Page 294 - Advanced Organic Chemistry Part B - Reactions & Synthesis

266 Allyl carbonate esters are also useful hydroxy-protecting groups and are intro-
duced using allyl chloroformate. A number of Pd-based catalysts for allylic depro-
CHAPTER 3 209 0
tection have been developed. They are based on a catalytic cycle in which Pd
Functional Group reacts by oxidative addition and activates the allylic bond to nucleophilic substitution.
Interconversion 210 211
by Substitution, Various nucleophiles are effective, including dimedone, pentane-2,4-dione, and
Including Protection and amines. 212
Deprotection
O 0 O –
R Pd R II Nu: 0 Nu
O O O O Pd ROH + CO 2 + Pd +

Table 3.1 gives the structure and common abbreviation of some of the most
frequently used hydroxy-protecting groups.

3.5.1.5. Protective Groups for Diols. Diols represent a special case in terms of appli-
cable protecting groups. 1,2- and 1,3-diols easily form cyclic acetals with aldehydes and
ketones, unless cyclization is precluded by molecular geometry. The isopropylidene
derivatives (also called acetonides) formed by reaction with acetone are a common
example.

RCH CHR
H +
RCHCHR + CH CCH 3 O O
3
HO OH O C
CH 3 CH 3

The isopropylidene group can also be introduced by acid-catalyzed exchange with
2,2-dimethoxypropane. 213

OCH 3 RCH CH 2
H +
RCHCH OH + CH CCH 3 O O + 2 CH OH
3
2
3
C
OH OCH 3
CH 3 CH 3
This acetal protective group is resistant to basic and nucleophilic reagents, but is
readily removed by aqueous acid. Formaldehyde, acetaldehyde, and benzaldehyde are
also used as the carbonyl component in the formation of cyclic acetals, and they
function in the same manner as acetone. A disadvantage in the case of acetaldehyde
and benzaldehyde is the possibility of forming a mixture of diastereomers, because of
the new stereogenic center at the acetal carbon. Owing to the multiple hydroxy groups
present in carbohydrates, the use of cyclic acetal protecting groups is common.

209 F. Guibe, Tetrahedron, 53, 13509 (1997).
210 H. Kunz and H. Waldmann, Angew. Chem. Int. Ed. Engl., 23, 71 (1984).
211
A. De Mesmaeker, P. Hoffmann, and B. Ernst, Tetrahedron Lett., 30, 3773 (1989).
212 H. Kunz, H. Waldmann, and H. Klinkhammer, Helv. Chim. Acta, 71, 1868 (1988); S. Friedrich-
Bochnitschek, H. Waldman, and H. Kunz, J. Org. Chem., 54, 751 (1989); J. P. Genet, E. Blart,
M. Savignac, S. Lemeune, and J.-M. Paris, Tetrahedron Lett., 34, 4189 (1993).
213
M. Tanabe and B. Bigley, J. Am. Chem. Soc., 83, 756 (1961).