Chemistry Reference
In-Depth Information
was performed in a conventional way the corresponding oxazolidinone 38
was obtained. However, when the same reaction was performed with an
alkyl-substituted aziridine 39, it afforded a mixture of oxazolidinones 40 and
41. Interestingly, the reaction under high speed ball milling produced only
one oxazolidinone 40 regioselectively within 17 h.
1.3.4 Boron-containing Heterocycles
Protection-deprotection is an important tool in organic synthesis as in many
cases the molecules such as bi-or poly-functional alcohols, acids, amines,
and thiols are required to be protected and subsequently deprotected to
obtain the target molecule. Both protection and deprotection step should be
simple and quantitative to make the synthetic procedure more economical
and industrially suitable. Boronic acid reacts in the solid state with suitably
substituted amines, amino acids, diols and polyhydroxy compounds like
mannitol and inositol under ball milling to form the corresponding five- or
six-membered boron containing heterocycles, which provides good pro-
tection to the functionalities.
1.3.4.1 Protection of Amines and Acid Functionality
Free amines were protected via the formation of six-membered diazabor-
inine by the reaction with phenyl boronic acid in solid state under ball
milling (Scheme 1.48).
46
The formed diazaborinine is stable towards strong
acid or strong base and it may be useful for electrophilic aromatic substi-
tutions. The amine in anthranilic acid, with its free carboxylic functional
group, was protected by reaction with boronic acid.
1.3.4.2 Protection of Aromatic 1,2-Diol Functionality
Suitably substituted diols are protected with boronic acid by the formation
of corresponding five-membered heterocycles. For example, pyrocatechol
HO
OH
B
NH
2
N
H
20-25 Hz
+
+
2H
2
O
B
ball milling
NH
2
NH
100%
H
HO
OH
B
NH
2
N
20-25 Hz
ball milling
B
+
+
2H
2
O
O
COOH
O
100%
Scheme 1.48 Quantitative solid state formation of six-membered boron containing
heterocycles.
Search WWH ::
Custom Search