Stereoselective synthesis of cyclobutyl GABA analogues and related compounds from (−)-(S)-verbenone

Biotransformation of (–)-Verbenone by Some Fungi

Biotransformation of (–)-verbenone by Aspergillus niger NRRL 330, Aspergillus wentii MRC 200316 and Penicillium digitatum MRC 500787 afforded (–)-10-hydroxyverbenone. Incubation with A. niger NRRL 330 resulted in with the highest yield.

A novel synthesis and antimicrobial activity of 1-[(Substituted-phenyl) sulfonyl]pyrrolidin-2-ones

Novel cyclization of 4-(substituted-phenylsulfonamido)butanoic acids to their corresponding 1-[(substituted-phenyl)sulfonyl]pyrrolidin-2-ones was successfully achieved by using polyphosphate ester (PPE). The reaction time was considerably reduced with corresponding increase in the yields, when polyphosphate ester (PPE) was used in combination with 4-(N,N-dimethylamino)pyridine (DMAP). All the synthesized compounds were screened for their antimicrobial activity. Minimum Inhibitory Concentration (MIC) values of synthesized compounds were also determined, and were found to be in the range of 0.09-1.0 mg.

Bioactive cyclobutane-containing alkaloids

The present review describes research on novel natural cyclobutane-containing alkaloids and synthetic compounds isolated from terrestrial and marine species. More than 210 compounds have been confirmed to show antimicrobial, antibacterial, anticancer, and other activities. Structures, origins, biosynthesis, photodimerization, and biological activities of a selection of cyclobutane-containing alkaloids and selected synthetic analogs of natural alkaloids are reviewed.

Diastereoselective Synthesis of Fulleropyrrolidines from Suitably Functionalized Chiral Cyclobutanes

The first diastereoselective synthesis of fulleropyrrolidines endowed with diastereomerically pure functionalized cyclobutanes is reported. The new C(60)-based cyclobutane derivatives 7a,b and 9 are suitably functionalized for further incorporation into peptide surrogates.

Unexpected Synthesis of Conformationally Restricted Analogues of γ-Amino Butyric Acid (GABA): Mechanism Elucidation by Electrospray Ionization Mass Spectrometry

From previous results with lower homologues, dehydroiodination of the three alkenyl-beta-enamino esters 3a-c was expected to provide six-membered N-heterocyclic products. The reactions of 3a-c with triethylamine are found to lead, however, to the unexpected stereoselective synthesis of the trisubstituted cyclopentane derivatives 4a-c, as confirmed by IR and NMR spectroscopy. Cyclopentanes 4a-c bear two chiral centers and a gamma-amino ester moiety, and are therefore conformationally restricted analogues of gamma-amino butyric acid (GABA), which is the major inhibitory neurotransmitter in the central nervous system. Use of electrospray ionization mass (ESI-MS) and tandem mass spectrometry (ESI-MS/MS) allowed the key iminium ion intermediates 5a-c(+), as well as the protonated molecules of both the reactant and final products, [3a-c + H](+) and [4a-c + H](+), to be intercepted and structurally characterized. From these findings a mechanism for this unexpected but synthetically attractive and efficient stereoselective reaction is proposed.

Synthesis of Optically Active Constrained 2-Substituted Norstatines: A Straightforward Application of Seebach's “SRS” Synthetic Principle

A straightforward two-step methodology of synthesis of optically active (2R)-substituted norstatines via addition of N-tert-butoxycarbonyl-substituted aldimines to (2S)-chiral enolates of 1,3-dioxolan-4-ones has been developed. In particular, the use of the natural (2S)-malic acid is examined for the synthesis of potential GABAergic spirocyclic gamma-lactams.

Stereoselective Rh-Catalyzed Hydrogenation of Cyclobutyl Chiral Enamides: Double Stereodifferentiation vs Catalyst-Controlled Diastereoselection

The hydrogenation reactions of several cyclobutyl enamides derived from (-)-alpha-pinene or (-)-verbenone have been investigated by using different catalysts. The chiralities of both the substrate and the catalyst as well as the Z/E stereochemistry of the double bond have been considered, and the observed diastereoselectivity has been rationalized. For enamides with the double bond separated from the cyclobutane by a methylene, the Wilkinson catalyst did not induce any diastereoselection, but excellent diastereoselectivity was observed when using Et-DuPHOS-Rh and ChiraPHOS-Rh. The configuration of the new stereogenic center was catalyst-dependent and can be rationalized according to the Halpern mechanism. For (Z)-enamides with the double bond directly linked to the cyclobutane ring, the chirality of the substrate governed the diastereoselection and the Halpern mechanism seemed not to be operative in the hydrogenation with ChiraPHOS, with the configuration of the new stereogenic center being determined by steric effects. On the contrary, the chirality of the catalyst was the factor determining the stereochemistry of the major products with alkyl-DuPHOS-Rh. Z/E stereochemistry influenced the stereodifferentiation, and a different behavior for each Z or E stereoisomer was found. For both (Z)- and (E)-enamides, some instances of match/mismatch between the chirality of the substrate and that of the catalyst were observed. As a result of all of these studies, a series of new cyclobutyl alpha-amino acids has been synthesized. These products are interesting to incorporate into conformationally constrained peptides.