Codex @codex  0

All-rac-alpha-tocopherol designates an alpha-tocopherol stereoisomer mixture containing all four pairs of enantiomers. The nomenclature does not require exactly equal proportions of the pairs.^[ref] It is a description of stereochemistry, rather than a different alpha ring-methylation pattern.

The three stereocenters at $2$, $4^{\prime }$ and $8^{\prime }$ permit the configurations RRR, RRS, RSR, RSS, SRR, SRS, SSR and SSS, with the letters read in that order. The enantiomer pairs are RRR/SSS, RRS/SSR, RSR/SRS and RSS/SRR. Inverting every center gives the partner; changing only some centers generally gives a diastereomer.

For an original equal-composition model, 80 micromoles of the mixture contains 10 micromoles of each configuration. The RRR-alpha-tocopherol fraction is $1/8$, while the total fraction with R at position 2 is $4/8$. These values follow from the additional equal-proportions assumption. A sample with four racemic pairs of unequal abundance can satisfy the all-rac description while having a different RRR fraction.

All eight configurations share a molecular formula and molar mass. A total-mass measurement cannot recover their distribution. A stereochemically discriminating separation or another validated selective measurement supplies additional information. Conversely, the chemical fraction of one configuration is not automatically its fraction of biological effect: that conversion would require a stated biological response model, including differences in transport, retention or metabolism.

The distinction is useful when comparing analytical specifications. “Alpha” fixes ring methylation; “all-rac” specifies a class of stereoisomer mixtures; a reported concentration specifies amount on the assay's chosen basis. None can replace the others.

Diastereomers are different stereoisomers that are not enantiomers. The meso compound form of tartaric acid is a diastereomer of either member of its enantiomer pair.

Diastereoselectivity is preferential formation or reaction of one diastereomer among competing possibilities. Unlike enantiomers, diastereomers can have different energies and physical properties even in an achiral environment.

For example, an addition reaction can favor one relative arrangement of two stereocenters, producing a $4:1$ diastereomeric ratio. This ratio describes product composition, while chemical yield describes how much product is obtained. Steric interactions, chelation, and conformational preferences can affect the competing transition-state energies, so the favored product depends on substrate and conditions.

Two sites are diastereotopic if replacing one or the other with the same test group gives diastereomers. In an achiral solvent they can have different chemical shifts, unlike a simple pair of symmetry-related sites. A rigid chiral metal chelate can make the two hydrogens of a linker methylene diastereotopic.

Enantiomers are a pair of nonsuperposable mirror-image molecules. The D and L configurations of alanine are an example; glycine has no such pair because its amino acid alpha carbon carries two identical hydrogen chemical atoms. ^[ref]

The word names a relation between two stereoisomers. Merely finding two different spatial arrangements does not make them enantiomers: arrangements that are different but not mirror partners are diastereomers.

Two epimers are diastereomers with opposite configurations at exactly one of the two or more tetrahedral stereocenters in their molecules. Their connectivity is the same, and configurations at the other centers agree. The definition is more specific than simply calling two compounds stereoisomers.^[ref]

D-glucose and D-mannose illustrate the relationship: their configurations differ at carbon 2 while the other corresponding stereocenters agree. This local change can alter recognition by enzymes without changing the elemental composition or the carbon-chain connectivity.

For a hypothetical molecule with corresponding stereocenters 2 and 3, the configurations $(2R,3R)$ and $(2S,3R)$ differ at one center and can form an epimeric pair. Changing both to $(2S,3S)$ is a different comparison: it does not satisfy the one-center condition. These labels assume the same molecular connectivity and consistently assigned centers.

A molecule with only one stereocenter does not produce an epimer pair merely by reversing that center: its two configurations are enantiomers, whereas the epimer definition requires a diastereomeric relationship. Epimerization names a process that interconverts an epimeric pair; it does not by itself specify the reaction mechanism, rate or equilibrium ratio.

Epimerization interconverts diastereomers differing at one stereocenter. It can change a product ratio after the initial bond-forming chemical reaction.

An endpoint experiment uses a polarimeter with a $1.0\mathrm{dm}$ sample cell and supplied $4\%$, $6\%$ and $8\%$ D-glucose chemical solutions. Collect about $15\mathrm{mL}$ of each. Fill the cell first with distilled water, dry its windows, move bubbles to the air trap, and retain its orientation. Scan light intensity against polarimeter analyzer angle through $360^{\circ }$. The protocol fits a Gaussian function to the upper third of a symmetric peak to locate its maximum. Rinse with the next sample, repeat for each glucose chemical solution, and subtract the water maximum's angle from each sample maximum. This is an operational peak-location method; the Gaussian function is not the physical transmission law.^[ref]

The existing optical activity and polarimetry articles explain the apparatus, Malus's law and instrument limitations. For an extension that observes mutarotation in time, freshly dissolve a specified crystalline anomer, record the mixing time, and repeat the optical rotation readout at fixed concentration, path length, wavelength and recorded temperature. Keep pH consistent and record the delay before the first reading. These are proposed additions to the endpoint protocol, not its published steps or a claim of measured results. Recheck the water blank, repeat the preparation, and compare with a separately equilibrated portion under the same conditions. A stable endpoint alone does not show the time evolution.

Interpret each concentration separately. If $\theta$ is blank-corrected optical rotation, $l$ is path length and $c$ is total glucose mass concentration, first calculate specific rotation $r=\theta /(lc)$. In an additive two-anomer approximation, let $r_{\alpha },r_{\beta }$ be reference specific rotations under the same conditions and $x_{\alpha }$ the mole fraction of alpha within that pair. Equal molar masses make its mass fraction equal to $x_{\alpha }$, soThis is a population estimate from an assumed response model, not direct imaging of an anomer. For a simple consistency check, doubling $c$ at fixed $l$ and unchanged composition doubles $\theta$ in this model while leaving $r$ unchanged. Averaging raw optical rotations at different concentrations would fail this test; normalize first.

Changing temperature, pH, contamination, incomplete dissolution, window stress, bubbles or instrumental drift can imitate or obscure a changing signal. Reference specific rotations must match the sample conditions. Minor open-chain and other ring forms limit the two-anomer approximation; a single optical rotation cannot separately determine every population. Also, these anomers are diastereomers, not an enantiomer pair: the equal-and-opposite cancellation formula for enantiomeric excess does not apply.

A is the predicted major diastereomer. At the carbon adjacent to the ketone, take phenyl as the large group, vinyl as medium, and hydrogen as small. There is no adjacent heteroatom donor that would force a five-membered chelate. The appropriate first model is therefore the non-chelating Felkin-Anh model.

View along the carbonyl-carbon–stereocentre bond. Rotate to place the large alpha phenyl group approximately perpendicular to the carbonyl plane. Of the two resulting approaches opposite that large group, the favourable Bürgi-Dunitz trajectory passes nearer hydrogen rather than vinyl. Propyl transfer to the carbonyl and protonation of its oxygen give the configuration labelled A: in the printed projection the new OH projects forward and the propyl bond backward, while the original vinyl bond remains backward. Rotation used to construct a Newman projection must not be mistaken for inversion of the original stereocentre.

This is a relative-selectivity prediction, not a numerical product ratio. Simple attack on the face that happens to look less crowded in the initial flat drawing is unreliable because the relevant C–C single bond can rotate. B is the competing product from the less favourable approach beside vinyl. The model and its preference for the trajectory beside the small group are described in the institutional treatment.^[ref]

For both substrates, the alkene pi bond attacks bromine and polarizes/breaks Br–Br. A bromine lone pair completes a three-membered bromonium ion, and bromide subsequently attacks a carbon from the opposite face, cleaving that C–Br bridge bond. The two new C–Br bonds are anti. A free carbocation that rotates before capture is not needed.

In trans-fused $A$, the fixed conformation makes the alternative anti-opening pathways diastereomeric. A chair-like, trans-diaxial opening is favored over a geometrically strained alternative that requires a twist/boat-like arrangement of the six-membered ring. The Fürst-Plattner rule describes this conformational preference. One depicted enantiomer of $A$ therefore gives essentially one dibromide diastereomer, with the two bromines occupying the trans-diaxial arrangement in its favored chair. If a racemic sample of $A$ were used, its other enantiomer would of course give the mirror product as well.

For cis-fused $B$, the accessible mirror-related folds and the equivalent anti pathways produce mirror-related dibromides with equal rates in an achiral environment. Adding the two bromines destroys the symmetry that made the starting configuration achiral. $B$ gives a racemic dibromide pair. The racemate follows from equivalent enantiomer-forming pathways, not from a loss of anti stereospecificity.

Only the low-coupling pair remains, so the detected product is the trans diastereomer, with approximate coupling 4 Hz. The reaction is highly diastereoselective within the spectrum's detection limit. Its possible absolute configurations are $(2R,3R)$ and $(2S,3S)$.

A conventional proton NMR spectrum in an achiral environment cannot distinguish those enantiomers: they have the same shifts and couplings. Thus a chiral catalyst plus one set of resonances does not establish an enantiomeric excess or an absolute configuration. Chiral chromatography, an appropriate chiral NMR method, or a calibrated stereochemical comparison would be needed.

At 400 MHz, a separation $\Delta \delta$ in ppm corresponds to $J=400\Delta \delta$ Hz. The two multiplets near 5.10 and 4.47 ppm each have a quoted separation of 0.02 ppm, giving $J\simeq 8$ Hz. The multiplets near 4.625 and 3.865 ppm each have separation 0.01 ppm, giving $J\simeq 4$ Hz. These are approximate values from the rounded peak labels.

The fused unsaturated/aromatic framework restricts the five-membered ring, as in the comparison in (a). Assign the larger coupling to cis and the smaller to trans. Matching equal couplings and integrals pairs the 5.10/4.47 signals with one another, and the 4.625/3.865 signals with one another. The integral ratio isWith an achiral substrate and achiral catalytic environment, each diastereomer is expected as a racemic enantiomeric pair: cis $(2R,3S)/(2S,3R)$ and trans $(2R,3R)/(2S,3S)$. The reported spectrum requires both diastereomers; the reagent label alone cannot override that evidence. Their ratio does not establish whether the selectivity arose in the initial surface addition or after product epimerization.

There are three largely independent constructions: a substituted Robinson-annulation core, a protected vicinal diol, and the ester acyl fragment. Disconnect the ester first to an allylic alcohol on the bicyclic ring and the acid $\mathrm{H}{\mathrm{O}}_2\mathrm{CCH}(\mathrm{Et})\mathrm{CH}(\mathrm{Me}{)}_2$. Disconnect the benzylidene acetal to a side chain $-\mathrm{C}{\mathrm{H}}_2\mathrm{CH}(\mathrm{OH})\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ and benzaldehyde. Reversing 1,2-reduction of the allylic alcohol gives the corresponding bicyclic enone; its six-membered enone ring is a Robinson disconnection back to a suitably alpha-substituted cyclohexanone and methyl vinyl ketone.

An efficient way to protect the side chain before annulation is to prepare 4-(bromomethyl)-2-phenyl-1,3-dioxolane from $\mathrm{BrC}{\mathrm{H}}_2\mathrm{CH}(\mathrm{OH})\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ and benzaldehyde with mild acid and water removal. The desired five-membered acetal protects the adjacent hydroxyl groups, while its external primary bromide remains an alkylating site. Alpha alkylation of the indicated cyclohexanone with this bromide gives a ring bearing $-\mathrm{C}{\mathrm{H}}_2\mathrm{CH}(\mathrm{O}-)\mathrm{C}{\mathrm{H}}_2\mathrm{O}-$ in the benzylidene-protected form. There is no need to expose the eventual ring enone to a nonselective late dihydroxylation.

Generate the enolate at the other, less substituted alpha position of the cyclohexanone and add methyl vinyl ketone. Hydrolysis of an enamine-mediated Michael variant or direct enolate trapping gives the appropriate 1,5-dicarbonyl. Intramolecular aldol condensation closes a six-membered ring onto the original cyclohexanone carbonyl. Dehydration gives the fused bicyclic enone, with the protected three-carbon side chain on the adjacent carbon of the original ring. Regioselecting the initial Michael addition is essential: reaction at the side-chain-bearing alpha site would place that substituent at the wrong fusion position.

Reduce the new enone carbonyl by 1,2-addition, for example sodium borohydride with cerium(III) chloride under Luche-type conditions, to retain the displayed ring C=C bond and produce the allylic alcohol. The benzylidene acetal should be maintained through these neutral/basic operations. The target has no wedge/dash assignments, so a constitutional solution does not require a claim of a unique fused-ring/alcohol diastereomer; isolate the required constitutional annulation product.

For the acyl branch, form the enolate of the indicated methyl butanoate, add acetone, then dehydrate the beta-hydroxy ester:Hydrogenate this isolated alkene before coupling it to the bicyclic substrate, giving $\mathrm{Me}{\mathrm{O}}_2\mathrm{CCH}(\mathrm{Et})\mathrm{CHM}{\mathrm{e}}_2$. saponification gives $\mathrm{H}{\mathrm{O}}_2\mathrm{CCH}(\mathrm{Et})\mathrm{CHM}{\mathrm{e}}_2$; activate it as its acid chloride or use a mild ester-coupling reagent, then acylate the bicyclic allylic alcohol. Performing the hydrogenation on the separate acyl branch avoids reducing the target ring alkene. Strong acid should be avoided during the final esterification because the benzylidene group is acid-labile.

The carbon audit is particularly useful: the fused rings contain six carbons from cyclohexanone plus four from methyl vinyl ketone; the side chain has three carbons from the protected bromodiol; the benzylidene unit supplies its acetal carbon and phenyl group. In the acyl fragment, the four butanoate acyl carbons plus three acetone carbons give seven; the starting ester methoxy carbon is lost during hydrolysis.

Both side chains have the order ring–$\mathrm{C}{\mathrm{H}}_2\mathrm{CH}=\mathrm{CHMe}$, and both printed double bonds have the Z geometry. The two carbon side chains are cis to one another; equivalently the labelled ring H and ring Me are cis on the opposite face. A useful common late precursor therefore has two cis ring–$\mathrm{C}{\mathrm{H}}_2\mathrm{CHO}$ substituents, which can undergo two Z-selective ethylidene Wittig reactions.

Use the starting 2-methylcyclohex-2-en-1-one in a reductive alpha-alkylation. Conjugate reduction under dissolving-metal/enolate-trapping conditions, followed by a primary electrophile $\mathrm{BrC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OTBS}$, installs a protected two-carbon alcohol side chain at the original methyl-bearing alpha carbon. This gives 2-methyl-2-(2-siloxyethyl)cyclohexanone. The carbon at that position is now quaternary, with its original methyl retained. Alternatively, reduce the enone to 2-methylcyclohexanone and deliberately form/select its more substituted enolate before alkylation; its opposite kinetic enolate would give the wrong regioisomer.

Convert the remaining ring ketone to an exocyclic enoate by Horner–Wadsworth–Emmons olefination with a phosphonoacetate:Hydrogenate this sole alkene to give ring–$\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_2\mathrm{Et}$ at that carbon. Select the diastereomer with this substituent cis to the pre-existing protected hydroxyethyl side chain, so the newly installed ring H is cis to ring Me. Less hindered-face hydrogenation can assist this relationship, but the route should include separation/verification of the desired cis product rather than assert complete stereoselectivity without data.

Reduce the ester with lithium aluminium hydride to ring–$\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ and remove the TBS group from the other side chain. Both appendages are now two-carbon primary alcohols. Oxidize them to aldehydes under non-overoxidizing conditions, for example Dess–Martin oxidation, obtaining the cis dialdehyde with two ring–$\mathrm{C}{\mathrm{H}}_2\mathrm{CHO}$ groups. Treat with excess salt-controlled, nonstabilized ethylidenetriphenylphosphorane under conditions favouring kinetic Z olefination; isolate the Z,Z product. Each aldehyde carbon becomes the inner vinylic carbon of its crotyl group, while the ylide supplies $\mathrm{CHMe}$.

This order postpones both stereodefined alkenes until after hydrogenation, so they are not accidentally saturated. It also avoids assuming that a crotyl grignard reagent adds without allylic transposition or that direct alcohol deoxygenation fixes the required ring stereochemistry. An achiral synthesis gives a racemic mixture pair with the specified relative configuration.

The left ring system is spiro, and the right-hand substituent is $\mathrm{CH}(\mathrm{OMe})\mathrm{Me}$, not OMe directly attached to the cyclohexene ring. Let $Q$ denote the right cyclohexenyl group bearing that methoxyethyl substituent cis to its linker. The connection to the left alkene carbon is $-(\mathrm{C}{\mathrm{H}}_2{)}_3-\mathrm{Q}$. These observations suggest two Diels–Alder constructions. A left-hand dienehas the original cyclopentane carbon as one terminal diene atom; cycloaddition with an ethylene equivalent gives the spiro six-membered ring. Its residual double bond is between the two internal diene atoms, putting the linker on the required alkene carbon.

Build $Q$ and its four-carbon alcohol appendage first. Protect the OH group of the indicated 4-Bromobutan-1-ol, for example as a THP ether. Convert the primary bromide to its grignard reagent and formylate with DMF, giving $\mathrm{THPOC}{\mathrm{H}}_2(\mathrm{C}{\mathrm{H}}_2{)}_3\mathrm{CHO}$. olefination with an allylidenetriphenylphosphorane, $\mathrm{P}{\mathrm{h}}_3\mathrm{P}=\mathrm{CHCH}=\mathrm{C}{\mathrm{H}}_2$, supplies a conjugated diene $\mathrm{THPOC}{\mathrm{H}}_2(\mathrm{C}{\mathrm{H}}_2{)}_3\mathrm{CH}=\mathrm{CHCH}=\mathrm{C}{\mathrm{H}}_2$; select the required diene geometry rather than assume a single E/Z outcome.

Cycloaddition with methyl vinyl ketone produces a cyclohexene with a protected four-carbon alcohol chain at one saturated ring carbon and an acetyl group at its adjacent ring carbon. Choose and isolate the adjacent-substituent cis regioisomer, matching the two dashed substituent bonds in the target. Endo preference and diene geometry can aid the selection, but an unsymmetrical diene is not guaranteed to give a unique regioisomer. Reduce the acetyl ketone to $\mathrm{CH}(\mathrm{OH})\mathrm{Me}$, then methylate that alcohol with sodium hydride/methyl iodide while the chain-end OH is protected. Remove the THP group under mild acid conditions to reveal $\mathrm{Q}(\mathrm{C}{\mathrm{H}}_2{)}_4\mathrm{OH}$, preserving the secondary methyl ether and the ring alkene.

Oxidize this primary alcohol to $\mathrm{Q}(\mathrm{C}{\mathrm{H}}_2{)}_3\mathrm{CHO}$. Add vinylmagnesium bromide to give $\mathrm{Q}(\mathrm{C}{\mathrm{H}}_2{)}_3\mathrm{CH}(\mathrm{OH})\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$, then oxidize the allylic alcohol to the enone $\mathrm{Q}(\mathrm{C}{\mathrm{H}}_2{)}_3\mathrm{C}(=\mathrm{O})\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$. This carbonyl carbon is the original alcohol-end carbon of 4-bromobutanol; after olefination it will be the linker-bearing carbon of the left ring alkene.

In the other branch, reduce the specified cyclopentanone to cyclopentanol, convert to bromocyclopentane, and react with triphenylphosphine to obtain the cyclopentylphosphonium salt. Deprotonation gives the cyclopentylidene phosphorane, whose ylidic carbon is part of the five-membered ring. Wittig olefination of the just-prepared enone gives the substituted diene displayed above. This carbonyl–ylide pairing keeps the ring carbon terminal in the diene and avoids accidentally constructing a fused ring instead of a spiro ring.

Finally, cycloadd an ethylene equivalent. Ethylene itself requires suitably forcing/high-pressure conditions; a more reactive alternative is phenyl vinyl sulfone, followed by reductive removal of the sulfone substituent using a method compatible with the methyl ether and retained C=C bonds. The latter installs the same two otherwise unsubstituted saturated carbons. Control or separate any additional cycloaddition isomers, and retain the previously selected cis configuration on $Q$.

The carbon count provides a strong check: cyclopentanone supplies five, bromobutanol four, DMF formylation one, the first allylidene ylide three, methyl vinyl ketone four, O-methylation one, vinylmagnesium two, and the final ethylene equivalent two, totalling 22 target carbons. Three original bromobutanol carbons form the linker; its fourth becomes the substituted carbon of the spiro ring's alkene. The proposal is a multistep constitutional/relative-stereochemical route with explicit regioisomer and diastereomer selection, not an unsupported claim of a single quantitative cycloaddition product.

Open the cyclic ester at its acyl–oxygen bond. The resulting delta-hydroxy acid isOxidation-state reversal at the benzylic alcohol gives $\mathrm{H}{\mathrm{O}}_2\mathrm{C}-\mathrm{CH}(\mathrm{nPr})-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}(\mathrm{iPr})-\mathrm{C}(=\mathrm{O})\mathrm{Ph}$. This identifies the key Michael disconnection between the central $\mathrm{C}{\mathrm{H}}_2$ and $\mathrm{CH}(\mathrm{iPr})$ groups:The nucleophile is the enolate or enamine of the phenyl ketone. The electrophile's propyl substituent must be alpha to its ester carbonyl; using a beta-propyl acrylate would move the substituent to the wrong ring position.

Disconnect $\mathrm{PhCOC}{\mathrm{H}}_2\mathrm{iPr}$ further to the indicated acetophenone. Crossed aldol addition to acetone followed by dehydration gives $\mathrm{PhCOCH}=\mathrm{CM}{\mathrm{e}}_2$; selective conjugate reduction or controlled alkene hydrogenation then gives $\mathrm{PhCOC}{\mathrm{H}}_2\mathrm{CHM}{\mathrm{e}}_2$. This avoids requiring an efficient SN2 alkylation with a secondary isopropyl halide. The alpha-propyl acrylate can be obtained by alpha-hydroxymethylation/dehydration of ethyl pentanoate, $\mathrm{Et}{\mathrm{O}}_2\mathrm{CC}{\mathrm{H}}_2\mathrm{nPr}$, using formaldehyde.

In the forward direction, perform the Michael coupling, reduce the phenyl ketone selectively with sodium borohydride while retaining the ordinary ester, hydrolyse the ester, and cyclize the delta-hydroxy acid by controlled acid-catalysed lactonization or an activated-acid coupling. The six-membered ring then has the order carbonyl–$\mathrm{CH}(\mathrm{nPr})$–$\mathrm{C}{\mathrm{H}}_2$–$\mathrm{CH}(\mathrm{iPr})$–$\mathrm{CH}(\mathrm{Ph})$–O. This atom order checks all three substituent positions. No relative configuration is specified, so the scheme does not imply a single diastereomer from the Michael addition or reduction.

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]

One valid asymmetric route uses N-Boc-(R)-proline as a chiral starting material. Convert its acid to the Weinreb amide with N,O-dimethylhydroxylamine and a coupling reagent such as EDC, then add phenylmagnesium bromide at low temperature. chelation of the magnesium tetrahedral intermediate prevents a second addition; aqueous workup releases the phenyl ketone. Reduce it with sodium borohydride, separate the resulting alcohol diastereomers, and retain the one with $R$ configuration at both pyrrolidine C2 and the benzylic carbinol. This is an asymmetric synthesis through a pre-existing enantiopure centre, even though the final reduction need not be perfectly diastereoselective. If the other protected alcohol is isolated, Mitsunobu esterification with a carboxylic acid followed by ester hydrolysis inverts just the carbinol centre. Keeping nitrogen Boc-protected during that inversion avoids neighbouring-amine participation. Mild Boc removal followed by reductive benzylation with benzaldehyde and sodium triacetoxyborohydride gives D, the $(R,R)$ amino alcohol. Use conditions that avoid epimerizing the amino ketone.

For ring expansion, activate D's alcohol with methanesulfonyl chloride and a tertiary amine base. Its own tertiary nitrogen attacks the benzylic carbon from the back, displacing mesylate with inversion and making a fused aziridinium ion. Acetate, supplied as a soluble acetate salt, attacks the original pyrrolidine C2 from the opposite face and breaks the old C2–N bond. The new N–benzylic-C bond is retained, incorporating that side-chain carbon into the six-membered ring. Inversion at the two successively attacked carbons gives the cis acetate/phenyl piperidine E in the displayed orientation. Attack at the benzylic carbon instead would simply open the three-membered ion back to a five-membered-ring derivative; specifying the attacked carbon is the crucial regiochemical step.

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.

A phosphorothioate diastereomer is a stereochemical form of a phosphorothioate substitution that differs in configuration without changing connectivity. In a nucleic-acid phosphate joining different nucleotide environments, replacing a non-bridging oxygen with sulfur can make phosphorus a stereogenic center. The resulting $R_P$ and $S_P$ forms have the same elemental composition but a different spatial arrangement at that center.

They are diastereomers in the usual nucleic-acid context because the rest of the RNA already contains fixed stereochemistry, including its ribose units. Changing one phosphorus configuration while preserving those other centers does not produce the mirror image of the entire RNA. It is therefore misleading to call such a pair enantiomers merely because one phosphorus label changes from R to S. Assignment of the labels uses the Cahn-Ingold-Prelog priority rules, not the order in which a nucleotide sequence is written.

Two independently stereogenic substituted phosphates give four combinations: $(R_P,R_P)$, $(R_P,S_P)$, $(S_P,R_P)$ and $(S_P,S_P)$. Their molecular masses are the same. A mass measurement alone cannot determine either the configuration at one site or the proportions in an unresolved mixture.

A simple activity model shows why mixture composition can matter. Suppose equal amounts of two purified forms separately give product fractions $0.8$ and $0.2$ in a stipulated assay. If molecules act independently and the assay response is linear, a mixture containing fraction $f$ of the first form gives $p=0.8f+0.2(1-f)$. At $f=1/2$, $p=0.5$. These invented numbers illustrate averaging; competition for limiting components can invalidate that model.

The U6 metal rescue experiment supplies an actual chemical distinction. Sulfur substitutions with opposite phosphorus configurations at the same U6 position showed differing metal-rescue behavior, including differences between branching and exon ligation.^[ref] Position, configuration and measured reaction therefore belong in the description of a phosphorothioate reagent.