CHAPTER-3
PERICYCLIC REACTIONS
T
he Pericyclic Reactions involve a cyclic redistribution of bonding electrons through
a concerted process i.e. without intermediates. It is concerted processes that
proceed via a cyclic transition state.
3.1. Classification of Pericyclic Reactions
1. Electrocyclic Reactions: Electrocyclic Reactions are reversible by nature, and usually
involve ring opening or ring closing. Electrocyclic Reactions could also be referred as
concerted cyclization of a conjugated π-electron system by converting one π-bond to a
ring forming σ-bond. It is an intramolecular reaction in which a new σ (sigma) bond is
formed between the ends of a conjugated π system. For e.g.
TRICK 101: Product is a cyclic compound that has one more ring and one less π bond than
the reactant.
In the reverse direction, σ bond in cyclic reactant breaks forming conjugated π system. It
has one more π bond than the cyclic reactant. For e.g.
2. Cycloaddition Reaction: In this reaction, two different π-bond containing molecules
react to form cyclic compound. It is designated as [A + B] where A and B refers to the
number of atoms containing π - electrons. Each of the reactant loses a π – bond and
resulting cyclic product has two new σ – bonds. For e.g.,
TRICK 101: The number of participating π-electrons in each component is given in
brackets preceding the name
3. Sigmatropic Reaction: In this reaction, a σ bond is broken in the reactant and a new σ
bond is formed in the product and the π – bond does not change but rearrange. σ bond
that is broken can be in the middle of π system or at the end of π – system.
TRICK 101: Electrocyclic and sigmatropic rearrangement are intramolecular reactions
while cyclo-addition is an intermolecular reaction.
3.2. Molecular Orbital Theory
1. Conservation of Orbital Symmetry Theory: It states that in – phase orbitals overlaps
during a pericyclic reaction. It was based on frontier orbital theory which is based on
molecular orbitals and orbital symmetry.
2. Frontier Orbital Method: Analysis is based on the interaction of HOMO of one
component with LUMO of other component.
HOMO – highest occupied molecular orbital – filled
LUMO – lowest unoccupied molecular orbital – empty
a. Molecular Orbital Description of Ethene: It has one π bond, it has two p atomic orbitals
that combine to form two π molecular orbital. The two p-orbitals in similar phase overlap
generating a π bonding molecular orbital, denotes as ψ1, it is occupied with two low energy
bonding molecular orbital. Π* antibonding molecular orbital is formed as two p-orbitals in
opposite phase combine generating a destabilizing node
b. Molecular Orbital Description of 1,3-Butadiene: It has two conjugated π – bond so it
has four p – atomic orbitals. These four different p-orbitals combine in different manner to
generate four different molecular orbital ranging from ϕ1- ϕ4. Two of these are bonding
molecular orbital namely ϕ1 and ϕ2, and lower in energy than the parent p-orbital. There
are two antibonding orbital namely ϕ3 and ϕ4 and these are higher in energy than the
parent p-orbital.
where, S = Symmetric Molecular Orbital, A = Asymmetric Molecular Orbitals
(i) Symmetric Molecular Orbitals: If the p – orbitals at the end of molecular orbital have
same sign (in-phase) then symmetric. For e.g. ψ1 in 1, 3 – Butadiene has same (+) sign of
upper lobe of first and fourth p – orbital and (–) sign of lower lobe
(ii) Asymmetric Molecular Orbital: If the p – orbitals at the end of molecular orbital have
opposite sign (out of phase). For e.g. - ψ2 has (+) and (–) sign respectively of upper lobe of
first and fourth p – orbital and hence asymmetric.
TRICK 101: Ground state HOMO and excited state HOMO always have opposite
symmetries; one is symmetric then other is asymmetric.
c. Molecular Orbital Description of 1, 3, 5 – hexatriene: The structure of hexatriene is as
follows:
Six p-orbitals combine differently to form six molecular orbitals denoted as ψ1- ψ6. Amongst
these three are bonding molecular orbital (ψ1- ψ3) and the other three are antibonding
molecular orbital (ψ4- ψ6). However, it must be noted that,
1. For the ground state electronic configuration, six π-electrons occupy three bonding
orbitals, hence, ψ3 is the HOMO and ψ4 is the LUMO.
2. For excited state, electrons from ψ3 go to ψ4, thus, ψ4 is the HOMO and ψ5 is the LUMO
3.3. Electrocyclic Ring Closure/Ring Opening
The following can happen in two manners. They are noted below:
1. Conrotatory Closure: In this case, the terminal p – orbitals rotate in the same direction.
The like phases of p-orbitals lie on the opposite side of the molecule, the two orbitals must
orient themselves in the same direction clockwise – or anticlockwise; For instance,
TRICK 101: If HOMO is asymmetric (end orbitals are different) then rotation has to be
conrotatory in order to achieve in – phase overlap on say bonding situation.
2. Disrotatory Closure: In this case, the terminal p – lobes rotate in opposite direction. The
like phases of p-orbitals lie on the same side of the molecule, the two orbitals must orient
themselves in the opposite direction one clockwise, another anticlockwise. For instance,
TRICK 101: If the HOMO is symmetric (the end orbitals are identical), rotation will be
disrotatory in order to achieve in – phase overlap i.e. bonding situation.
IMPORTANT:
Case 1: If HOMO is symmetric
Then Disrotatory ring closure is symmetry allowed, whereas conrotatory ring closure is
symmetry forbidden.
Case 2: If HOMO is asymmetric
Then Disrotatory ring closure is symmetry forbidden while conrotatory ring closure is
symmetry allowed.
3. a. Stereochemistry of Product and Mode of rotation:
Consider the following case:
❖ For (2E, 4Z, 6E) – octatriene: It has three π – bonds, so 6 p – orbitals and HOMO is
ψ3 in the ground state which is symmetric, thus disrotatory ring closure is allowed.
3.3.1. Photochemical & Thermal Reactions
1. Photochemical Reactions: It takes place when a reactant absorbs light. It involves
excited state molecular orbitals (HOMO and LUMO).
2. Thermal Reactions: It takes place at room temperature and ground state molecular
orbital (HOMO and LUMO) are involved.
❖ What is the product of Ring Closure of (2E, 4Z, 6Z) – octatriene if reaction takes
place under thermal conditions?
Ans: In Thermal condition, ground state HOMO ψ3 is involved which is symmetric, so it is
disrotatory ring closure.
❖ What is nature of product when the ring closure of (2E, 4Z, 6Z) – octatriene takes
place under photochemical condition?
Ans: In photochemical condition, excited state HOMO is involved which is ψ4 and it is
asymmetric, so conrotatory mode of ring closure is allowed.
For (2E, 4Z) – hexadiene: It is similar to 1, 3 – Butadiene but here it is methyl substituted. So
its ground state HOMO ψ2 is asymmetric so ring closure is by conrotatory mode under
Thermal condition.
❖ What is the product when (2E, 4E) – hexadiene undergo thermal reaction?
Ans: The product has been illustrated below:
3.3.2. Woodward-Hoffmann Rules for Electrocyclic Reactions
Number of Conjugated π-
Reaction Conditions
bonds
Even Number
Odd Number
Allowed Mode of Ring
Closure
Thermal
Conrotatory
Photochemical
Disrotatory
Thermal
Disrotatory
Photochemical
Conrotatory
3.6. Configuration of the product in an Electrocyclic Reaction
Substituent in the Reactant
Mode of Ring Closure
Allowed Mode of Ring
Closure
Point in the Opposite
Disrotatory
Cis
Direction
Conrotatory
Trans
Point in the Same Direction
Disrotatory
Trans
Conrotatory
Cis
3.4. Cycloaddition Reaction
It is the reaction of two components to form a cyclic compound or a new ring. It is
designated as [A + B] where A & B refers to number of atoms containing π – electrons.
They can be intermolecular or intramolecular.
SAMPLE
3.4.1. Classification of Cycloaddition Reaction
3.4.3. Diels Alder Reaction
It is a reaction between a conjugated diene and dienophile. It is a concerted [4 + 2]
Cycloaddition reaction. Both enthalpy and entropy decreases. Many Diels – Alder reactions
are accelerated by Lewis acid catalyst. For Diels Alder reaction to occur proper alignment
of frontiers orbital of diene and dienophile is necessary. The thumb rule should be the
positive end of one reactant lines up with negative end of the other reactant.
3.4.4. Classification of Diels Alder Reaction
1. Normal [4 + 2]: Diene is electron rich and Dienophile is electron poor.
2. Inverse electron – demand [4 + 2]: In this case, Diene is electron poor and dienophile is
electron rich.
3. Hetero [4 +2]: In it heteroatom can be a part of diene or dienophile or both.
3.4.5. Criteria for Diene in a Diels Alder Reaction
1. Basic Requirements to be a Diene:
(i) The diene could be cyclic or open chain.
(ii) The diene should be electron-rich and reactivity should be enhanced by electron
donating group substituent.
2. Order of Reactivity of Diene towards maleic anhydride (Dienophile):
3. Conformation of Dienes: Open chair diene can acquire two conformations. For instance,
The diene must adopt an s – cis conformation to be reactive. Cyclic diene which adopts s –
cis conformation are very reactive. For e.g.,
TRICK 101: Cyclic dienes which are permanently in s – Trans conformation are
unreactive in Diels – Alder Reaction. For e.g.,
3.4.6. Criteria for Dienophile in Diels Alder Reaction
1. Basic Requirements to be a Dienophile:
(i) Please note that a dienophile must not always require to be an alkene, the sole criteria
should be containing a π-bond. It could very well be an alkyne. Infact N=N could also
participate in the process as long as it carries an electron poor group.
(ii) Dienophile should be electron deficient and reactivity enhanced by electron
withdrawing substituents. For e.g.,
2. Order of Reactivity of Dienophile:
a. With Respect to Electron-Withdrawing Substituents: Electron withdrawing substituent
increases the rate of reactions. For e.g.,
b. With Respect to Angle-Strain:
The cyclic alkenes and alkynes with angle strain are reactive dienophile. The driving force
here is the reduction in angle strain. For e.g.,
3.4.7. Stereochemistry of Dienes and Dienophiles
SAMPLE
1. Stereochemistry of Dienes:
(i) Inside substituents of diene will become β – configuration (above the plane) in the
product.
(ii) Outside substituents of diene will become α – configuration in the product (below the
plane).
2. Stereochemistry of Dienophile:
Endo Approach of dienophile is preferred over Exo approach due to secondary orbital
overlap between dienophile activating substituent and diene.
3. In a condition where both dienes and dienophiles are substituted, diastereomers are
formed which are usually referred as “exo” or “endo”
3.4.8. Regiochemistry of Diels Alder Reaction
Please note that when non-symmetrical dienes react with non-symmetrical dienophiles,
there occurs a probability of two regioisomers. Dienes with substituents on the terminus
(“1-substituted dienes”) tend to give “1,2” products (“ortho”). Dienes with substituents on
the 2-position (“2-substituted dienes”) tend to give the “1,4” product (“para”). In general,
“1,3” products (“meta”) are only minor byproducts.
a. Substituted diene react to give mainly [1, 2] – product:
b. Substituted diene react to give mainly [1, 4] – substituted product:
❖ What is the product of following reaction? Indicate with appropriate with
appropriate Stereochemistry.
Ans: The product would be:
The reaction occurs in a following manner:
❖ What is product of following reaction?
Ans: The desired product will be:
❖ Predict the structure of the product for the following reaction.
Ans: The structure of the product is:
❖ What would be the structure of Product (A) and Product (B)?
Ans:
❖ Elucidate the reaction mechanism and structure of the product for the reaction
mentioned below
Ans:
3.4.9. Intramolecular Diels-Alder Reaction (IMDA)
1. Type I IMDA: Dienophile is attached to C – 1 atom of diene. It results in the formation of
fused bicyclic system. It can be elucidated in the following manner:
A few examples of Type I IMDA is illustrated below:
2. Type II IMDA:
SAMPLE
3.4.10. Hetero Diels-Alder Reaction
Hetero-Diels-Alder (HDA) reaction is one of most powerful available methodologies to
synthesize optically active six-membered heterocycles, with extensive synthetic
applications in natural or unnatural products with a wide range of biological activity. The DA
reaction has high regioselectivity and endo-stereoselectivity. In this case, hetero atom is
part of the dienophile. For instance,
3.4.11. Retro Diels-Alder or Inverse electron demand D – A Reaction
The Diels Alder Reaction is usually reversible in nature. The equilibrium lies by usually
toward the Diels-Alder adduct at lower temperature and, at higher temperature, toward
the diene and the dienophile. For Retro Diels-Alder reaction, the diene is electron –
deficient and the dienophile is electron – rich. High temperatures are normally required for
Retro Diels Alder Reaction.
❖ What would be the structure of the product for the reaction mentioned below?
Ans:
❖ What would be the structure of the product for the reaction given below?
Ans:
❖ Elucidate the structure of the product for the reaction mentioned below.
Ans:
3.4.12. Photoenolization
When the reactive carbonyl function and a γ-hydrogen are conjugated via an aromatic ring
or double bond, the 1,4-diradical created by hydrogen abstraction quickly relaxes to a
conjugated enol tautomer.
3.4.13. [2+2] Cycloaddition
When compared to [4+2] Cycloaddition Reaction, the [2+2] cycloaddition reaction does not
occur under thermal condition (except under revised circumstances), but takes place
photochemically. This has been further explained in the sections exploiting the symmetry
of HOMO and LUMO of alkene reactant. For [2+2] Cycloaddition to occur the following
rules should be taken into account:
[2s + 2s] addition
not allowed
Where s = Suprafacial bond formation
[2s + 2s] addition
allowed
3.4.15. Stereospecificity of [2+2] Cycloaddition
Please note that, Least hindered transition state is observed.
3.4.16. Regioselectivity of [2+2] Cycloaddition
3.4.18. Photochemical [2+2] Cycloaddition Reaction
SAMPLE
For Photochemical [2+2] Cycloaddition to occur, the light must excite an electron from the
ground state HOMO to generate an excited state HOMO which inturn interacts with the
LUMO of second alkene to allow the overlap of like phases of p-orbitals. The interaction
occurs via suprafacial pathway.
3.5. Sigmatropic Rearrangement
1. Suprafacial Rearrangement: If the migrating group remains on the same face of π –
system then rearrangement is Suprafacial.
2. Antarafacial Rearrangement: If the migrating group moves to the opposite face of π
system, the rearrangement is Antarafacial.
3.5.1. Classification of Sigmatropic Rearrangements
1. [3,3] Sigmatropic Rearrangement: For [3,3] Sigmatropic Rearrangements, both ends of
the relocating σ bond migrate three atoms. These could be broadly divided into two
categories, firstly, 'all carbon' version is known as the Cope rearrangement, and an oxygen
in the appropriate position changes this to the Claisen rearrangement. [3,3] Sigmatropic
Rearrangement can be illustrated through following examples.
2. [1,3] Sigmatropic Rearrangement: [1, 3] Sigmatropic rearrangement involves 2 pair of
electrons (one π + one 6 pair of electrons). In it, a sigma bond is broken in the reactant and
new 6 – bond is formed and π electrons rearrangement occurs. Thermal [1,3] H-shift
reaction cannot take place suprafacially while maintaining bonding interaction between the
H and the ends of the π system. Thermal suprafacial [1,3]-sigmatropic H shifts
are forbidden.
Photochemical [1,3] H-shift reaction can take place suprafacially and maintain
simultaneous bonding interactions between the H and the ends of the π system. A
photochemical suprafacial [1,3]-sigmatropic rearrangement is allowed (and of course, being
suprafacial, is physically possible).
TRICK 101: σ – bond that breaks is allylic to carbon. It can be carbon – hydrogen, carboncarbon, carbon – nitrogen bond etc.
3. [2,3] Sigmatropic Rearrangement: For [2,3]- Sigmatropic Rearrangement, five-membered
rings in the transition states is observed. This reaction offers great stereoselectivity. The
[2,3] Sigmatropic Rearrangement could be illustrated in the following manner:
TRICK 101: The E-alkene will favor the formation of anti product, while Z-alkene will favor
formation of syn product
4. [1,5] Sigmatropic Rearrangement: For [1,5] Sigmatropic Rearrangement, a six
membered transition state ring, with the H atom shifting to a new position is expected .
This is a sigmatropic rearrangement since one sigma bond breaks and another forms. The
[1,5] hydrogen shift in cyclopentadiene can be observed already at room temperature. At
60°C, migration is so fast that only one signal appears for all hydrogens in 1H-NMR.
The [1,5] Sigmatropic Rearrangement could be illustrated in the following manner:
IMPORTANT:
If the number of pair of electron = even then ground state HOMO is asymmetric.
If the number of pair of electron = odd then ground state HOMO is symmetric.
4. [1,7] Sigmatropic Rearrangement: A thermal, concerted [1,7]-H shift is sometimes
observed in acyclic systems because the 1,3,5-triene system is floppy enough to allow the
H to migrate from the top face to the bottom face (making the triene the antarafacial
component). For e.g.,
3.5.2. Woodward – Hoffmann Rules for Sigmatropic Reaction
Number of Electron-Pairs
Reaction Conditions
Allowed Mode of Ring
Closure
Even Number
Thermal
Antarafacial
Photochemical
Suprafacial
Thermal
Suprafacial
Photochemical
Antarafacial
Odd Number
3.5.3. Summary of Thermal Sigmatropic Hydrogen Shift
[1,3] H- Shift
[1,5] H- Shift
[1,7] H- Shift
Stereochemistry
Antarafacial
Suprafacial
Antarafacial
Feasibility
Impossible
Easy
Possible
3.5.4. [1,3] Sigmatropic Rearrangement
The photochemical [1, 3] Suprafacial shift of an alkyl group is allowed with retention of
configuration at the migrating carbon. The thermal [1, 3] Suprafacial shift of an alkyl group
is allowed with inversion of configuration at migrating carbon and [1 ,3] H shift is
forbidden under thermal conditions. [1, 3] – Sigmatropic rearrangements are not common
and only seen in system that are highly strained.
❖ What would be the structure of the following product from the reaction given
below?
Ans:
❖ Elucidate the structure of the product when the following compound is heated.
Ans:
3.5.5. Cope Rearrangement
It is [3, 3] Sigmatropic rearrangement of a cyclic hexa – 1, 5 – Dienes.
A Cope
rearrangement is known to take place through a rearrangement of overlap between a
group of orbitals around this ring as shown in the example below. Two orbitals forming a
sigma bond tilt away from each other while two orbitals that are pi bonding tilt toward
each other. It takes place at around 250°C and the activation energy is determined to be
35k cal / mol.
3.5.6. Factors affecting Cope Rearrangement
1. Substituent that can be brought into conjugation during the rearrangement favours it in
one direction and lowers the activation energy of such a process.
2. Introduction of alkoxy substitution (oxy – cope system)
3. Release of Strain upon Rearrangement
3.5.7. Stereochemistry of Cope Rearrangement
SAMPLE
3.5.11. Stereochemistry of Claisen Rearrangement
The Claisen rearrangement is known to be concerted (bond cleavage and recombination),
exothermic, pericyclic reaction. The Woodward - Hoffmann rules predict a suprafacial,
stereospecific reaction pathway. The kinetics for Claisen Rearrangement are of the first
order and it proceeds through a highly ordered cyclic transition state. Crossover
experiments tend to eliminate the possibility of the rearrangement which occur through
an intermolecular reaction mechanism and are found to be consistent with an
intramolecular process.
Solvent effects have also been observed in the Claisen rearrangement, the polar solvents
tend to accelerate the reaction. Hydrogen-bonding solvents are usually associated with the
highest rate constants. For example, ethanol/water solvent mixtures give rate constants
10-fold
higher
than
sulfolane.
Trivalent
organoaluminium
reagents,
such
as
trimethylaluminium, have been shown to accelerate this reaction.
Relative Stereochemistry across new carbon – carbon single bond is established as a result
of chair like transition state and it depends upon geometry of double bond in the starting
material. For e.g.,
❖ What would be the structure of the product when the following compound is
heated?
Ans: The above mentioned compound when heated would undergo Claisen Rearrangement
and the structure of the product would be following.
3.5.11. Aromatic Claisen Rearrangement
Aromatic Claisen Rearrangement involves rearrangement of allyl phenyl ether via [3,3]sigmatropic rearrangement. The intermediate generated in this case, very quickly
undergoes tautomerization to form ortho substituted phenol. For e.g.,
There could be various probabilities with substitution at ortho, meta and para- positions.
The substitution at meta position effects the regioselectivity of the rearrangement. When
the ortho position is blocked of allyl phenyl ether, the reaction proceeds in the following
manner:
TRICK 101: When Para and ortho position filled, then no reaction will occur.
3.5.12. Ireland- Claisen Rearrangement
The Ireland-Claisen Rearrangement is another approach of Claisen-Rearrangement
wherein an allyl ester of carboxylic acid is used instead of allyl vinyl ether. The ester is
further converted to silyl-stabilized enolate. The reaction begins with deprotonation of the
α-hydrogen of the ester using LDA to form an enolate which then attacks TMSCl to
stabilize the charge and produce LiCl salt. The two olefin groups are ideally positioned for
a claisen rearrangement, which results in the formation of a carbonyl group. Deprotection
of the TMS group with base and protonation then results in the final carboxylic acid product,
proceeds in the following manner:
3.6. Group Transfer Reaction
1. Diimide and Related Reductions: Diimide is generated by oxidation of hydrazine and it is
involved in the transfer of two groups. Concerted delivery of two hydrogen takes place
with syn stereospecificity in Suprafacial way in the manner shown below:
A few examples of Diimide and Related Reductions would be:
1.
2.
2. Ene Reaction: The amalgamation of double and triple bond to an alkene reactant
carrying a transferable allylic hydrogen is known as ene-reactions. Ene reactions are
favored when the hydrogen accepting reagent, the "enophile", is electrophilic. The ene
reactions involve only one sigma bond (C – H) to break. For e.g.,
Additionally, Electron donating group on the exe and electron withdrawing group on the
exophile speed up the reaction. The molecular orbital approach could be elucidated in the
following manner:
❖ What would be the structure of the product for the following reactions?
Ans: The structure of the product will be:
❖ What would be the structure of the expected product for the following reactions?
Ans: The structure of the product will be:
3.6.1. Metallo-Ene Reactions
The metallo-ene reactions are mechanistically similar when compared to classic enereactions.
The transition state for metallo-ene reactions is a cyclic six membered
transition state which is an amalgamation of allylic and alkene species which together
undergo rearrangement. A C-C σ bond is formed between allylic and alkene species due to
migration of allylic species to one of the terminus of alkene reactant. Allylic metal
reagents (Example: metals Mg, Zn, Li, Ni, Pd, and Pt) takes part readily by migration of
metal atom (instead of hydrogen atom) and result in formation of a new carbon – metal
bond.
❖ What would be the structure of the product for the reaction mentioned below?
Ans: The structure of the product will be
❖ Elucidate the structure of the product when the following compound is heated
Ans: The structure of the expected product would be:
3.7. Cheletropic Process
Cheletropic Process involve the generation of transition state with a cyclic array of atoms
and an associated cyclic array of interacting orbitals which occurs as a result of
reorganization of π and σ bonds. It is a concerted process in which two sigma bonds are
made (or broken) which terminate at a single atom. For example,
There is generation of singlet carbene followed by suprafacial addition as illustrated below.
Also, one must know the fact that cheletropic reactions are stereospecific and reversible.
3.7.1. Molecular Orbital Description of Cheletropic Process
A carbene is a neutral molecule containing a divalent carbon with six electrons in its
valence shell. Due to this, carbenes are highly reactive electrophiles and generated as
reaction intermediates. A singlet carbene contains an empty p orbital and a roughly sp2
hybrid orbital that has two electrons. Singlet carbenes add stereospecifically to alkenes,
and alkene stereochemistry is retained.
For [2+1] Cheletropic Process, the olefin reacts with singlet carbene, the molecular orbital
approach has been shown below:
The singlet carbene approach occurs in two different ways namely Linear Approach and
Non-Linear Approach.
a. Case I: Linear Approach: The Linear Approach involves 2 HOMO – LUMO interactions. In
the linear approach, the electrons in the orbital of the small molecule are pointed directly
at the π-system. The rotation will be disrotatory if the small molecule approaches linearly.
b. Case II: Non-Linear Approach:
SAMPLE
3.7.2. Elimination of N2 and CO
Here shown below, are a few examples of elimination of CO and N2. In the second example,
the carbonyl carbon gets eliminated as CO. The final outcome is making two new bonds to
one atom and these are classical examples of Cheletropic Extrusions, wherein a stable
molecule is eliminated. Infact, this provides entropic benefits of gaseous evolution and
serves as the driving force for the reaction.
3.8. Ketene [2+2] Cycloaddition Reaction
One of the few [2+2] allowed thermal cycloadditions allowed includes [2+2] Ketene
Cycloadditions. The [2+2] Ketene addition is allowed as the transition state is stablised by
the pi orbital of the Carbonyl stablising the electron deficient carbon on the alkene.
The most nucleophillic atom on the diene adds to most electrophillic atom on ketene and
is geometry of junction comes from cis double bond of Cyclopentadiene