Page 177 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry
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Encyclopedia of Physical Science and Technology EN007F-314 July 6, 2001 16:59
Heterocyclic Chemistry 333
m-CPBA
O
dichloromethane
X X X
5-exo-trig product 6-endo-trig product
Ring expansion of aziridine to a tetrahydrooxazole is NaOCl O
manganese salen
accelerated by the release of angle strain in the former.
Ph complex Ph
H O O
N
+ n-C 6 H 13 n-C 6 H 13
O
H N t i
H Bu OOH, Ti(O Pr) 4
a ring expansion of aziridine OH (-)-diethyl tartrate OH
R 1 R 1
O + CH 2 N 2 O
R 2 R 2
V. MAJOR CLASSES OF
synthesis of epoxides
HETEROCYCLIC COMPOUNDS
The conversion of oxirane into thiirane proceeds via as
A. Three- and Four-Membered Rings
S N ANRORC mechanism.
1. Synthesis
N _
The saturated three-membered rings containing a nitro- N
C
gen, oxygen, or sulfur atom are respectively referred to as O KSCN O _ S
aziridine, oxirane, and thiirane. They are usually readily O S
formed by a ring closure that is entropically favored. The oxirane
analogous formulation of four-membered rings (n = 2) is
NCO
less favored and, unsurprisingly, less common. _
+ NCO
S
_ S
preparation of thiirane
(CH 2 ) n (CH 2 ) n
HZ CH 2 Br Z CH 2
The formation of aziridine is illustrated by the following
Na 2 S
Br Br thietane
S sequence.
cyclizations that give small heterocyclic rings
+
H 3 N H
NaOH N
Epoxides (oxiranes) are extremely useful synthetic in- _ H 2 O
SO 3
termediates (see Section V.A.2 below). Most double bonds
formation of aziridine
can be epoxidized by reaction with m-chloroperoxy-
benzoic acid (m-CPBA). Electron-deficient double bonds
Important routes to four-membered rings containing
are an exception, but can be epoxidized using alkaline
one (or more) heteroatoms are [2 + 2]cycloadditions, as
hydrogen peroxide.
previously discussed. Ring expansion of cyclopropanone
Some alkenes undergo enantioselective epoxidation
upon treatment with hypochlorite in the presence of a chi-
ral manganese catalyst (Jacobsen reaction). The epoxida- O O
HCHO PhCH=NPh
tion of allylic alcohols by means of an alkylhydroperoxide O C C O NPh
attached to a titanium alkoxide–tartrate system is known Ph
an azetidinone
as the Sharpless asymmetric epoxidation and is one of
the most general and widely used asymmetric processes
known; high enantioselectivity is usually observed. to azetidinones is also practical.
Carbonyl compounds react with diazoalkanes with loss
of nitrogen to give epoxides. Indeed, addition of a car- Cl
O HO NHCH 2 COOH HO NHCH 2 COOR O
bene (R 2 C:) or a nitrene (RN:) (or a carbene or nitrene t BuOCl Ag +
equivalent) to a double bond is another general route to NCH 2 COOR
three-membered heterocycles. azetidinones via ring expansion