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Encyclopedia of Physical Science and Technology EN007F-314 July 6, 2001 16:59
332 Heterocyclic Chemistry
A general means of deoxygenating ROH into RH in-
volves conversion of the hydroxy group into a thiocar- S
bonylimidazolide followed by reductive cleavage using + _ S
tributyltin hydride in the presence of the radical initiator COOEt COOEt
azobisisobutyronitrile (AIBN).
m-CPBA NaH
O
NHCOOEt NHCOOEt O O
N N O Bu 3 SnH O S C 2 Cl 6
O
EtOOCN H AIBN EtOOCN H COOEt
OR OR
- SO 2
O O
Benzotriazole ia an inexpensive and stable heterocycle S COOEt
that is soluble in a variety of common solvents. It is easily
COOEt
introduced into a molecule and can act as a satisfactory
leaving group. Such properties enable its use in a wide
variety of synthetic transformations, including displace-
ments to form unsymmetrical secondary aliphatic amines.
IV. SYNTHESIS OF HETEROCYCLES
N
R 3 MgX R 3
N + NHR 2 The synthesis of heterocyclic rings has much in common
N R 1
NHR 2 with carbocyclic rings. Ring formation can be via cycload-
R 1
dition, ring closure, ring expansion or contraction, ring-
atom interchange (ANRORC reactions), or insertion of
When borane becomes bonded to certain chiral oxaz- reactive intermediates into double bonds.
aborolidines, hydride transfer to ketones takes place, giv-
ing mainly oneenantiomerofthe alcohol. Thelarger group a c hν a d b a f b a f
of the ketone points away from the heterocyclic ring, min- b + d b c c + e c e
d d
imizing steric hindrance.
[2 + 2] cycloaddition [4 + 2] cycloaddition
+
Ph
H Ph _ b b
O H OH
BH 3 .THF a c a c
Ph Cl Ph + N - O Me Ph Cl d e d e
H Ph B
B - + O Cl 1,3-dipolar cycloaddition
O H H
N H Ph
B
CH 3
The Woodward–Hoffmann rules predict the regio- and
stereochemistry of substituents in rings formed by cy-
The ring opening of certain modified carbohydrates by
cloaddition reactions. In practice, a number of cycload-
mercury(II) acetate proceeds with subsequent cyclization
dition reactions are rarely or never observed, and these
(Ferrier rearrangement) to give a substituted cyclohex-
are forbidden by these important rules. The extremely
anone that can be reduced to an inositol derivative. Inos-
useful Diels–Alder reaction is a [4 + 2]cycloaddition and
itol phosphates are key messenger molecules in cellular
was originally applied to carbocycles (atoms a–f = C), but
signaling.
it has been adapted to the synthesis of heterocycles.
OMe OH
Ph Ph
RO RO OCOCH 3 N ∆ N
O i Hg(OCOCH 3 ) 2 +
ii NaCl O O
OCOCH 3
RO RO O
OR OR synthesis of a dihydro-1,2-oxazine
Sulfur heterocycles readily form sulfur ylides that can Ring-closure reactions follow well-established pat-
undergo electrocyclic ring expansion to give a medium- terns, which are summarized in Baldwin’s rules. For ex-
size ring. The sulfur can then be used to link two carbon ample, the ring closure of the terminal radical below leads
atoms with extrusion of sulfur dioxide to form a cyclic totheso-called5-Exo-Trigproductratherthanthe6-Endo-
alkene (Ramberg–B¨acklund reaction). Trig product.