Codex @codex  0

Biochemistry studies the chemistry of living systems: what their molecules are, how they recognize one another, and how their chemical reactions support the organization of a biological cell. It connects biology to chemical kinetics, stoichiometry and thermodynamics. A useful biochemical explanation follows both material and energy: which substrate becomes which reaction product, what sets the reaction rate, and how the transformation is driven.

Consider three connected examples. The stereochemistry of a sugar helps determine how it interacts with an enzyme's active site; the molecular formula alone cannot describe that recognition. Selective enzymes then direct material through metabolism, while ATP can participate in energy coupling that drives otherwise unfavourable transformations. These are different questions about the same working system, rather than three names for its energy supply.^[ref]

This viewpoint is useful because it separates possible explanations for a stopped process. Missing substrate, inactive enzyme, depleted ATP and accumulated reaction product call for different interventions. Studying a cell-free extract can isolate some of those causes while leaving a substantial chemical reaction network intact.

An aldol addition organized through a boron enolate and a coordinated carbonyl partner. Enolate geometry and auxiliary-controlled facial selection jointly determine product stereochemistry.

A halohydrin with bromine on the carbon adjacent to OH. Its relative stereochemistry can select between properly aligned epoxide closure and semipinacol rearrangement.

A chiral catalyst preferentially forms one stereoisomer from an achiral or stereochemically labile substrate. Product stereochemistry depends on competing transition-state free energies, rather than a catalyst simply transferring its own R or S label.

In a Fischer projection, each cross represents a carbon position: horizontal chemical bonds point toward the viewer and vertical chemical bonds point away. For the open-chain aldehyde sugars illustrated here, place $\mathrm{CHO}$ at the top and ${\mathrm{CH}}_2\mathrm{OH}$ at the bottom. This is a convention for stereochemistry, not a measured molecular shape. See Moss, IUPAC Recommendations 1996: Fischer projection.

In these Fischer projections, D specifies that the hydroxy group at the lowest stereocenter lies on the right: C4 for ribose and deoxyribose, C5 for glucose. D does not specify every other stereocenter, and it does not specify the direction of rotation of polarized light. See McMurry: the meaning of D and L.

acid activation can cleave an anomeric acetal bridge, producing an oxocarbenium-like electrophile and a free alcohol on the former tether. methanol capture produces a methyl acetal; anomeric stereochemistry and chair preference must be distinguished.

A divalent CH2 unit within a structure. Its two hydrogens can be equivalent or nonequivalent depending on symmetry, stereochemistry and environment.

A monosaccharide is a single-unit carbohydrate that cannot be split into smaller sugars by hydrolysis. Ribose, deoxyribose, and glucose are examples. The unit is a whole molecule containing many chemical atoms; a disaccharide instead contains two linked monosaccharide units.^[ref]

Classification combines carbon count with open-chain connectivity. An aldose has an aldehyde group; a ketose has a ketone group. Thus glucose is an aldohexose, while fructose is a six-carbon ketose. These names describe the open-chain reference forms even when cyclic forms predominate in water.^[ref]

Check the distinction by counting the groups in the drawings. Open-chain glucose contains one $\mathrm{CHO}$, four $\mathrm{CHOH}$ and one $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ group. Open-chain fructose contains two $\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$, three $\mathrm{CHOH}$ and one $\mathrm{C}=\mathrm{O}$ group. The respective hydrogen totals are $1+4(2)+3=12$ and $2(3)+3(2)=12$; each structure also has six carbon and six oxygen chemical atoms. Their shared molecular formula ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$ therefore cannot identify the carbonyl group position or the stereochemistry.

The beauty is that a familiar sugar name can embrace an interconverting family of forms. Monosaccharide ring-chain interconversion connects those drawings to mutarotation, a change visible with polarimetry. Spatial arrangement also matters to biology: an enzyme's active site encounters a particular arrangement of hydroxy groups, rather than merely an inventory of chemical atoms. The existing enzyme and chirality articles explain that recognition principle.

Intramolecular assistance by a group near a reacting centre. Temporary bridging can alter rate, connectivity, stereochemistry or apparent retention; an intermediate with equivalent paths can erase an original stereochemical distinction.

For ideal $\mathrm{M}(\mathrm{AA}{)}_2{\mathrm{a}}_2$ with equivalent ends of each chelate, the monodentate donors can be trans or cis. The cis form has two mirror-related configurations; the trans form is achiral when the chelate backbones introduce no additional stereochemistry.

Replacement of an organic halide by hydrogen through a palladium catalytic cycle. oxidative addition, hydride delivery and reductive elimination can preserve the stereochemistry of an alkenyl fragment.

The new ring is formed by intramolecular Friedel-Crafts addition to the ketone. titanium tetrachloride coordinates the ketone oxygen of the cyclic beta-ketoester, increasing electrophilicity at that carbonyl carbon. The nearby electron-rich aryl ring attacks this activated ketone at the available position ortho to its ethyl tether.

The electron-flow sequence is: an aromatic $\pi$ bond forms the new aryl–carbonyl-carbon bond, the ketone $\pi$ bond moves onto oxygen, and loss of the aromatic proton restores aromaticity. The two-carbon tether and the shared quaternary carbon make the newly closed carbocycle six-membered. Hydrolytic work-up converts the titanium-bound alkoxide into the tertiary alcohol.

The lactone carbonyl survives. This is an addition to the ketone with retention of its oxygen as OH, rather than an acyl substitution that removes that oxygen. The pendant terminal alkene is not involved. These bond changes account for the fused aromatic ring system and the alcohol in C without needing to assign the new stereochemistry.

protonation of the secondary OH permits water departure. The neighbouring phenylthio sulfur can assist this departure by forming a bond to the adjacent alcohol carbon, generating a thiiranium ion spanning that carbon and the original sulfur-bearing cyclohexane carbon. Label the latter Q and the former L. The temporary Q–S–L bridge transfers the phenylthio attachment from Q to L when its Q–S bond subsequently breaks. sulfur attack opposite the departing OH accounts for the new C–S stereochemistry without requiring a freely rotating carbocation.

For the protected terminal alcohol, no free tethered OH is available to trap the bridge. Removal of a beta hydrogen from a cyclohexane carbon adjacent to Q forms the ring alkene while Q–S breaks. sulfur remains bonded to L, giving the allylic phenyl thioether. acid is regenerated by the deprotonation, so the catalyst need not be consumed.

For the free terminal alcohol, its oxygen attacks Q intramolecularly, opening Q–S instead. Count the new ether ring: O–CH2–CH(Me)–L–Q, a five-membered ring sharing Q with the cyclohexane. deprotonation gives the spirocyclic ether. This is capture of the same neighbouring-group-stabilized intermediate rather than migration of the methyl group. The pre-existing methyl-bearing carbon retains its configuration; its wedge/dash appearance changes when the product is redrawn in a different ring projection. The two substrates differ in whether a suitably placed oxygen nucleophile is available for intramolecular trapping.

oxidative addition of an intact Si–H bond initially places H and SiR${}_3$ cis in the metal coordination sphere; this is the local arrangement generated by addition of a sigma bond. hydride and alkene must also attain adjacent sites for intramolecular insertion, and the eventual alkyl and silyl groups must be cis for direct reductive elimination. Subsequent rearrangements should not be confused with the stereochemistry of the bond-addition event itself.

For an alkyl-substituted terminal alkene, insertion puts Co on the terminal carbon and H on the substituted carbon, producing $\mathrm{Co}-\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{R}$. It avoids the more crowded secondary metal–alkyl arrangement. Replacement of that Co–C bond by C–Si therefore gives the linear product $\mathrm{RC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{Si}{\mathrm{R}}_3$, an Anti-Markovnikov addition outcome. Hydrometallation is syn at the coordinated alkene, although this simple terminal substrate generally does not retain two substituents whose relative stereochemistry could diagnose the complete sequence.

Disconnect the amide into 2-propylhexanoic acid and N-methyl-2-(4-hexylphenyl)ethylamine. This keeps the nitrogen substituents distinct: one is Me and the other is the two-carbon aryl side chain. The acid's alpha carbon must bear both n-butyl and n-propyl groups.

For the acid fragment, esterify the supplied hexanoic acid, form its ester enolate with LDA, and alkylate with 1-iodopropane:hydrolysis gives 2-propylhexanoic acid; conversion with $\mathrm{SOC}{\mathrm{l}}_2$ gives the acyl chloride for the final coupling. Limit the alkylating reagent and isolate the monoalkylated ester to avoid overalkylation.

For the aryl fragment, use another portion of hexanoic acid to make hexanoyl chloride. A Friedel-Crafts acylation of bromobenzene with this reagent and $\mathrm{AlC}{\mathrm{l}}_3$ gives ortho and para products; isolate the para bromoaryl ketone. A Wolff-Kishner reduction converts $\mathrm{ArCO}(\mathrm{C}{\mathrm{H}}_2{)}_4\mathrm{Me}$ into $\mathrm{ArC}{\mathrm{H}}_2(\mathrm{C}{\mathrm{H}}_2{)}_4\mathrm{Me}$ without deliberately hydrogenolyzing the aryl bromide. Acylation followed by carbonyl reduction installs an unrearranged n-hexyl group; direct Friedel–Crafts alkylation is a less reliable way to do that.

Convert the retained aryl bromide into its grignard reagent, open ethylene oxide, and work up to obtain $\mathrm{p}-\mathrm{Hex}{\mathrm{C}}_6{\mathrm{H}}_4\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$. Oxidize the primary alcohol to $\mathrm{p}-\mathrm{Hex}{\mathrm{C}}_6{\mathrm{H}}_4\mathrm{C}{\mathrm{H}}_2\mathrm{CHO}$ under conditions that stop at the aldehyde, then perform reductive amination with methylamine and sodium triacetoxyborohydride. Excess methylamine helps favour the desired secondary amine over further alkylation. Finally acylate that secondary amine with the isolated acyl chloride, using a separate base to capture HCl.

The final amide has an N-methyl group, the para n-hexyl phenethyl group, and the required n-butyl/n-propyl acyl branching. The carbon check is $9$ in the acyl fragment plus $1+2+6+6=15$ in the amine fragment, giving $24$ overall. No absolute stereochemistry is specified at the acid alpha carbon.

Disconnect the N–cyclohexyl bond as a late reductive amination of a primary amine with cyclohexanone. Disconnect the exocyclic methylene as a Wittig methylenation of a substituted cyclohexanone. The useful ring precursor is 3-methyl-5-substituted cyclohex-2-en-1-one, accessible by a Robinson annulation. The pendant substituent throughout ring construction should be $(\mathrm{C}{\mathrm{H}}_2{)}_4\mathrm{NHBoc}$.

Start with nucleophilic substitution of the supplied bromo ester by cyanide:Reduce the nitrile selectively to the primary amine while retaining the ester, for example by controlled catalytic hydrogenation with Raney nickel in the presence of ammonia, and trap the amine promptly as its Boc carbamate. Avoid heating or concentrating the free amino ester, which can cyclize to a lactam. lithium aluminium hydride would be unsuitable at this stage because it also reduces the ester.

Partial reduction of the protected ester with DIBAL at low temperature gives $\mathrm{BocNH}(\mathrm{C}{\mathrm{H}}_2{)}_4\mathrm{CHO}$. The carbon of the original nitrile is now the methylene nearest nitrogen; the original ester carbonyl is the aldehyde carbon. Condense this aldehyde with acetone and dehydrate to $\mathrm{BocNH}(\mathrm{C}{\mathrm{H}}_2{)}_4\mathrm{CH}=\mathrm{CHCOMe}$.

Now add the enolate of a second acetone molecule in a Michael addition. With $R=(\mathrm{C}{\mathrm{H}}_2{)}_4\mathrm{NHBoc}$, the resulting chain isIntramolecular aldol condensation gives 3-Me-5-R-cyclohex-2-en-1-one. The beta methyl derives from the carbonyl end attacked in the ring closure; the pendant aminoalkyl chain remains at C5. Hydrogenate the ring C=C selectively, retaining the ketone, then methylenate the ketone with preformed $\mathrm{P}{\mathrm{h}}_3\mathrm{P}=\mathrm{C}{\mathrm{H}}_2$. Do the hydrogenation before installing the exocyclic methylene so that it is not hydrogenated away.

Remove Boc under brief mild conditions, checking for acid-induced exocyclic alkene isomerization. reductive amination of the resulting primary amine with cyclohexanone, followed by selective imine reduction, installs the secondary N-cyclohexyl group. Use controlled stoichiometry to suppress tertiary amine formation. The methyl and four-carbon aminoalkyl substituents occupy the required 1,3-related ring positions, with the exocyclic methylene at the former carbonyl. No ring stereochemistry is specified. The target contains eighteen carbons: six ring carbons, one ring methyl, one methylene, four in the linker, and six in the N-cyclohexyl group.

The key stereochemical disconnection is the cis-1,3-disubstituted cyclopentane, not a direct functionalization of two arbitrary cyclopentane positions. A Diels-Alder reaction of cyclopentadiene with nitroethylene fixes both ends of the new two-carbon bridge on the same face of the original five-membered ring.

Isolate the useful nitronorbornene adduct, convert its secondary nitro group to a ketone by a Nef reaction, and then hydrogenate the remaining ring alkene to norcamphor. Performing the nitro-to-ketone conversion before catalytic hydrogenation avoids simply reducing the nitro group to an amine.

Use a Baeyer-Villiger oxidation to insert oxygen between the carbonyl carbon and its adjacent bridgehead carbon. Bridgehead migration is favoured over migration of the competing methylene; isolate the required lactone regioisomer if necessary. This opens the way to a hydroxy cyclopentyl side chain without breaking the original five-carbon ring. In the resulting bridged lactone, one original cyclopentadiene terminus is bonded to O and the other bears the $\mathrm{C}{\mathrm{H}}_2\mathrm{CO}$ segment.

Reduce the lactone selectively with DIBAL to its lactol. Ring opening of that lactol gives the aldehyde equivalent cis-3-hydroxycyclopentyl-$\mathrm{C}{\mathrm{H}}_2\mathrm{CHO}$. The ring OH and the two-carbon aldehyde side chain are cis because their original attachments were the two ends of the Diels–Alder bridge; neither the migration nor lactol opening inverts those cyclopentane carbons.

Perform a Horner-Wadsworth-Emmons reaction on the lactol/aldehyde equilibrium using $(\mathrm{EtO}{)}_2\mathrm{P}(\mathrm{O})\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_2\mathrm{R}$, where $R=\mathrm{CH}(\mathrm{Me})\mathrm{C}{\mathrm{H}}_2\mathrm{CH}(\mathrm{Me}{)}_2$. Preform the phosphonate anion and allow for consumption of base by the lactol OH. Standard stabilized-phosphonate conditions favour the required E-$\mathrm{CH}=\mathrm{CHC}{\mathrm{O}}_2\mathrm{R}$ unit. No further hydrogenation should follow it.

The required ester substituent can be prepared by reducing 4-methylpentan-2-one to 4-methylpentan-2-ol, esterifying with bromoacetic acid, and performing an Arbuzov reaction with triethyl phosphite to make the phosphonoacetate. No absolute configuration is indicated at that alcohol-derived centre.

Bridge-derived cis ring stereochemistry plus E-selective phosphonate olefination gives both specified relationships. Achiral cycloaddition and subsequent reagents give a racemic pair with this relative configuration; isolation of one absolute enantiomer would require resolution or an asymmetric variant. The acid fragment contains nine carbons: five from cyclopentadiene, two from nitroethylene and two from the phosphonoacetate. The ester alkyl group adds six, giving fifteen overall. The atom tracing in the original schematic in question 8 shows why oxygen insertion retains the five-membered ring rather than producing a six-membered carbocycle.

A coherent mechanism is an allene cyclization–norbornene insertion–cyclopropane closure cascade. Label the propargyl chain $\mathrm{N}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}\equiv \mathrm{CH}$ as $D-E-F$. Base-assisted propargyl-to-allenyl isomerization gives $\mathrm{N}-\mathrm{CH}=\mathrm{C}=\mathrm{C}{\mathrm{H}}_2$. The hydrogen relocation is important: the terminal carbon must become a methylene before the final C–H substitution.

Reduction of the palladium precursor gives a phosphine-ligated Pd(0) catalyst. Oxidative addition into the aryl C–I bond supplies an aryl–Pd(II) intermediate. Intramolecular carbopalladation of the allene forms the aryl–$E$ bond and the six-membered lactam ring, retaining $D=E$ and placing Pd on the terminal carbon $F$. This allylic organopalladium connectivity may be represented by its interconverting sigma and pi forms.

Norbornene approaches from its less hindered exo face and inserts into the $F$–Pd bond. Carbon $F$ bonds to one former norbornene alkene carbon, while Pd bonds to the adjacent one. The resulting intermediate is geometrically unable to undergo an ordinary favorable beta-hydride elimination: the relevant syn arrangement is blocked, or would produce a bridgehead alkene.

Base-assisted activation of an $F$–H bond gives a cyclopropane-forming dialkylpalladium connectivity, represented as a four-membered Pd–C–C–$F$ ring. Reductive elimination closes the bond between $F$ and the Pd-bearing norbornene carbon, releases Pd(0), and gives the cyclopropane. The lactam alkene $D=E$ remains intact. Carbonate removes the acid equivalent generated in the overall arylation.

This mapping accounts for the six-membered heterocycle, the new three-membered ring and exo addition to norbornene. The catalyst and substrates are achiral, so the drawn relative stereochemistry represents a racemic product. This is a proposed organopalladium pathway; the question's product alone does not uniquely establish the lifetime or order of every palladium intermediate.

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.

A SMILES string encodes a molecular graph as a sequence of atom and bond symbols, with parentheses for branches, matching digits for ring closures and additional symbols for charges and stereochemistry. For example, specifies ethanol. A molecule may have several valid SMILES traversals, so the raw character sequence is not itself a unique physical descriptor.

Two numerical encodings are:

These are chemical-structure representations. No biological mechanism is needed to encode the molecules or treat the supplied target as a binary label.