Josep Bonjoch

Biosynthesis and Structure of Aeruginoside 126A and 126B, Cyanobacterial Peptide Glycosides Bearing a 2-Carboxy-6-Hydroxyoctahydroindole Moiety

Aeruginosins represent a group of peptide metabolites isolated from various cyanobacterial genera and from marine sponges that potently inhibit different types of serine proteases. Members of this family are characterized by the presence of a 2-carboxy-6-hydroxyoctahydroindole (Choi) moiety. We have identified and fully sequenced a NRPS gene cluster in the genome of the cyanobacterium Planktothrix agardhii CYA126/8. Insertional mutagenesis of a NRPS component led to the discovery and structural elucidation of two glycopeptides that were designated aeruginoside 126A and aeruginoside 126B. One variant of the aglycone contains a 1-amino-2-(N-amidino-Delta(3)-pyrrolinyl)ethyl moiety at the C terminus, the other bears an agmatine residue. In silico analyses of the aeruginoside biosynthetic genes aerA-aerI as well as additional mutagenesis and feeding studies allowed the prediction of enzymatic steps leading to the formation of aeruginosides and the unusual Choi moiety.

Total Synthesis of (−)‐Strychnine via the Wieland–Gumlich Aldehyde

Fifteen steps suffice for an enantioselective total synthesis of (-)-strychnine (1) from 1,3-cyclohexanedione. The key steps are the easy generation of the enantiopure intermediate 2, the closure of the piperidine ring by a reductive Heck reaction, and the elaboration of the indoline nucleus in an advanced synthetic stage. TBDMS=tert-butyldimethylsilyl.

Total Synthesis of (−)-Anominine

The first total synthesis of anominine has been achieved, and the absolute configuration of the product has been determined. The key features include the development of a new, highly efficient organocatalyzed method for the asymmetric synthesis of Wieland-Miescher ketone building blocks, an unusual selenoxide [2,3]-sigmatropic rearrangement, and a ZrCl(4)-catalyzed indole coupling as well as several chemoselective transformations controlled by the structurally congested nature of the bicyclic core.

First total syntheses of aeruginosin 298-A and aeruginosin 298-B, based on a stereocontrolled route to the new amino acid 6-hydroxyoctahydroindole-2-carboxylic acid.

The first total syntheses of aeruginosin 298-A (1) and aeruginosin 298-B (3) are described. The syntheses of the alternative putative structures 2 and 4 were also accomplished. The key common strategic element is the stereo-controlled synthesis of (2S,3aS,6R,7aS)-6-hydroxyoctahydroindole-2-carboxylic acid (L-Choi, 5) from L-tyrosine. The synthesis of this new bicyclic alpha-amino acid, which is the core of aeruginosins, involves Birch reduction of O-methyl-L-tyrosine (6) and aminocyclization of the resulting dihydroanisole 7 in acid medium, followed by N-benzylation to give the diastereoisomers 12 and 13. Upon acid treatment with HCl-MeOH, the last two produce an equilibrium mixture in which the endo isomer 13 significantly predominates. Hydrogenation of 13 in the presence of (Boc)2O gives 16, which on reduction with LS-Selectride furnishes the alcohol 22, a protected L-Choi. Successive couplings of 22 with D-leucine, protected (R)-(4-hydroxyphenyl)lactic acid, and L-arginine fragments, followed by reduction to the argininol level and a deprotection end step complete the synthetic sequence to produce aeruginosin 298-A (1). Spectral comparison showed that peptide 2, with the structure previously proposed for aeruginosin 298-A, was different from the natural product. However, synthetic 1 was found to be identical to the isolated natural sample of aeruginosin 298-A. These results unequivocally establish that the absolute stereochemistry of aeruginosin 298-A, formerly assigned incorrectly, is D-Hpla-D-Leu-L-Choi-L-Argol, as shown by structure 1. Aeruginosin 298-B was also synthesized and shown to be a mixture of rotamers of D-Hpla-D-Leu-L-ChoiNH2 (3), rather than an epimeric mixture of 3 and the L-Leu-incorporating 4.

Enantioselective total synthesis of Wieland-Gumlich aldehyde and (-)-strychnine

A total synthesis of (-)-strychnine in 15 steps from 1,3-cyclohexanedione in 0.15% overall yield is described. The sequence followed in the assembling of rings is: E-->AE [2-(2-nitrophenyl)-1,3-cyclohexanedione]-->ACE (3a-aryloctahydroindol-4-one)-->ACDE (arylazatricyclic core)-->ABCDE (strychnan skeleton)-->ABCDEF (Wieland-Gumlich aldehyde)-->ABCDEFG (strychnine). The key steps of the synthesis are the enantioselective construction of the 3a-(2-nitrophenyl)-octahydroindol-4-one ring system and the closure of the piperidine ring by a reductive Heck cyclization to generate the pivotal intermediate (-)-14. In contrast, a Lewis acid promoted a-alkoxypropargylic silane-enone cyclization did not lead to synthetically useful azatricyclic ACDE intermediates. The introduction of C-17 and the closure of the indoline ring by reductive amination of the alpha-(2-nitrophenyl) ketone moiety complete the strychnan skeleton from which, via the Wieland-Gumlich aldehyde, the synthesis of (-)-strychnine is achieved.

Synthesis of 2-azabicyclo[3.3.1]nonanes by means of (carbamoyl)dichloromethyl radical cyclization

Abstract A new procedure for the synthesis of 2-azabicyclo[3.3.1]nonanes by intramolecular carboradical cyclization of 4-(trichloroacetamido)cyclohexenes substituted with an electron withdrawing substituent (ester or nitrile) is described. The procedure allows the preparation of synthetically interesting azabicyclos 14 and 15 in nearly 70% yield.

cis-Decahydroquinolines via Asymmetric Organocatalysis: Application to the Total Synthesis of Lycoposerramine Z

A concise synthesis of the Lycopodium alkaloid lycoposerramine Z is reported. Key to the strategy is a one-pot organocatalyzed Michael reaction followed by a domino Robinson annulation/intramolecular aza-Michael reaction promoted by LiOH, leading to enantiopure cis-decahydroquinolines.

3A-(O-NITROPHENYL)OCTAHYDROINDOL-4-ONES : SYNTHESIS AND SPECTROSCOPIC ANALYSIS

Abstract A short entry to 3a-( o -nitrophenyl)octahydroindol-4-ones employing ozonolysis and double reductive amination of 2-allyl-2-( o -nitrophenyl)-1,3-cyclohexanedione ( 9 ) is described. The symmetric dione 9 is synthesized in a 50% overall yield from 1,3-cyclohexanedione by means of o -nitroarylation followed by O -allylation and subsequent Claisen rearrangement. Configurational and conformational aspects of azabicyclic derivatives 1 (a-k) are discussed.

Asymmetric synthesis of 2-azabicyclo[3.3.1]nonanes by a microwave-assisted organocatalysed tandem desymmetrisation and intramolecular aldolisation

The six-membered nitrogen-containing ring of the morphan scaffold, ubiquitous in natural products, is formed by an intramolecular aldol process of an aza-tethered dicarbonyl compound, leading to the first asymmetric synthesis of a morphan derivative using organocatalysis.

Synthesis of the octahydroindole core of aeruginosins: a new bicyclic α-amino acid

Abstract The first synthesis of 6-hydroxyoctahydroindole-2-carboxylic acid derivatives with a cis fusion in the azabicyclic nucleus is described. The synthesis involves a Birch reduction of O -methyl-L-tyrosine followed by an aminocyclization in acid medium to give a mixture of 6-oxo-octahydroindole-2-carboxylic acids 3 and 4 , which could be separated after dibenzylation. Transesterification, hydrogenolysis in the presence of acetic anhydride and reduction of the amido ketones 9 and 10 provides the four stereoisomeric alcohols from which alcohol 14 showed nmr data very close to the aeruginosin 298-A core.