IChO-2025_2.2
Type: Structure Construction
Points: 19
Question: context: After a few transformations,  C can be converted into an aldehyde D which reacts with E to form F.  F can be further transformed into rapamycin. Using the Zimmerman-Traxler model, predict the arrangement of the reactants as they come together to form the six-membered ring transition state in the reaction to form F. Draw the substituents at positions labelled with numbers. If a position does not have a substituent, put an X in the box. Boxes 1 and 2 have been filled in as an example. You may use the abbreviation  R^{2} as given in the scheme above. Draw the structure of F showing all stereochemistry. You may use the abbreviation  R^{2} as given in the scheme above.
requirement: Using the Zimmerman-Traxler six-membered ring transition state model, fill in the boxes (3-8) to show the arrangement of groups in the transition state that leads to the formation of product F. Draw the structure of compound F with correct stereochemistry, including the newly formed stereocenters from the aldol reaction.
Answer: model: Six-membered ring Zimmerman-Traxler Model
transition_state_positions: position_3: group: Evans auxiliary with Ph and Me groups
description: Phenyl group on nitrogen-containing oxazolidinone auxiliary with Me substituent
position_4: group: X
description: Position marker or variable group
position_5: group: —H
description: Hydrogen on the enolate carbon
position_6: group: Cyclohexyl fragment with OMe and OTBS
description: Six-membered ring with methoxy and tert-butyldimethylsilyl ether substituents
position_7: group: R²
description: Alkyl chain from aldehyde (polyene fragment)
position_8: group: —H
description: Hydrogen on the aldehyde carbon
key_features: chair_conformation: Six-membered ring chair
boron_coordination: Boron center coordinates enolate oxygen and aldehyde oxygen
Z_enolate: Z-configuration of enolate (positions 5/6)
aldehyde_orientation: Aldehyde at back with R² group (position 7/8)
points: 15
grading: full_credit: condition: Aldehyde approach (aldehyde is at the back instead of the front)
points: 5
condition: Z enolate (flip 5/6)
points: 5
condition: Aldehyde conformation (flip 7/8)
points: 5
partial_credit: condition: For swapping positions 3 and 4
deduction: -2
condition: For each trivial mistake
deduction: -1
structure_description: Aldol product with Evans auxiliary still attached
key_features: auxiliary: Phenyl oxazolidinone with Me group remains attached
new_stereocenters: Two newly formed stereocenters from aldol reaction
polyene_chain: R² represents the polyene chain from compound C
protected_groups: OMe and OTBS groups on cyclohexyl fragment
hydroxyl_group: New OH group formed at aldol position
smiles: [Complex structure with Evans auxiliary, new stereocenters, and polyene chain - exact SMILES depends on R² definition]
points: 4
grading: full_credit: condition: Structure of F with stereochemistry fully correct
points: 4
partial_credit: condition: Absolute stereochemistry of newly formed stereocenters is correct
points: 2
additional_note: If incorrect, but the relative stereochemistry is correct
additional_points: 1
condition: For each error of functional groups not undergoing reaction (trivial error)
deduction: -1
notes: No error carried forward awarded even if part 2.2a is incorrect as stereochemistry can be inferred from the final structure of rapamycin.