useful, however, in protection strategy. During a lengthy synthesis, it may be necessary to protect a double bond so that it does not undergo any undesired reactions. Bromine can be added to form the dibromide and removed later by denomination in order to restore the functional group. 

Furthennore, the presence of electnxi-withdrawing substituents, such as an allylic hydroxy group, is deactivating, an effect intensified by esterification to a degree such that acylation of an allylic alcohol may be sufficient to protect the double bond during photooxidation at another site. ... 

Metal hydrides reduce preferably polar double bonds, whereas catalytic hydrogenation is somewhat selective for non-polar double bonds. Selective protection of amino groups in amino acids. 

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). 

These oxazolines have cationic surface-active properties and are emulsifying agents of the water-in-oil type. They ate acid acceptors and, in some cases, corrosion inhibitors (see Corrosion). Reaction to oxazoline also is useful as a tool for determination of double-bond location in fatty acids (2), or for use as a protective group in synthesis (3). The oxazolines from AEPD and TRIS AMINO contain hydroxyl groups that can be esterified easily, giving waxes (qv) with saturated acids and drying oils (qv) with unsaturated acids. 

Methylation of avermectins B and B2 leads to the corresponding derivatives of the A series (49). A procedure involving the oxidation of the 5-methoxy group with mercuric acetate and NaBH reduction of the 5-keto-intermediate allows the conversion of the A to the B components (50). The 23-hydroxy group of the "2" components, after selective protection of the other secondary hydroxy groups, is converted to a thionocarbonate, which can be elirninated to give the 22,23-double bond of the "1" components alternatively it can be reduced with tributyltin hydride to the 22,23-dihydro derivatives (= ivermectins) (51). 

The C. 100-C. 101 diol group, protected as an acetonide, was stable to the Wit-tig reaction used to form the cis double bond at C.98-C.99, and to all the conditions used in the buildup of segment C.99-C. 115 to fully protected palytoxin carboxylic acid (Figure 1,1). 

To control the stereochemistry of epoxidation at the 10,11-double bond in intermediates in prostaglandin synthesis, a bulky protective group was used for the C15-OH group. Epoxidation of the tribenzylsilyl ether yielded 88% a-oxide epoxidation of the tri-/ -xylylsilyl ether was less selective. ... 

In a synthesis of sativene a carbonyl group was protected as a 2,4-DNP while a double bond was hydrated with BH3/H202/0H . Attempted protection of the carbonyl group as a ketal caused migration of the double bond protection as an oxime or oxime acetate was unsatisfactory since they would be reduced with BH3. 

An oxazolidine was used to protect the carbonyl group in an a,/3-unsaturated aldehyde during reduction of the carbon-carbon double bond by H2/Raney Ni. It... 

A thiol, usually under basic catalysis, can undergo Michael addition to an activated double bond, resulting in protection of the sulfhydryl group as a substituted 5-ethyl derivative. 

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

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