Synthesis of Heterocyclic Rings 1(Introduction & Substitution)
Ring SynthesisCyclization Reactions Cycloaddition Reactions Ring transformation
Nu
E
Ring enlargement
ring contraction
Baldwin's rules for ring closures
• Baldwin's rules in organic chemistry are a series of guidelines outlining the relative favourabilities of ring closure reactions in alicyclic compounds. They were first proposed by Jack Baldwin in 1976.
• The rules classify ring closures in three ways:• the number of atoms in the newly formed ring• into exo and endo ring closures, depending whether the bond broken during the ring closure is inside (endo) or outside (exo) the ring that is being formed
• into tet, trig and dig geometry of the atom being attacked, depending on whether this electrophilic carbon is tetrahedral (sp3 hybridised), trigonal (sp2 hybridised) or digonal (sp hybridised).
Cyclization Reactions• Displacement at saturated carbons
NH2
Br
NH2
Br
NH
NH
NH
NH2
Br
NH2
Br
NH
3-exo-tet 4-exo-tet 5-exo-tet 6-exo-tet
Examples
O OBr OO
-
The Darzens reaction (also known as the Darzens condensation or glycidic ester condensation) is the chemical reaction of a ketone or aldehyde with an α-haloester in the presence of base to form an α,β-epoxy ester,
This reaction was discovered by the organic chemist Auguste George Darzens in 1904.[3]
N
R
XR
R
Br
Br
X
R
RNH
Br
X
RNH2
Feist-Benary Furane Synthesis
O
RR1
OH
Cl
CO2Et
R1O
Cl O
R
CO2Et
RR1
CO2Et
OO
RR1
OH
Cl
CO2Et
Pyridine
+
-
-
-Haloketone -Ketoester
N
OPh
N
OPh
Cl
(CH3)3CO- K+