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The first step is an endo-selective Diels-Alder reaction of the pyrone diene with the electron-poor acrylamide. Number the pyrone $O1-C2(=O)-C3(OH)=C4-C5=C6$, and the acrylamide carbons $C_{\alpha }$ and $C_{\beta }$, with $C_{\alpha }$ next to its carbonyl. The favored polar connection is C3–C${}_{\alpha }$ and C6–C${}_{\beta }$, leaving C4=C5 and the original C2–O1 lactone bridge intact.

Lithium allyloxide, generated from n-Butyllithium and allyl alcohol, attacks the bridging lactone carbonyl. Collapse of the tetrahedral intermediate cleaves its acyl–oxygen bond. proton transfer converts the former O1 bridge into an OH substituent at C6 and leaves the former pyrone carbonyl as an allyl ester attached to C3. The morpholine amide is retained.

Thus C is allyl 1,4-dihydroxy-2-(morpholine-4-carbonyl)cyclohex-5-ene-1-carboxylate, with the stereochemistry described below. In this product numbering C1 is the former pyrone C3, C2 is the acrylamide $C_{\alpha }$, C3 is $C_{\beta }$, and C4 is the former pyrone C6.

The bridge fixes the former C3 carboxyl group and C6 oxygen on the same face. An endo acrylamide places its amide substituent on that face too. Accordingly CO${}_2$allyl at C1, the C2 amide and the C4 OH are cis; the C1 OH is on the opposite face. The alternative face of attack gives the enantiomer, so achiral reagents give a racemate. The $1731\mathrm{c}{\mathrm{m}}^{-1}$ band is consistent with the newly formed ester, while the strong lower-frequency band is consistent with the retained, strongly polarized amide carbonyl. These bands support carbonyl-class retention; they do not independently prove the predicted regioisomer or stereochemistry.

Syndiotactic polypropylene is the expected principal product:The bridged cyclopentadienyl–fluorenyl metallocene has approximately $C_s$ site symmetry. Activation by methylaluminoxane gives a cationic zirconium–alkyl polymerization-active site. Repeated coordination chemistry and migratory insertion of propene extend the chain. The two environments encountered during successive insertions favor alternating stereochemistry, giving syndiotactic tacticity. There is no stoichiometric small molecule eliminated in the propagation step; initiator fragments and chain-transfer products belong to the end groups.

This is an Ireland–Claisen rearrangement. Number the ester carbonyl carbon $C_0$, its enolizable carbon $C_{\alpha }$, and the three allylic ring carbons $C_1-C_2=C_3$, starting at the carbon bearing the ester oxygen.

lithium hexamethyldisilazide removes an $\alpha$ proton and trimethylsilyl chloride traps the oxygen of the resulting ester enolate, giving a silyl ketene acetal. Heating permits a six-electron, [3,3]-sigmatropic rearrangement through the arrayThe electron-pair movements are $\pi (C_{\alpha }=C_0)\to C_0=O$, $\sigma (O-C_1)\to \pi (C_1=C_2)$, and $\pi (C_2=C_3)\to \sigma (C_3-C_{\alpha })$. Acidic work-up cleaves the silyl ester to the carboxylic acid. The benzyl ether remains a protecting group; it is not the migrating bond.

The suprafacial reaction transfers the starting allylic stereochemistry to $C_3$, where the new side chain is delivered from the original oxygen face. In the product's printed orientation, the new C${}_3$–C${}_{\alpha }$ bond is behind the ring and the C${}_3$ methyl group is toward the viewer. At that ring star, the Cahn-Ingold-Prelog priority rules orders the groups as side chain, alkenyl ring direction, saturated ring direction, methyl, giving $R$.

The second star is $C_{\alpha }$. Its priorities are carboxyl, substituted ring carbon, $\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OBn}$, H. A chair-like transition state normally places the bulky side chain pseudoequatorial. However, its absolute configuration also depends on the E/Z geometry of the trapped silyl ketene acetal: opposite geometries give opposite configurations at this star under the same suprafacial ring transfer. The printed conditions do not state solvent or the ketene-acetal geometry. Thus the secure assignment is the ring-star $R$ configuration and conditional $\alpha$-star stereochemistry, rather than an unqualified absolute assignment to both stars. Drawing the assumed ketene-acetal geometry and its chair is necessary before selecting one $\alpha$ epimer.

Write the alkyne as $\mathrm{Me}{\mathrm{s}}_2\mathrm{P}-{\mathrm{C}}_{\alpha }\equiv {\mathrm{C}}_{\beta }-\mathrm{Ph}$. The product places Al at $C_{\alpha }$ and H at $C_{\beta }$:A useful Lewis acid-assisted model begins with a side-on or bridged alkyne–Al interaction. As in the unsymmetrical bonding of aryl-bridged aluminium aggregates, the electrophilic Al center can interact with more than one carbon while a donating group biases the contact. The phosphine can additionally associate transiently with Al, organizing the alkyne near an Al–H bond. Transfer of hydride to the distal carbon while the proximal carbon retains Al gives the observed regiochemistry.

The drawing requires syn addition of H and Al. In the printed alkene, $\mathrm{Me}{\mathrm{s}}_2\mathrm{P}$ and Ph are opposite; consequently Al and the newly added H are on the same side. A cyclic hydroalumination transition state produces this relationship. Steric congestion subsequently inhibits a persistent P$\to$Al bond, leaving the two sites available as a frustrated Lewis pair. A long-lived free vinyl cation or an anti hydride attack is unnecessary, and would not naturally explain the stereochemistry.

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 first two operations invert the allylic hydroxyl group. Triphenylphosphine adds to the azodicarboxylate, generating the reagent pair that converts the alcohol into an alkoxyphosphonium leaving group. Benzoate displaces it with inversion at carbon, giving the inverted benzoate ester. Methoxide then cleaves that ester by nucleophilic acyl substitution, retaining the newly inverted carbon–oxygen stereochemistry because cleavage occurs at the acyl group, not by substitution at the stereogenic carbon.

The inverted allylic alcohol exchanges with the orthoacetate under acid catalysis. Ethanol loss produces an allyl ketene acetal, schematically $\mathrm{C}{\mathrm{H}}_2=\mathrm{C}(\mathrm{OEt})-\mathrm{O}-(\mathrm{allyl})$. A six-membered suprafacial Johnson-Claisen rearrangement breaks the allylic C–O bond, shifts the ring alkene by one position and forms the new bond from the original alkene terminus to the ketene-acetal CH${}_2$. The result is the $-\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_2\mathrm{Et}$ substituent at the carbon that was the remote end of the original double bond. A chair-like transition state placing the larger substituents away from axial crowding explains the stereochemical transfer; the inverted alcohol configuration selects the corresponding face.

Finally, TiCl${}_4$ activates the anomeric methoxy group and methoxide departure creates an oxocarbenium ion. Hydride transfer from triethylsilane replaces that methoxy substituent by hydrogen. The ring oxygen and pendant OTBS group remain intact. The sequence is inversion, stereospecific allylic rearrangement, then reductive removal of the anomeric methoxy group, rather than direct reduction or hydrolysis of the ester introduced by the Claisen step.

A consistent proposed sequence is desilylation, five-membered hemiketal formation, retro hetero-Diels–Alder opening, intramolecular aldol addition and hemiacetal closure. The essential carbon rearrangement is loss of the original ketone–side-chain bond and formation of a bond between its two neighbouring carbons; it is not a simple Michael addition.

For an explicit atom map, label the starting six-membered ring $A-B=C-D-E-O_r-A$, with the ketone at $D$, methyl substituents at $A$ and $B$, the hydroxy group at $A$, and the side chain $E-X-T-\mathrm{Et}$. Here $X$ is the methylene and $T$ is the carbon bearing the protected alcohol. Let $O_D$ be the original ketone oxygen and $O_T$ the protected alcohol oxygen. These labels identify atoms, not new substituents.

First, Fluoride removes the silyl protecting group from $O_T$. The resulting alcohol attacks the protonated ketone at $D$, forming the five-membered hemiketal $D-E-X-T-O_T-D$. The original oxygen $O_D$ is now a hydroxy group; the original pyran ring remains intact at this stage.

Next, The original pyran ring can undergo a retro hetero-Diels-Alder reaction. The three electron-pair movements are $B=C\to C=D$, $D-E\to E=O_r$ and $O_r-A\to A=B$: break $D-E$ and $O_r-A$, and form the three indicated double bonds. The result is the tethered aldehyde/dienol esterThe temporary $D-O_T$ bond keeps both reacting ends within the same molecule. At $D$, both oxygen substituents remain attached, and the new $D-O_T$ tether prevents separation into independent fragments.

Then The $C=D$ enol attacks the tethered aldehyde at $E$. Formation of $C-E$, restoration of $D=O_D$, and proton transfer give the six-membered lactone ring $C-D-O_T-T-X-E-C$, with $E-O_{r}H$. This is an intramolecular aldol reaction, and it supplies the crucial new carbon–carbon bond without losing any carbon atom.

Finally, Keto-enol tautomerism at $A=B$ gives the methyl ketone at $A$ and introduces the hydrogen at $B$. The $E-O_{r}H$ alcohol then attacks that ketone, closing the five-membered hemiacetal ring $A-B-C-E-O_r-A$. The resulting fused rings have exactly the product's connectivity: $D$ is the lactone carbonyl, $O_T$ is its ring oxygen, and $O_r$ is the five-membered-ring oxygen.

The stereocentre at $T$ survives throughout this sequence. The configuration initially at $E$ is erased at the aldehyde stage, and those at $A$, $B$, $C$ and $E$ arise during the later cyclization and proton-transfer steps. A folded intramolecular aldol reaction followed by reversible hemiacetal formation is consistent with the drawn cis ring junction, but connectivity and orbital bookkeeping alone do not prove that it is the unique selected diastereomer. In particular, a claim of simple retention at $E$ would contradict this atom map.

A related pyranone-to-fused-lactone rearrangement under aqueous hydrogen fluoride is experimentally reported; that account also explicitly qualifies the relative stereochemistry as assigned on mechanistic grounds. The sequence above is a proposed explanation of the examination structure, rather than a mechanism claimed to have been experimentally established in that paper.^[ref]

Both free primary allylic alcohols undergo Sharpless asymmetric epoxidation. With slightly more than two equivalents of hydroperoxide, the expected fully oxidized product is the diepoxide, with both original C=C bonds converted to epoxides and both primary alcohols retained. The two central pre-existing stereocentres and their ether substituents are retained.

The stereochemical construction is unambiguous by applying the L-(+)-tartrate mnemonic separately at each end: orient that alkene horizontally with its CH${}_2$OH group at lower right, and deliver oxygen from the back face. In the source's original in-plane orientation this gives the left epoxide oxygen behind the plane and the right epoxide oxygen in front. The methyl and hydroxymethyl groups retain their original relative arrangement about each former double bond. One cannot simply draw both oxygens on the same face of the original extended chain: the two allylic-alcohol ends have opposite drawing orientations. titanium binding to each free hydroxyl directs oxygen transfer; the pre-existing stereocentres can create matched/mismatched rate differences, but the stated reagent control predicts the stereochemistry of both epoxidations. Molecular sieves maintain a dry active titanium–tartrate system.^[ref]

Number the epoxide-bearing ring carbon carrying CH${}_2$OH as C1, then proceed through the other epoxide carbon C2 around the ring to C6. Heating with water opens the epoxide at the less substituted C2 by backside attack: the incoming oxygen becomes C2–OH on the front face, while the original epoxide oxygen remains as C1–OH on the back face. Proton transfers complete formation of the triol. Thus the two original epoxide oxygens' roles are distinct: one is retained from B and the new hydroxyl oxygen comes from water.

Iodine next activates the remaining C4=C5 bond as an iodonium ion. The pendant primary alcohol oxygen attacks intramolecularly at C5 to make the five-membered cyclic ether with path O–CH${}_2$–C1–C6–C5, opening the iodonium anti and leaving iodine at C4 on the opposite face. Bicarbonate removes the oxonium proton and neutralizes acid; it is not the source of the ether oxygen. The folded tether selects the face of closure compatible with the bridged structure, accounting for the ether/iodine stereochemistry.

The remaining two hydroxyl groups react with acid-activated 2,2-dimethoxypropane. Successive oxygen attack, methanol departure and intramolecular closure form the isopropylidene ketal of C, protecting that diol without changing its carbon configurations. The terminal primary hydroxyl has already become the cyclic ether and is not one of the two acetonide oxygens. This atom tracking distinguishes the epoxide hydrolysis, iodoetherification and protection stages.

Silver(I) promotes iodide departure, with precipitation of AgI. The oxygen of dimethyl sulfoxide substitutes at the iodine-bearing carbon to form an alkoxydimethylsulfonium intermediate. Triethylamine removes the hydrogen on that carbon; collapse forms C=O and breaks O–S, releasing dimethyl sulfide. The result is ketone D by a Kornblum oxidation. In arrow terms, the C–H electrons form the carbonyl pi bond as the O–S bond breaks. The carbon bearing iodine becomes the carbonyl carbon; the existing bridged ether and acetonide are spectators. The iodine centre loses its tetrahedral stereochemistry because it becomes trigonal planar.

Acid-promoted union of a carbonyl-derived oxocarbenium ion with a tethered alkene to form a ring. Cation trapping and the preferred cyclization conformation determine the final substitution and stereochemistry.

Ribose is a monosaccharide with molecular formula ${\mathrm{C}}_5{\mathrm{H}}_{10}{\mathrm{O}}_5$. Its biologically familiar form is D-ribose, an aldopentose whose open-chain Fischer projection has $\mathrm{CHO}$ at the top, ${\mathrm{CH}}_2\mathrm{OH}$ at the bottom, and hydroxy groups on the right at C2, C3, and C4. The opposite horizontal substituents are hydrogen. See McMurry, Organic Chemistry, 25.3: D-ribose and D-glucose configurations.

Read the drawing by counting groups: $\mathrm{CHO}$ contributes $\mathrm{CHO}$, three $\mathrm{CHOH}$ groups contribute ${\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{O}}_3$, and ${\mathrm{CH}}_2\mathrm{OH}$ contributes $\mathrm{C}{\mathrm{H}}_3\mathrm{O}$. Their sum is ${\mathrm{C}}_5{\mathrm{H}}_{10}{\mathrm{O}}_5$. The count checks composition; the right/right/right pattern additionally specifies the stereochemistry of D-ribose.

In RNA, ribose occurs as a ring within each nucleotide, joining a nucleobase to the phosphate-linked backbone. Its C2 position retains a hydroxy group, unlike deoxyribose in DNA. In a nucleotide, the sugar positions carry primes, so this is C2′. See Alberts et al.: nucleotide sugars in RNA and DNA. Free ribose in water is also predominantly cyclic, with several ring forms in chemical equilibrium and very little open-chain form; the Fischer projection is valuable for comparing configurations, not for estimating their abundances. See Mason et al., The Conformation of a Ribose Derivative in Aqueous Solution: A Neutron Scattering and Molecular Dynamics Study (2013), introduction.

Ring inversion interconverts ring conformations while preserving covalent connectivity and configurational stereochemistry. In a cyclohexane chair flip, axial and equatorial labels exchange, while the up/down face of a given substituent is retained.

Enantioselective oxygen transfer to an allylic alcohol using a titanium–tartrate complex and a hydroperoxide. With the alkene horizontal and its hydroxymethyl substituent at lower right, L-(+)-tartrate directs delivery from the back face and D-(−)-tartrate from the front face. Rotate a structure in its plane to apply the mnemonic; reflecting it changes its stereochemistry.

A linear string encoding atoms, bonds, branches, ring closures and optionally stereochemistry of a molecular graph.

Stereochemistry studies the spatial arrangement of chemical atoms in molecules. A Fischer projection records this information at successive stereocenters, whereas a molecular formula only counts chemical atoms. See McMurry: representing carbohydrate stereochemistry.

A substituent is an attached chemical atom or group considered relative to a chosen molecular framework. Testing whether two substituents are identical may require comparing their full chemical bond connectivity and stereochemistry.

In chemical usage, sugars commonly means small carbohydrates, including single-unit monosaccharides and two-unit disaccharides. Glucose, ribose, and deoxyribose are monosaccharides. Cellulose is instead a polysaccharide assembled from many glucose units. The distinction counts chemically linked units; it does not classify every carbohydrate by sweetness or biological role. See Clark, Douglas and Choi, Biology 2e, 3.2: carbohydrates.

A useful first description of a monosaccharide asks how many carbon chemical atoms it contains and what its open-chain structure is. Ribose is an aldopentose, a five-carbon aldehyde sugar; glucose is an aldohexose, with six carbon chemical atoms. A Fischer projection then records the stereochemistry, information missing from the molecular formula. See McMurry, Organic Chemistry, 25.1: carbohydrate classification.

Primary synthetic study. Pages 5220–5221, Schemes II–III, report aqueous hydrogen fluoride conversion of related pyranones into a fused lactone. The accompanying discussion qualifies the relative stereochemistry as a mechanistic assignment rather than an unambiguous assignment from the reported NMR spectroscopy.

A six-membered chair-like transition-state model for an aldol-type bond-forming step. It predicts relative stereochemistry by comparing substituent placement and nonbonded interactions, but must be adapted to the actual enamine or metal-enolate catalyst.