A facile synthesis of bispyrroloquinone and bispyrroloiminoquinone ring system of marine alkaloids

Radiosynthesis of 4-[(18)F]fluoro-L-tryptophan by isotopic exchange on carbonyl-activated precursors

Several (18)F-labeled aromatic amino acids have been developed primarily for tumor imaging with positron-emission-tomography (PET). Also, (18)F-labeled tryptophan derivatives were synthesized by electrophilic (18)F-fluorination or by introducing a [(18)F]fluoroalkyl group. Here, a 3-step method for a nucleophilic radiosynthesis of 4-[(18)F]fluoro-L-tryptophan was developed. A carbonyl activated precursor containing a chiral amino acid building block was radiofluorinated by isotopic exchange, followed by removal of the activating formyl group by reductive decarbonylation and subsequent cleavage of the building block under acidic conditions. The title compound was obtained within 100 min with a radiochemical yield of about 13%, a molar activity of >70 MBq/mmol and an enantiomeric excess of >99%.

Total Synthesis of Calothrixins A and B via Oxidative Radical Reaction of Cyclohexenone with Aminophenanthridinedione

Bioactive indolo[3,2-j]phenanthridine alkaloids, calothrixin B and its N-oxide derivative calothrixin A have been synthesized via an oxidative free radical reaction. calothrixin B is generated from the commercially available 2,4,5-trimethoxybenzaldehyde in only 7 steps. The key step in this synthesis is the Mn(OAc)₃ mediated oxidative free radical reaction of 9-(benzylamino)phenanthridine -7,10-dione with cyclohexenone to form 12-benzyl-12H-indolo[3,2-j]phenanthridine-7,13-dione.

Total synthesis of zyzzyanones A-D

Zyzzyanones A-D is a group of biologically active marine alkaloids isolated from Australian marine sponge Zyzzya fuliginosa. They contain a unique bispyrroloquinone ring system as the core structure. The first total synthesis of all four zyzzyanones is described here. The synthesis of these alkaloids started from a previously known 6-benzylamino indole-4,7-quinone derivative and involves 6-7 steps. The key step in the synthesis involves the construction of a pyrrole ring in one step using a Mn(OAc)3 mediated oxidative free radical cyclization reaction of a 6-benzylamino indole-4,7-quinone derivative with 4-benzyloxyphenyl acetaldehyde diethyl acetal in CH3CN.

Synthesis of Pyrroloquinones via a CAN Mediated Oxidative Free Radical Reaction of 1,3-Dicarbonyl Compounds with Aminoquinones

Pyrroloquinone ring systems are important structural units present in many biologically active molecules including a number of marine alkaloids. For example, they are found in a series of marine metabolites, such as tsitsikammamines, zyzzyanones, wakayin, and terreusinone. Several of these alkaloids have exhibited antimicrobial, antimalarial, antifungal, antitumor, and photoprotecting activities. Synthesis of pyrroloquinone unit is the key step in the synthesis of many of these important organic molecules. Here, we present a ceric (IV) ammonium nitrate (CAN) mediated oxidative free radical cyclization reaction of 1,3-dicarbonyl compounds with aminoquinones as a facile methodology for making various substituted pyrroloquinones. 1,3-dicarbonyl compounds used in this study are ethyl acetoacetate, acetylacetone, benzoyl acetone, and N,N-dimethyl acetoacetamide. The aminoquinones used in this study are 2-(benzylamino)naphthalene-1,4-dione and 6-(benzylamino)-1-tosyl-1H-indole-4,7-dione. The yields of the synthesized pyrroloquinones ranged from 23–91%.

Marine pyrroloiminoquinone alkaloids

Recent reports on the synthetic studies of marine pyrroloiminoquinone alakloids and their analogs are reviewed.

13C NMR discrimination of regioisomeric bispyrroloquinone/bispyrroloiminoquinone ring systems

The structural assignment of bispyrroloquinone and bispyrroloiminoquinone regioisomers was achieved using (13)C NMR spectral data. In the case of bispyrroloiminoquinones, the carbonyl group in the regioisomer possessing a nitrogen atom in both alpha-positions was systematically less deshielded than the carbonyl group in the other regioisomer. In the case of bispyrroloquinones, the most deshielded carbonyl group in the regioisomer with a nitrogen atom in both alpha-positions was more deshielded than the same carbonyl group in the other regioisomer.