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A cycloaddition joining a three-atom 1,3-dipole to a two-atom unsaturated unit and forming a five-membered ring. The atom order of the dipole fixes which heteroatoms become adjacent.

A three-atom dipolar electronic system participating in a [3+2]-cycloaddition.

The reverse of a cycloaddition, splitting a ring into unsaturated fragments.

The large negative activation entropy is consistent with a strongly organized bimolecular cycloaddition. Two separately translating and rotating molecules must approach with a restricted orientation to form a compact ring-forming transition state. Losing relative positional and orientational freedom lowers entropy.

Internal conformational restrictions and solvent rearrangement can contribute too. In a nonpolar solvent, a simple association picture is more plausible than assigning the whole effect to strong ionic solvation, but the numerical value is not a direct count of molecular orientations alone.

The negative activation volume means the transition state occupies less partial molar volume than the two separated reactants. Pressure therefore favours reaching it and increases the forward rate, consistent with a compact associative cycloaddition in which new bonds begin to form.

The magnitude, about $25\mathrm{c}{\mathrm{m}}^3\mathrm{mo}{\mathrm{l}}^{-1}$, is molecular in scale. It includes packing and solvent reorganization as well as intrinsic bond-length changes, so it should not be interpreted as a simple geometric volume carved out of two rigid molecular surfaces.

Number the cyclopentenone carbonyl carbon $C_1$, its alkene carbons $C_2$ and $C_3$ with OR at $C_3$, and its saturated carbons $C_4$ and $C_5$. The ester alkene has alpha carbon $X$, bearing Me and CO2Et, and beta carbon $Y$, bearing the protected carbon substituent. lithium diisopropylamide generates the ordinary ketone enolate at the saturated alpha carbon $C_5$. The acetonide is retained.

The carbon end of this enolate attacks ester beta carbon Y. Its pi system shifts through X onto the ester oxygen, forming the first new bond $C_5-Y$ and an ester enolate at X. That enolate now attacks the original cyclopentenone beta carbon $C_3$ intramolecularly, giving the second new bond $X-C_3$. The original enone electrons move onto its ketone oxygen, and protonation of the final ketone enolate completes the product. The OR substituent is retained at the new bridgehead; it need not leave in either Michael addition.

The atom map provides a useful structural check. Between bridgeheads $C_3$ and $C_5$, the three paths are $C_3-C_2-C_1-C_5$, $C_3-C_4-C_5$, and $C_3-X-Y-C_5$. Their numbers of internal atoms are 2, 1 and 2, establishing the norbornane skeleton. Both original carbonyls survive and both initial alkenes are consumed. The two successive conjugate additions give this atom map without requiring a concerted cycloaddition.

Number the cyclopentanone ring $p_1$ at its carbonyl, then $p_2$ at the methyl-bearing carbon, $p_3$ at the mesylate-bearing carbon, $p_4$ at the carbon joined to cyclohexane $h_2$, and $p_5$ at the remaining methylene. B has a $p_2=p_3$ enone and the $h_1=h_6$ enamine. triethylamine removes the hydrogen at $p_2$ while the $p_3$–O bond breaks to release mesylate. An E2 elimination, with approximately antiperiplanar geometry in the puckered ring, gives the conjugated cyclopentenone. Elimination towards $p_4$ instead would give a nonconjugated alkene.

The enamine carbon $h_6$ now attacks enone beta carbon $p_3$ intramolecularly. nitrogen donates into $h_1$ to give the iminium ion, while the enone electrons move onto the ketone oxygen. protonation of the ketone enolate gives C. The second new bond is $h_6-p_3$; the first was $h_2-p_4$. These two connections account for the bridged skeleton without a concerted cycloaddition or an aldol step. The formula of B is ${\mathrm{C}}_{16}{\mathrm{H}}_{23}\mathrm{NO}$: relative to A, elimination removes $\mathrm{C}{\mathrm{H}}_4{\mathrm{O}}_3\mathrm{S}$ overall, namely methanesulfonic acid after proton transfers.

Write the reacting terminus as $R-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$, where $R$ includes the pre-existing stereocentres and the intact $\mathrm{OC}{\mathrm{H}}_2\mathrm{C}(=\mathrm{O})\mathrm{NM}{\mathrm{e}}_2$ substituent. ozone undergoes 1,3-dipolar cycloaddition across the terminal alkene to give a molozonide. In a resonance description of ozone as ${\mathrm{O}}^{-}-{\mathrm{O}}^{+}=\mathrm{O}$, the alkene pi pair bonds to one terminal oxygen while the other terminal oxygen forms the second C–O bond and the ozone pi bond shifts onto its central oxygen. Cleavage of this unstable primary ozonide produces a carbonyl compound and a carbonyl oxide, which recombine to form the secondary ozonide. Reductive workup breaks the peroxide bonds and gives the carbonyl fragments $R-\mathrm{C}{\mathrm{H}}_2\mathrm{CHO}$ and formaldehyde.

Excess sodium borohydride also reduces the newly formed aldehyde: hydride attacks its carbonyl carbon, shifting the pi electrons onto oxygen, and the final acidic workup protonates the alkoxide. Thus the retained chain becomes $R-\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$. The terminal alkene carbon becomes formaldehyde and can be reduced to methanol; it is no longer part of the main product. This accounts for the one-carbon shortening of the retained skeleton.

The original amide carbonyl is present in both H and I and is not reduced under the ordinary borohydride workup. The existing stereocenters and benzyl ether are retained. Stopping the mechanism at an aldehyde, or deleting the amide C=O when reading the smaller printed scheme, would give the wrong product. The new terminal alcohol carbon is CH2 and therefore adds no stereocenter.

Disconnect the two new bonds of the isoxazolidine as a nitrone [3+2] cycloaddition. Suitable achiral partners are $\mathrm{PhCH}={\mathrm{N}}^{+}(\mathrm{Ph}){\mathrm{O}}^{-}$ and the trans enone $(2-\mathrm{pyridyl}\mathrm{N}-\mathrm{oxide})\mathrm{CO}-\mathrm{CH}=\mathrm{CHPh}$.

A practical catalyst family is Cu(II) with an enantiopure bis(oxazoline) Ligand in coordination chemistry, for example a Cu(OTf)${}_2$–BOX complex. The enone can bind through its ketone oxygen and pyridine N-oxide oxygen, giving an O,O-chelated assembly. Chelation both activates the conjugated double bond and fixes its orientation in the chiral pocket. Highly enantioselective cycloadditions of this substrate class with Cu(II)–BOX complexes are experimentally established.^[ref]

The nitrone carbon bonds to the enone $\alpha$ carbon and the nitrone oxygen bonds to the $\beta$ carbon. Thus the carbonyl-bearing carbon lies between the two phenyl-bearing ring carbons, as in the target. The trans enone gives the required trans relationship of its original acyl and phenyl substituents. Selection between endo/exo dipole approaches gives the remaining relative stereochemistry; the sterically preferred chelated transition-state arrangement places the two ring phenyl substituents on the same face and the acyl group on the opposite face, matching the drawing.

One enantiomer of the BOX Ligand in coordination chemistry exposes one enone face; its mirror image reverses the product's absolute stereochemistry. The exact Ligand in coordination chemistry substituents, metal counterion and temperature require optimization. The proposal identifies an experimentally supported catalyst platform and its stereochemical control elements, rather than claiming that an arbitrary BOX complex must give a specified numerical enantiomeric excess.

The sydnone is a mesoionic 1,3-dipole. Its two cycloaddition termini add across the alkyne to give a bicyclic adduct. Subsequent retrocycloaddition expels the ring oxygen and carbonyl carbon together as carbon dioxide, leaving an aromatic pyrazole.

The product is 1-$R_1$-4,5-di-$R$-pyrazole, plus CO${}_2$. The sydnone nitrogen bearing $R_1$ becomes pyrazole N1; the adjacent nitrogen becomes N2; the sydnone CH becomes C3; and the two alkyne carbons become C4 and C5. Since both alkyne substituents are the same, reversing its orientation gives the same constitutional product.

Formation of the aromatic pyrazole and release of CO${}_2$ favor the fragmentation. The net transformation can be understood entirely as a cycloaddition–cycloreversion sequence; no biological application is needed.

Reductive ozonolysis cuts the unsaturated backbone into cyclopentane-1,3-dicarbaldehyde units, apart from end fragments. Both alkene carbons carry a hydrogen, so the carbonyl products are aldehydes rather than ketones. Ring substituent stereochemistry is inherited from the polymer repeat; backbone E/Z geometry does not survive cleavage.

Ozone adds across a backbone alkene by cycloaddition, giving a primary ozonide. Its fragmentation produces a carbonyl group and a carbonyl oxide, which recombine to a secondary ozonide. Dimethyl sulfide reduces the peroxide-containing intermediate, forming dimethyl sulfoxide and the two carbonyl termini. Repetition at every backbone double bond liberates the dialdehyde. An oxidizing work-up would instead risk oxidation to carboxylic acids, so the specified reductive work-up is essential.

The acidic ethanolic step converts the terminal carboxylic acid to its ethyl ester, leaving the pre-existing thiazole ester. Azide then adds across the alkyne by 1,3-dipolar cycloaddition, followed by protonation at work-up. F contains an NH-1,2,3-triazole replacing the alkyne, with the thiazole and $(\mathrm{C}{\mathrm{H}}_2{)}_3\mathrm{C}{\mathrm{O}}_2\mathrm{Et}$ attached to its two carbon atoms. The thiazole's own CO${}_2$Et group remains.

The formula check is decisive:One can represent the cycloaddition with cyclic movement of the azide resonance-form electrons and the alkyne $\pi$ electrons into the two new C–N bonds. Alkali azide initially gives a triazolide; aqueous work-up provides the NH. Interchanging formal 4/5 labels for the carbon substituents in an N-unsubstituted triazole does not establish a separate permanently fixed N-substituted regioisomer.

Tin tetrachloride coordinates the enone carbonyl, lowering the dienophile pi-star energy. Butadiene reacts in an endo-oriented Diels–Alder cycloaddition: its terminal carbons bond to the enone's two alkene carbons, its central bond becomes the new cyclohexene double bond, and the enone C=C pi electrons become a new sigma bond. All three bond reorganizations occur in a suprafacial six-electron cycle.

Approach from the face opposite the methyl substituent avoids steric crowding and gives the indicated relationship between methyl and the non-enolizable junction hydrogen. The immediate stereospecific cycloaddition gives a cis-fused junction relationship. The junction carbon next to the ketone can subsequently enolize; reprotonation on either face epimerizes this centre without breaking either newly formed C–C bond. This accounts for the mixture at the wavy-bond position while the other junction and methyl relationship remains fixed. The modest 1.5:1 ratio should not be mistaken for failure of suprafaciality in the cycloaddition itself. Later oxime formation removes the ketone's ordinary enolization pathway and furnishes the depicted isolated junction diastereomer.

The secondary hydroxylamine condenses with formaldehyde. nitrogen attacks the carbonyl carbon, giving a carbinolamine; proton transfer and loss of water generate the tethered nitrone $\mathrm{R}-{\mathrm{N}}^{+}({\mathrm{O}}^{-})=\mathrm{C}{\mathrm{H}}_2$, where $R$ is the entire bicyclic substituent attached to nitrogen in D. Molecular sieves remove water, favouring this condensation.

The nitrone undergoes an intramolecular 1,3-dipolar cycloaddition with the retained cyclohexene double bond. To define E precisely, label the former oxime carbon $C_A$, the adjacent upper ring junction $C_B$, the other junction $C_C$, and the alkene carbons $C_D$ and $C_E$ such that the chain from $C_A$ to the more remote alkene terminus is $C_A-C_B-\mathrm{C}{\mathrm{H}}_2-C_D=C_E-\mathrm{C}{\mathrm{H}}_2-C_C$. Let $C_F$ be the new formaldehyde-derived nitrone methylene. The cycloaddition forms $C_F-C_D$ and O–$C_E$, retains N–O and N–$C_A$, and changes N=$C_F$ and $C_D=C_E$ into single bonds. The new five-membered heterocycle is N–$C_F$–$C_D$–$C_E$–O. These connections, together with the original bicycle, specify the bridged isoxazolidine E.

The inwardly tethered cycloaddition is suprafacial at the alkene and preserves the existing carbon stereocentres; the C–C and C–O additions occur on the same accessible alkene face. The pre-existing N-substituent orientation restricts which fold can bring both reacting ends together without crossing the bridge. This is a genuine two-new-bond cycloaddition, not a Mannich reaction followed by unrelated oxygenation.

methyl fluorosulfonate alkylates the tertiary isoxazolidine nitrogen, giving a quaternary N-methyl ammonium salt; the methyl remains attached to nitrogen in F. hydride from lithium aluminium hydride then cleaves the N–O bond, releasing the corresponding oxygen as an aluminium-bound alkoxide while restoring neutral tertiary amine nitrogen. Aqueous workup protonates the alkoxide to give the alcohol.

Thus the C–O bond made in the cycloaddition survives as the alcohol-bearing C–O bond, and the formaldehyde-derived methylene survives as the extra carbon between nitrogen and the carbon framework. The N–O bond is the one removed. N-methylation followed by reductive N–O cleavage gives the bridged amino alcohol F without requiring cleavage of its newly formed C–C bond or hydrogenation of a nonexistent residual alkene.

A mesoionic five-membered heterocycle that can undergo cycloaddition and carbon-dioxide extrusion to form a pyrazole.