Page 274 - Advanced Organic Chemistry Part B - Reactions & Synthesis

246 HO CH 3 CO 2
O OH O OH
CHAPTER 3 Lu(O SCF )
3
3 3
Functional Group HO HO
2
3
Interconversion (CH CO) O
by Substitution, HO H O H O
Including Protection and PhCO 2 O 2 CCH 3 HO PhCO 2 O 2 CCH 3
Deprotection
Scandium triflimidate, Sc N SO CF
, is also a very active acylation catalyst.
2
3 2 3
CH
CH 3 3
SCF ) ]
2 mol % Sc[N(O 2 3 2 3
(PhCO) O
2
OH 25°C, 3 h O CPh
2
)
CH(CH ) CH(CH 3 3 98% Ref. 112
3 3
Bismuth(III) triflate is also a powerful acylation catalyst that catalyzes reactions
with acetic anhydride and other less reactive anhydrides such as benzoic and pivalic
anhydrides. 113 Good results are achieved with tertiary and hindered secondary alcohols,
as well as with alcohols containing acid- and base-sensitive functional groups.
CH 3 CH
CH 3 CH 3 3 mol % CH 3
CH 3 + [(CH ) CCO] O Bi(OTf) 3 3
3 3
2
O CC(CH )
OH 2 3 3
97%
Trimethylsilyl triflate is also a powerful catalyst for acylation by anhydrides.
Reactions of alcohols with a modest excess (1.5 equival) of anhydride proceed in inert
solvents at 0 C. Even tertiary alcohols react rapidly. 114 The active acylation reagent

is presumably generated by O-silylation of the anhydride.

CH 3 OH (CH CO) O CH 3 O CCH
3
2
5 equiv 2 3
(CH ) SiO SCF 3
3
3 3
5 mol %
In addition to acyl halides and acid anhydrides, there are a number of milder and
more selective acylating agents that can be readily prepared from carboxylic acids.
Imidazolides, the N-acyl derivatives of imidazole, are examples. 115 Imidazolides are
isolable substances and can be prepared directly from the carboxylic acid by reaction
with carbonyldiimidazole.
O O
RCO H+ N N C N N RC N N + HN N + CO 2
2

112
K. Ishihara, M. Kubota, and H. Yamamoto, Synlett, 265 (1996).
113 A. Orita, C. Tanahashi, A. Kakuda, and J. Otera, J. Org. Chem., 66, 8926 (2001).
114 P. A. Procopiou, S. P. D. Baugh, S. S. Flack, and G. G. A. Inglis, J. Org. Chem., 63, 2342 (1998).
115
H. A. Staab and W. Rohr, Newer Methods Prep. Org. Chem., 5, 61 (1968).