Synthesis of 2-methyl- and 2-methylenecyclobutane amino acids

Enhancing lathyrane structural diversity and MDR activity by combinatorial modification of lathyrane nucleus and ester side chain: A case study of Euphorbia Factor L1 and Euphorbia Factor L3

The chemical transformation of lathyrane nucleus through reduction and oxidation reactions using Euphorbia Factor L1 (EFL1) and Euphorbia Factor L1 (EFL3) as examples were investigated, along with a co-modification strategy of lathyrane nucleus and its side ester chain. A total of 38 lathyrane derivatives (5-42) including 34 new compounds were obtained, which greatly enriched the structural diversity of the lathyrane-type diterpenoids. Cytotoxicity against drug-sensitive and drug (adriamycin, ADM) resistant MCF-7 cells showed that 23 out of 38 transformed derivatives possessed obvious cytotoxic activity with IC50 values ranging from 7.0 to 41.1 μM and 3.2 to 45.5 μM, respectively, against both cells, compared to the noncytotoxic EFL1 and EFL3. The multidrug resistance (MDR) reversing activities of these lathyrane derivatives were further evaluated in MCF-7/ADM. Three transformed compounds (reversal fold, RF = 151.33, 62.94 and 47.3 for 27, 37 and 42) showed markedly higher activity than EFL1 (RF = 32.92) and EFL3 (RF = 39.68). Structure-activity relationship study revealed an essential role of C-6/17 and C-12/13 double bonds on lathyrane nucleus for exerting MDR reversal activity. Western blotting analysis showed that 42 could reduce the expression level of P-glycoprotein (P-gp) in MCF-7/ADM cells; however, the most active compound 27 with an unnatural 5/7/7/4 fused-ring diterpenoid skeleton, had no inhibitory effect on P-gp expression.

A 1H NMR and theoretical investigation of the conformations of some monosubstituted cyclobutanes

The complete analysis of the complex (1)H NMR spectra of some monosubstituted cyclobutanes was achieved to give all the (1)H chemical shifts and (n)J(HH) (n = 2, 3 and 4) coupling constants in these molecules. The substituent chemical shifts of the substituents in the cyclobutane ring differ significantly from those in acyclic systems. For example, the OH and the NH(2) groups in cyclobutanol and cyclobutylamine produce a large shielding of the hydrogens of the opposite CH(2) group of the ring compared with little effect on the comparable methylene protons of butane. These effects and the other (1)H shifts in the cyclobutanes were modelled successfully in the CHARGE program. The RMS error (calculated vs observed shifts) for the 34 (1)H shifts recorded was 0.053 ppm. The conformational equilibrium in these compounds between the axial and the equatorial conformers was obtained by comparing the observed and the calculated (4)J(HH) couplings. These couplings in cyclobutanes, in contrast to the corresponding (3)J(HH) couplings, show a pronounced orientation dependence; (4)J(eq-eq) is ca 5 Hz and (4)J(ax-ax) ca 0 Hz. The couplings in the individual conformers were calculated at the B3LYP/EPR-III level. The conformer energy differences ΔG(ax-eq) vary from 1.1 kcal mol(-1) for OH to 0.2 kcal mol(-1) for the CH(2)OH substituent. The values of the conformer energy differences are compared with the previous IR data and the corresponding theoretical values from molecular mechanics (MM) and DFT theory. Generally, good agreement is observed although both the MM and the DFT calculations deviate significantly from the observed values for some substituents.

Cyclobutane amino acid analogues of furanomycin obtained by a formal [2 + 2] cycloaddition strategy promoted by methylaluminoxane

The synthesis and conformational analysis of a new type of conformationally restricted alpha-amino acid analogue of the amino acid antibiotic furanomycin is presented. The restriction involves the cis-fused cyclobutane and tetrahydrofuran units, generating the unusual 2-oxabicyclo[3.2.0]heptane core, which is found in a great number of biologically active natural products. The synthetic strategy is based on a formal [2 + 2] cycloaddition between 2-(acylamino)acrylates as acceptor alkenes and 2,3-dihydrofuran as a donor alkene, promoted by bulky aluminum-derived Lewis acids, particularly by methylaluminoxane (MAO). Additionally, following the same strategy, the synthesis of furanomycin analogues incorporating the 2-oxabicyclo[4.2.0]octane is reported.

Recent Progress on the Stereoselective Synthesis of Cyclic Quaternary alpha-Amino Acids

The most recent papers describing the stereoselective synthesis of cyclic quaternary alpha-amino acids are collected in this review. The diverse synthetic approaches are classified according to the size of the ring and taking into account the bond that is formed to complete the quaternary skeleton.

Conformational Analysis of 2-Substituted Cyclobutane-α-amino Acid Derivatives. A Synergistic Experimental and Computational Study

An extensive conformational study of different 2-substituted cyclobutane-alpha-amino acid derivatives in the solid state, in the gas phase, and in solution has been carried out. The study combines experimental techniques, such as X-ray diffraction and NMR spectroscopy, and computational methods, such as DFT calculations and molecular dynamics (MD) simulations, in a set solvent. The study reveals that the substituent at C2 in the cyclobutane ring, when fixed in an equatorial position, modulates the conformational preference of the ring-puckering.