Conformational Control by Proline Derivatives and Their Medicinal Chemistry Effects: Ring Puckering, Amide Cis–Trans Isomerization, and Drug Properties
Conformational Control by Proline Derivatives and Their Medicinal Chemistry Effects: Ring Puckering, Amide Cis–Trans Isomerization, and Drug Properties
Proline and its derivatives are widely found in small-molecule drugs, peptidomimetics, and therapeutic peptides. Recent medicinal chemistry studies have shown that structures such as 4-fluoroproline, 4-(trifluoromethyl)proline, α-substituted proline, and bicyclic proline can alter molecular properties including target binding, lipophilicity, solubility, and metabolic stability [1].
These effects are related to the unique backbone structure of proline. The proline side chain cyclizes back to the backbone nitrogen atom to form a five-membered pyrrolidine ring. The puckering state of this ring is coupled to the spatial orientation of adjacent amide bonds. Consequently, modification of a substituent on the ring can propagate to the neighboring backbone and ultimately affect the three-dimensional conformation of the entire molecule.
This article focuses on this relationship and addresses one central question: How can a local structural change on the proline ring propagate step by step and ultimately influence the behavior of an entire drug molecule?
1. Why Can the Proline Ring Control Backbone Conformation?
1.1 The Five-Membered Ring Structure of Proline
Taking L-proline (CAS 147-85-3) as an example, its side chain is connected to the amino nitrogen to form a five-membered pyrrolidine ring. Compared with most proteinogenic amino acids, this cyclic structure restricts the spatial orientations accessible to the backbone, while the pyrrolidine ring itself can still undergo conformational puckering.
In proline, C4 corresponds to Cγ in amino acid nomenclature and is an important modification site discussed in this article. Introduction of substituents such as F, CF₃, OH, or NH₂ at the 4-position can alter the puckering preference of the pyrrolidine ring through stereoelectronic and steric effects, thereby further influencing the adjacent amide bond and backbone conformation.
In the proline structure, the γ-carbon is the C4 atom. According to the direction in which Cγ deviates from the principal plane of the ring, two conformations are commonly used to describe the major puckering states:
① Cγ-endo: Cγ deviates toward the side containing the carboxyl group;
② Cγ-exo: Cγ deviates toward the opposite side.
Unsubstituted proline shows only a weak preference for either state, and the two states can interconvert rapidly. Model studies have estimated the energy barrier for ring-pucker interconversion to be approximately 10–13 kJ·mol⁻¹ [2].
Proline therefore provides a five-membered ring system in which the accessible conformational space is already restricted but can still be further tuned. Substitution at the 4-position represents an important means of controlling this system.
Major pathway through which substitution at the 4-position of proline produces structural effects
C4 substituent and its stereochemistry
↓
Stereoelectronic effects + steric effects
↓
Changes in the relative stability of Cγ-endo / Cγ-exo
↓
Changes in the spatial orientation of adjacent carbonyl groups and the backbone
↓
Changes in amide cis–trans equilibrium and rotational kinetics
This pathway describes local conformational effects. In a complete drug molecule, additional factors such as other functional groups, solvation, and protein binding are superimposed on these local effects.
2. How Does 4-Fluoroproline Alter Ring Puckering?
2.1 The C—F Bond Exhibits Pronounced Stereoelectronic Effects
4-Fluoroproline is one of the most extensively studied model systems for conformational control in proline.
The C—F bond is strongly polar. In 4-fluoroproline, the F—C—C—N fragment is influenced by the gauche effect (a vicinal gauche conformational effect), causing the C—F bond to preferentially adopt particular spatial arrangements. Experimental studies have shown that this effect can stabilize specific pyrrolidine ring-puckering states [2].
For the L-proline configuration, the two commonly studied 4-fluoro stereoisomers exhibit different conformational preferences:
Structure | Major Ring-Puckering Preference |
(2S,4S)-4-Fluoroproline | Cγ-endo |
(2S,4R)-4-Fluoroproline | Cγ-exo |
NMR studies by Kubyshkin showed that these conformational preferences persist in solvents of different polarities, supporting the conclusion that they arise primarily from C—F stereoelectronic effects rather than simply from solvent-induced dipolar interactions [2].
2.2 Ring Puckering Further Influences the Backbone
The distinction between Cγ-endo and Cγ-exo is not confined to the pyrrolidine ring itself.
In studies using fluoroproline and other proline analogues to investigate backbone conformation, Shoulders et al. found that ring-puckering preferences induced by side-chain substitution can propagate to backbone dihedral angles, thereby altering the spatial arrangement of carbonyl groups and neighboring backbone functionalities [3].
Consequently, two fluoroproline derivatives that differ only in their stereochemistry at the 4-position may exhibit differences within a complete drug molecule in terms of:
① Backbone conformation;
② Substituent orientation;
③ Complementarity with the protein binding pocket;
④ Amide conformational equilibrium.
3. How Does 4-CF₃-Proline Differ from 4-Fluoroproline?
Both CF₃ and F are fluorine-containing substituents, but they control proline ring conformation through different mechanisms.
3.1 Monofluorination Primarily Exhibits Stereoelectronic Effects
In 4-fluoroproline, the single fluorine atom tends to adopt a pseudoaxial orientation, and the C—F gauche effect has a pronounced influence on ring puckering [2].
3.2 CF₃ Is More Strongly Governed by Steric Effects
The CF₃ group is substantially larger than a single F atom. Model studies have shown that in 4-CF₃-proline, the C—CF₃ bond tends to position the CF₃ group in a pseudoequatorial orientation in order to minimize steric crowding [2].
The two types of substitution can be summarized as follows:
Substituent | Major Influencing Factors | Ring-Conformational Characteristics |
F | C—F stereoelectronic effect | Relatively pronounced conformational preference with strong stereochemical dependence |
CF₃ | Steric effects, accompanied by electronic and dipolar effects | Tends toward a pseudoequatorial orientation, with a relatively weaker ring-puckering preference |
Kubyshkin's experiments further showed that although 4-CF₃-proline exhibits a degree of ring-conformational preference, this preference is weaker than that of the corresponding 4-fluoroproline derivatives [2].
This difference is particularly important for the amide cis–trans equilibrium discussed below.
4. How Do Proline Derivatives Affect Adjacent Amide Bonds?
Two distinct properties of an amide adjacent to proline need to be considered separately:
1. What proportions of trans and cis are present at equilibrium;
2. How rapidly trans and cis interconvert.
The former is a thermodynamic equilibrium property, whereas the latter is a kinetic process.
4.1 What Does the trans/cis Equilibrium Mean?
The amide bond in a peptidyl-proline structure can form two major rotational isomers:
· trans: trans amide
· cis: cis amide
The trans/cis equilibrium constant can be expressed as:
K(trans/cis) = equilibrium concentration of the trans rotamer ÷ equilibrium concentration of the cis rotamer
For example:
If the equilibrium population is 80% trans and 20% cis:
K(trans/cis) = 80 ÷ 20 = 4
The larger the value of K(trans/cis), the higher the proportion of the trans form at equilibrium.
4.2 Why Does Ring Puckering Affect the trans/cis Ratio?
In fluoroproline models, stereoisomers that preferentially adopt the Cγ-exo conformation generally increase the relative population of the trans amide, whereas stereoisomers that favor Cγ-endo tend to increase the relative proportion of the cis amide [2].
This phenomenon arises because ring puckering alters the spatial relationship between neighboring backbone groups.
The literature frequently invokes an n→π* interaction between adjacent carbonyl groups to explain the stabilizing effect of Cγ-exo on particular backbone arrangements. However, the physical origin of this stabilization remains subject to different interpretations involving orbital interactions and dipolar effects. Therefore, in drug design, it is more appropriate to understand this phenomenon as an overall difference in conformational energy arising from changes in local geometry [2].
4.3 4-CF₃ Substitution Has a Relatively Limited Effect on Equilibrium Populations
4-CF₃-proline exhibits a particularly useful phenomenon.
In N-acetyl amino acid methyl ester models, the trans/cis ratios of the two 4-CF₃-proline stereoisomers are relatively close to that of unsubstituted proline. The changes in energy differences derived from these ratios do not exceed approximately 1.1 kJ·mol⁻¹ [2].
This indicates that 4-CF₃ substitution can significantly influence other molecular properties while exerting only a relatively small effect on the steady-state trans/cis population.
5. Why Are Amide Rotational Kinetics Important?
5.1 The trans/cis Ratio and Rotation Rate Describe Different Processes
Schematic free-energy diagram of cis/trans amide isomerization. The energy levels and barrier heights shown are illustrative and do not represent actual values for a specific compound.
The diagram contains two distinct types of information.
The energy difference between the two low-energy cis and trans states
determines their relative populations at equilibrium, that is, K(trans/cis).
The energy difference from the cis or trans state to the top of the transition-state barrier
corresponds to the activation free energy of amide rotation, denoted ΔG‡, which determines the rate of conformational interconversion.
Therefore, a modification may exert only a small effect on K(trans/cis) while significantly changing ΔG‡ and the rate of amide rotation.
The cis and trans states correspond to two relatively stable amide rotamers. Their relative free-energy difference determines the cis/trans ratio at equilibrium, whereas the activation free energy from each stable state to the rotational transition state determines the rate of cis/trans interconversion. Different proline derivatives can affect these two parameters separately; therefore, changes in conformational populations and changes in isomerization rates need to be evaluated independently.
5.2 Proline Amide Rotation Is Intrinsically a Relatively Slow Process
The amide C—N bond has partial double-bond character, and rotation around the C—N bond therefore requires the molecule to overcome a relatively high energy barrier.
The typical rotational barrier of a peptidyl-proline bond in water is approximately 80–90 kJ·mol⁻¹, and cis/trans interconversion can occur on the millihertz timescale [2].
Experimentally, exchange spectroscopy (EXSY) NMR is commonly used to measure the exchange rate between two rotamers, after which the rotational activation barrier can be calculated using the Eyring relationship.
As ΔG‡ decreases, the rate of amide rotation increases accordingly. Because the exchange rate depends exponentially on the activation barrier, a change of only several kilojoules per mole can produce a substantial difference in the rate.
5.3 Cγ-exo May Participate in the Transition State for Amide Rotation
By comparing 4-F-, 4-CF₃-, and other proline analogues, Kubyshkin found systematic differences in amide rotation rates among different stereoisomers substituted at the 4-position.
These data support the following mechanistic model:
The major transition state for peptidyl-proline amide rotation tends to adopt a Cγ-exo ring conformation. [2]
According to this model:
① Proline analogues that readily adopt Cγ-exo require less conformational adjustment to reach the rotational transition state;
② Analogues that disfavor formation of Cγ-exo require a greater conformational energy input;
③ Dipolar relationships between the substituent and the neighboring carbonyl group may further alter the energy of the transition state.
In models of the two 4-CF₃-proline stereoisomers, the ground-state trans/cis ratios differ only slightly, yet the cis→trans rotational barriers still differ by approximately 2.0–2.6 kJ·mol⁻¹ [2].
This result demonstrates that ring conformation can control not only the populations of stable conformations but also the process of conformational interconversion.
6. How Do Local Conformational Changes Affect Complete Drug Molecules?
The studies discussed above primarily reveal the relationships between the local structure of proline, ring puckering, amide conformation, and rotational kinetics. In a complete drug molecule, these local effects must propagate through multiple levels before ultimately being expressed as changes in binding and drug properties.
6.1 Target Binding: Altering the Accessibility of Productive Binding Conformations
Protein–ligand binding requires a drug molecule to adopt a three-dimensional conformation that is complementary to the binding pocket.
If a particular proline substitution increases the population of conformations resembling the bound state, less conformational adjustment may be required as the drug transitions from the unbound state to the bound state. This phenomenon is commonly referred to as conformational preorganization.
The design of proline analogues therefore requires simultaneous consideration of:
① The ring-puckering preference of the unbound molecule;
② The ring-puckering state in the protein-bound conformation;
③ The orientation of neighboring carbonyl groups;
④ The spatial position of the substituent within the binding pocket;
⑤ Whether a certain degree of local conformational adaptability needs to be retained.
Studies by Shoulders et al. demonstrated that stereoelectronic and steric effects originating from proline side chains can propagate to the backbone structure, and therefore ring substitution can alter the degree of backbone preorganization [3].
6.2 Lipophilicity: The Substituent Itself and Three-Dimensional Dipole Arrangement Jointly Determine the Outcome
Simple statements such as “F increases polarity” or “CF₃ increases hydrophobicity” are insufficient to describe the physicochemical properties of complete proline derivatives.
In N-acetyl proline methyl ester models, Kubyshkin measured the following changes:
Structural Change | Change in logP Relative to the Unsubstituted Model |
4-F substitution | Approximately −0.2 to −0.4 |
4-CF₃ substitution | Approximately +0.7 |
A difference of approximately 0.18 logP units was also observed between the two 4-F diastereomers [2]. These compounds have the same molecular formula and functional-group composition, differing only in substituent orientation and overall dipole arrangement. Lipophilicity is therefore jointly influenced by multiple factors:
Intrinsic substituent properties + ring conformation + bond-dipole orientation + intramolecular dipole arrangement + solvation
This is also why, once a proline analogue is incorporated into a complete drug molecule, its physicochemical properties need to be evaluated through experimentally measured logP or logD under defined pH conditions.
For a drug to move from the solid state into an aqueous phase, two important processes are involved:
1. Overcoming intermolecular interactions within the crystal;
2. Achieving adequate solvation after entering the aqueous phase.
Solubility is simultaneously affected by the degree of exposure of polar groups; the overall lipophilicity of the molecule; intermolecular hydrogen bonding; crystal-packing patterns; melting point and lattice stability; and the relative stability of different conformations in the solid state and in solution.
Changes in amide rotation may alter both the conformations accessible to the molecule and its crystal-packing arrangements. This relationship has been examined through relatively systematic experimental observations in studies of ibuzatrelvir [4].
6.4 Metabolic Stability: The Electronic Properties of Substituents and Their Three-Dimensional Exposure Jointly Influence Metabolism
Fluorination is frequently used to reduce oxidative metabolism, but the introduction of F does not produce the same result in every molecule.
Proline modification can potentially influence metabolism through multiple pathways:
① Altering the electronic properties of specific C—H sites;
② Altering molecular lipophilicity;
③ Altering the exposure of potential metabolic sites;
④ Altering the binding conformation of the molecule to cytochrome P450 enzymes.
Therefore, metabolic stability should be evaluated using data from the complete molecule, including liver microsomal assays, hepatocyte assays, and metabolite identification. It cannot be inferred directly from the presence of a single substituent.
7. Ibuzatrelvir: From the P2 Proline Scaffold to Whole-Drug Properties
PF-07817883, with the generic name ibuzatrelvir, is a second-generation severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) main protease (Mpro) inhibitor developed through further optimization of nirmatrelvir.
One of Pfizer's major research objectives was to reduce the susceptibility of nirmatrelvir to cytochrome P450 3A (CYP3A)-mediated oxidative metabolism while maintaining antiviral activity and oral absorption [4]. PF-07817883 ultimately demonstrated greater metabolic stability than nirmatrelvir.
7.1 What Structural Changes Occurred in the P2 Region?
The P2 region of nirmatrelvir contains a highly constrained 3-azabicyclo[3.1.0]hexane scaffold.
At the corresponding position, ibuzatrelvir uses (2S,4R)-4-CF₃-L-proline.
At the same time, structural optimization was also carried out in the P4 region. Therefore, differences between the two molecules in overall lipophilicity, solubility, metabolic stability, and pharmacokinetics arise from the combined effects of multiple structural modifications [4].
This point is particularly important when analyzing the role of the P2 proline unit: the P2 region can be used to explain local conformation and binding-pocket complementarity, but all ADME changes of the complete molecule cannot be attributed solely to 4-CF₃-Pro.
7.2 How Does 4-CF₃-Pro Contribute to Mpro Binding?
The crystal structure of SARS-CoV-2 Mpro complexed with ibuzatrelvir shows the formation of a stable enzyme–inhibitor complex. The overall binding conformations of ibuzatrelvir and nirmatrelvir are highly similar [5].
The P2 4-CF₃-proline residue occupies the S2 region of Mpro and forms favorable hydrophobic and steric complementarity with this region.
The nitrile group of ibuzatrelvir forms a reversible covalent adduct with the catalytic residue Cys145. Structural studies show that nucleophilic attack of the nitrile group by the Cys145 thiol produces a thioimidate-type adduct [5].
The P2 4-CF₃-Pro unit therefore has two types of structural roles:
① Adjusting the local P2 scaffold and ring conformation;
② Providing hydrophobic volume and spatial shape that complement the S2 region.
7.3 P2/P3 Amide Rotation Changes Markedly
One important difference between PF-07817883 and nirmatrelvir is the extent to which amide rotation in the P2/P3 region is restricted.
Allerton et al. compared the two compounds using NMR and solid-state analysis and found that observable amide rotamers are present in both compounds. However, rotamer exchange is substantially faster and the rotational barrier is lower for PF-07817883 [4]. The original study explicitly reported pronounced differences between the two compounds in rotamer interconversion rates and rotational barriers.
Representative parameters reported in the study are as follows:
Parameter | Nirmatrelvir | PF-07817883 |
P2/P3 amide rotational half-life, t½ | Approximately 10.54 min | Approximately 0.21 min |
Energy of Rotation | Approximately 20.4 kcal·mol⁻¹ | Approximately 18.9 kcal·mol⁻¹ |
The rotational half-life decreases from approximately 10.5 min to approximately 0.21 min, corresponding to an approximately 50-fold difference in timescale [4].
Although the reported energies of rotation differ by only approximately 1.5 kcal·mol⁻¹, amide rotation rates are highly sensitive to rotational barriers, and this difference can therefore still produce a pronounced kinetic effect.
This result corresponds well with the model studies discussed above:
Changes in the local proline scaffold → changes in ring-conformational constraints → changes in the rotational barrier of the adjacent amide → changes in the exchange rate between cis/trans rotamers
7.4 Why Might Amide Dynamics Affect Solid-State and Dissolution Behavior?
Allerton et al. also investigated the solid-state properties of the two molecules and discussed the relationship between the extent of restricted amide rotation, crystal structure, and dissolution behavior [4].
When amide rotation is slower, certain conformations may persist for longer periods in the solid state and participate in the formation of stable crystal-packing arrangements. When the rotational barrier is lowered, the rate of conformational exchange increases, and crystal-packing patterns and lattice stability may consequently change.
It should be emphasized that this relationship involves multiple factors:
Amide dynamics + overall molecular structure + intermolecular interactions + crystal form jointly determine the final solid-state properties.
7.5 Results of Whole-Molecule Optimization
The original study reported a logD of approximately 0.9 for PF-07817883, compared with approximately 1.9 for nirmatrelvir. PF-07817883 also exhibited lower intrinsic clearance in human liver microsomes and favorable oral pharmacokinetic characteristics [4].
These results reflect coordinated changes at multiple levels during drug optimization:
Level | Observed Change |
Local P2 structure | Bicyclic scaffold replaced with 4-CF₃-Pro |
Local dynamics | Faster P2/P3 amide rotation |
Target binding | Effective Mpro-binding conformation maintained |
Overall physicochemical properties | Reduced logD |
Metabolism | Improved metabolic stability |
Pharmacokinetics | Preclinical results support an improved oral pharmacokinetic profile as a single agent |
A relatively direct experimental relationship can be established in that measurable changes were observed in structure, amide dynamics, protein-binding mode, and whole-drug properties. Nevertheless, each overall ADME outcome remains jointly controlled by the structure of the entire molecule.
8. How Can These Mechanisms Be Applied to Drug Design?
The value of proline analogues lies in their ability to provide a structural variable through which both local conformation and drug properties can be investigated simultaneously.
In practical projects, the corresponding experiments can be selected according to the specific question being addressed.
Design Question | Parameters to Observe | Common Methods |
Changes in target activity | Bound-state ring conformation, substituent orientation | X-ray crystallography, activity assays |
Different activities of stereoisomers | Cγ-endo / Cγ-exo preference | NMR, structural analysis |
Abnormal cis/trans ratio | K(trans/cis) | ¹H NMR, ¹⁹F NMR |
Amide exchange too slow or too fast | Exchange rate, ΔG‡ | EXSY NMR, variable-temperature NMR |
Unsuitable lipophilicity | logP, logD | Partitioning experiments |
Insufficient solubility | Solubility, melting point, crystal form | Solid-state analysis, solubility measurements |
Rapid metabolic clearance | Sites of metabolism, intrinsic clearance | Liver microsomes, hepatocytes, metabolite identification |
If a lead compound already contains a Pro residue, a series of matched molecular pairs can be designed according to the specific research question, for example:
Pro → (2S,4R)-4-F-Pro → (2S,4S)-4-F-Pro → 4,4-diF-Pro → 4-CF₃-Pro
Comparisons should not be based solely on IC₅₀. Only by jointly analyzing activity, conformation, rotational kinetics, logD, solubility, and metabolic stability can it be determined which molecular process has actually been altered by a given substituent.
Based on the conformational-control mechanisms described above, different proline derivatives can be selected in practical research as synthetic building blocks or conformational probes according to the position of substitution, stereochemistry, and protecting-group pattern. Representative chemicals relevant to the research directions discussed above are listed below.
9. Related Proline Derivatives and Chemical Building Blocks for Research
Table 1. Parent Proline and α-Methylproline
Detailed Classification | CAS No. | Aladdin Catalog No. | Name | Specification or Purity | Product Features and Applications |
Parent proline · free form | 147-85-3 | L-Proline | ≥99% | Parent reference compound for conformational studies of proline derivatives; used to compare pyrrolidine-ring conformations, adjacent amide cis–trans equilibria, and peptide-chain conformational changes before and after ring substitution; also suitable for the synthesis of proline-containing peptides and peptidomimetics. | |
Parent proline · ring-N Boc-protected form | 15761-39-4 | Boc-L-proline | ≥99% | Basic proline building block with ring-nitrogen protection; suitable for solution-phase peptide synthesis, peptidomimetic construction, and derivatization at proline sites, and can serve as the parent control in matched molecular pair studies of substituted prolines. | |
Parent proline · ring-N Fmoc-protected form | 71989-31-6 | Fmoc-L-proline | ≥98% | Suitable for introducing proline residues in solid-phase peptide synthesis; can be used to study backbone conformation, amide cis–trans isomerization, and sequence dependence in proline-containing peptides. | |
α-Methylproline · free form | 42856-71-3 | α-Methyl-L-proline | ≥96%(T) | The α-methyl group alters the steric environment at C2; used to investigate the effects of α-substitution on local backbone conformation, amide behavior, and proteolytic stability of peptide molecules, and can also serve as a building block in drug and peptidomimetic design. | |
α-Methylproline · ring-N Boc-protected form | 103336-06-7 | (2S)-1-[(tert-Butoxy)carbonyl]-2-methylpyrrolidine-2-carboxylic acid | ≥97% | Protected α-methylproline synthetic building block; facilitates incorporation of 2-methylproline into peptide or small-molecule scaffolds and can be used to compare the effects of α-position steric restriction on conformation and metabolic stability. |
Table 2. 4-Hydroxyproline
Classification | CAS No. | Aladdin Catalog No. | Name | Specification or Purity | Product Features and Applications |
trans-4-Hydroxy · free form | 51-35-4 | L-Hydroxyproline | Moligand™, ≥99% | Corresponds to trans-4-hydroxy-L-proline; used to study the effects of a 4-hydroxy group on pyrrolidine-ring puckering, hydration, hydrogen bonding, and peptide-chain conformational stability, and is a representative proline analogue in collagen and collagen-model peptide research. | |
trans-4-Hydroxy · ring-N Boc-protected form | 13726-69-7 | Boc-L-hydroxyproline | ≥98% | Protected trans-4-hydroxyproline building block; suitable for solution-phase peptide synthesis, subsequent functionalization of the hydroxy group, and studies of the conformational effects of 4-hydroxy substitution. | |
trans-4-Hydroxy · ring-N Fmoc-protected form | 88050-17-3 | Fmoc-L-hydroxyproline | ≥98%, sum of enantiomers | trans-4-Hydroxyproline building block for solid-phase peptide synthesis; used to construct hydroxyproline-containing peptides and collagen-model peptides and to study the effects of hydroxy substitution on local puckering and peptide-chain stability. | |
cis-4-Hydroxy · free form | 618-27-9 | cis-4-Hydroxy-L-proline | ≥98% | Used in paired comparisons with trans-4-hydroxy-L-proline to investigate the effects of stereochemistry at the 4-position on ring puckering, backbone geometry, hydrogen bonding, and the stability of collagen-like structures. | |
cis-4-Hydroxy · ring-N Boc-protected form | 87691-27-8 | N-Boc-cis-4-hydroxy-L-proline | ≥97% | Protected cis-4-hydroxyproline building block; suitable for peptide and peptidomimetic synthesis, derivatization of the 4-hydroxy group, and structure–activity relationship comparisons between cis- and trans-4-hydroxyproline. |
Table 3. Fluorinated Prolines
Classification | CAS No. | Aladdin Catalog No. | Name | Specification or Purity | Product Features and Applications |
4-Monofluoro · trans · free form | 2507-61-1 | trans-4-Fluoro-L-proline | ≥97% | 4-Fluoroproline with the (2S,4R) configuration; associated with a Cγ-exo ring-puckering preference and suitable for studies of C—F stereoelectronic effects, amide cis–trans equilibria, backbone conformation, and conformational responses monitored by ¹⁹F NMR. | |
4-Monofluoro · trans · ring-N Boc-protected form | 203866-14-2 | (2S,4R)-1-(tert-Butoxycarbonyl)-4-fluoropyrrolidine-2-carboxylic acid | ≥96%(T) | Protected 4-fluoroproline building block with the (2S,4R) configuration; used to introduce 4-fluoroproline with a Cγ-exo conformational preference into peptide and drug scaffolds for studies of conformational and activity relationships. | |
4-Monofluoro · cis · free form | 2438-57-5 | H-cis-4-Fluoro-proline | ≥97% | 4-Fluoroproline with the (2S,4S) configuration; associated with a Cγ-endo ring-puckering preference and suitable for paired studies with the trans isomer to investigate the effects of stereochemistry at the 4-position on amide equilibrium, backbone conformation, and physicochemical properties. | |
4-Monofluoro · cis · ring-N Boc-protected form | 203866-13-1 | (2S,4S)-1-(tert-Butoxycarbonyl)-4-fluoropyrrolidine-2-carboxylic acid | ≥97% | Protected 4-fluoroproline building block with the (2S,4S) configuration; suitable for solution-phase synthesis and matched molecular pair studies with the (2S,4R) isomer to examine the effects of ring puckering and stereochemistry on structure–activity relationships. | |
4-Monofluoro · cis · ring-N Fmoc-protected form | 203866-19-7 | (2S,4S)-1-(((9H-Fluoren-9-yl)methoxy)carbonyl)-4-fluoropyrrolidine-2-carboxylic acid | ≥98% | 4-Fluoroproline building block with the (2S,4S) configuration for solid-phase peptide synthesis; used to construct peptides containing cis-4-fluoroproline and to study ring puckering, amide isomerization, and the stereochemical effects of fluorine substitution. | |
4,4-Difluoro · C2 configuration unspecified · free form | 1073555-68-6 | 4,4-Difluoropyrrolidine-2-carboxylic acid | ≥97% | Proline scaffold bearing gem-difluoro substitution at the 4-position; used to study the effects of difluoro substitution on ring electronic properties, ring puckering, amide isomerization, and the physicochemical properties of fluorinated molecules, and can also be used in ¹⁹F NMR-related studies. | |
4,4-Difluoro-L-proline · ring-N Boc-protected form | 203866-15-3 | N-Boc-4,4-difluoro-L-proline | ≥97% | Protected L-configured 4,4-difluoroproline building block; suitable for peptide and peptidomimetic synthesis and for comparing the effects of monofluoro and difluoro substitution on local electronic properties, conformation, and amide behavior. | |
4-Trifluoromethyl · (2S,4R) · hydrochloride form | 2940879-29-6 | (2S,4R)-4-(Trifluoromethyl)pyrrolidine-2-carboxylic acid hydrochloride | ≥97% | 4-(Trifluoromethyl)proline salt with the (2S,4R) configuration; used in the synthesis of drugs and peptidomimetic scaffolds containing 4-CF₃-proline and suitable for studying CF₃ steric occupancy, ring conformation, and the rotational kinetics of adjacent amide bonds. | |
4-Trifluoromethyl · (2S,4R) · ring-N Boc-protected form | 470482-44-1 | (2S,4R)-1-[(tert-Butoxy)carbonyl]-4-(trifluoromethyl)pyrrolidine-2-carboxylic acid | ≥97% | Protected 4-CF₃-proline building block with the (2S,4R) configuration; suitable for drug and peptidomimetic synthesis and for studying the effects of CF₃ substitution on amide rotation, lipophilicity, conformation, and complementarity with hydrophobic protein pockets. | |
4-Trifluoromethyl · (2S,4S) · free form | 1283107-66-3 | (2S,4S)-4-(Trifluoromethyl)pyrrolidine-2-carboxylic acid | ≥95% | 4-CF₃-proline with the (2S,4S) configuration; used in fundamental studies of conformation, acid–base properties, lipophilicity, and amide rotation, and can be compared with the (2S,4R) series to investigate stereochemical effects at the 4-position. | |
4-Trifluoromethyl · (2S,4S) · ring-N Boc-protected form | 470482-41-8 | (2S,4S)-1-[(tert-Butoxy)carbonyl]-4-(trifluoromethyl)pyrrolidine-2-carboxylic acid | ≥97% | Protected 4-CF₃-proline building block with the (2S,4S) configuration; suitable for peptide and small-molecule synthesis and can be combined with the corresponding protected (2S,4R) compound to form a stereochemically matched series for studying changes in ring puckering, amide dynamics, and physicochemical properties. |
Table 4. 4-Aminoproline and Bicyclic Proline Derivatives
Classification | CAS No. | Aladdin Catalog No. | Name | Specification or Purity | Product Features and Applications |
4-Amino · trans · free form | 16257-88-8 | trans-4-Amino-L-proline | ≥95% | 4-Aminoproline with the (2S,4R) configuration; the amino group at the 4-position can serve as a site for subsequent acylation, urea formation, and coupling reactions and can be used to study the effects of 4-amino substitution, protonation state, and local charge on ring conformation and molecular recognition. | |
4-Amino · trans · 4-amino Boc-protected form | 1279034-98-8 | (2S,4R)-4-{[[(tert-Butoxy)carbonyl]amino}pyrrolidine-2-carboxylic acid | ≥97% | Bifunctional building block in which the 4-amino group is selectively protected while the ring nitrogen remains available for reaction; used to construct 4-aminoproline-containing peptides, peptidomimetics, and multifunctional derivatives, while allowing subsequent deprotection of the 4-amino group for conjugation. | |
4-Amino · trans · ring-N protected/carboxyl methyl ester hydrochloride form | 334999-32-5 | N-Boc-trans-4-amino-L-proline methyl ester hydrochloride | ≥97% | The ring nitrogen is Boc-protected, the carboxyl group is methyl-esterified, and the 4-amino group is present in salt form; the 4-amino group can be used for selective acylation and conjugation reactions, making this compound suitable for constructing proline derivatives with side-chain attachment sites. | |
4-Amino · cis · free form | 16257-83-3 | (2S,4S)-4-Aminopyrrolidine-2-carboxylic acid | ≥95% | 4-Aminoproline with the (2S,4S) configuration; suitable for comparison with the trans isomer to investigate how the orientation of the 4-amino group affects ring conformation, local charge, hydrogen bonding, and peptide-chain geometry, and can also be used for side-chain functionalization. | |
4-Amino · cis · 4-amino Boc-protected form | 1279034-78-4 | (2S,4S)-4-{[(tert-Butoxy)carbonyl]amino}pyrrolidine-2-carboxylic acid | ≥97% | (2S,4S) building block with selective protection of the 4-amino group; used to construct cis-4-aminoproline derivatives and can be compared with the (2S,4R) series in stereochemical and structure–activity relationship studies. | |
4-Amino · cis · ring-N protected/carboxyl methyl ester hydrochloride form | 171110-72-8 | (2S,4S)-1-tert-Butoxycarbonyl-4-aminopyrrolidine-2-carboxylic acid methyl ester hydrochloride | ≥97% | cis-4-Aminoproline salt with Boc protection on the ring nitrogen and methyl esterification of the carboxyl group; retains the 4-amino group as a coupling site and can be used for amidation, linker installation, and construction of multifunctional peptidomimetic scaffolds. | |
[3.1.0] Bicyclic · (1S,3S,5S) · ring-N Boc-protected form | 197142-36-2 | (1S,3S,5S)-2-(tert-Butoxycarbonyl)-2-azabicyclo[3.1.0]hexane-3-carboxylic acid | ≥98% | Cyclopropane-fused, conformationally constrained proline analogue; used to restrict the geometry of the pyrrolidine scaffold, reduce local conformational freedom, and study the effects of the bicyclic structure on amide orientation, binding conformation, and structure–activity relationships. | |
[3.1.0] Bicyclic · (1S,3R,5S) · ring-N Boc-protected form | 1417743-41-9 | (1S,3R,5S)-2-Boc-2-azabicyclo[3.1.0]hexane-3-carboxylic acid | ≥97% | Bicyclic building block with a different C3 stereochemical configuration from the (1S,3S,5S) isomer; used to construct conformationally constrained drugs and peptidomimetics and to compare the effects of bicyclic stereochemistry on backbone geometry and spatial complementarity in protein binding. |
Note: The products listed above are representative Aladdin products for scientific research. Specific applications should be determined according to the product specifications, batch COA, and the intended reaction or evaluation system. Additional information on product specifications, grades, and COAs can be retrieved from the Aladdin website using the “product name/CAS/catalog number.”
References
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[2] Kubyshkin V. Polarity Effects in 4-Fluoro- and 4-(Trifluoromethyl)prolines. Beilstein Journal of Organic Chemistry. 2020;16:1837–1852. doi:10.3762/bjoc.16.151.
[3] Shoulders MD, Satyshur KA, Forest KT, Raines RT. Stereoelectronic and Steric Effects in Side Chains Preorganize a Protein Main Chain. Proceedings of the National Academy of Sciences of the United States of America. 2010;107(2):559–564. doi:10.1073/pnas.0909592107.
[4] Allerton CMN, Arcari JT, Aschenbrenner LM, et al. A Second-Generation Oral SARS-CoV-2 Main Protease Inhibitor Clinical Candidate for the Treatment of COVID-19. Journal of Medicinal Chemistry. 2024;67(16):13550–13571. doi:10.1021/acs.jmedchem.3c02469.
[5] Chen P, Van Oers TJ, Arutyunova E, et al. A Structural Comparison of Oral SARS-CoV-2 Drug Candidate Ibuzatrelvir Complexed with the Main Protease (Mpro) of SARS-CoV-2 and MERS-CoV. JACS Au. 2024;4(8):3217–3227. doi:10.1021/jacsau.4c00508.
For more related articles, see below:
Technical Comparison and Method Selection for Hydroxyproline Determination
Cyclic isomers--Azabicyclic molecular building blocks to aid drug design
危险品化学品经营许可证（带存储）