George Varvounis

Pyrrolo[1,4]benzodiazepines are tricyclic compounds that are considered “privileged structures” since they possess a wide range of biological activities. The first encounter with these molecules was the isolation of anthramycin from cultures of Streptomyces, followed by determination of the X-ray crystal structure of the molecule and a study of its interaction with DNA. This opened up an intensive synthetic and biological study of the pyrrolo[2,1-c][1,4]benzodiazepines that has culminated in the development of the dimer SJG-136, at present in Phase II clinical trials. The synthetic efforts have brought to light some new synthetic methodology, while the contemporary work is focused on building trimeric pyrrolo[2,1-c][1,4]benzodiazepines linked together by various heterocyclic and aliphatic chains. It is the broad spectrum of biological activities of pyrrolo[1,2-a][1,4]benzodiazepines that has maintained the interest of researchers to date whereas several derivatives of the even less studied pyrrolo[1,2-d][1,4]benzodiazepines were found to be potent non-nucleoside HIV-1 reverse transcriptase inhibitors. The present review is an update on the synthesis of pyrrolo[2,1-c][1,4]benzodiazepines since the last major review of 2011, while the overview of the synthesis of the other two tricyclic isomers is comprehensive.

A simple and efficient one-pot cascade reaction for the regioselective synthesis of trans or cis/trans 2,3-disubstituted 2,3-dihydrobenzofurans is reported. The method involves fluoride-induced desilylation, generation of o-quinone methide (o-QM) by chloride or nitrate anion elimination, Michael addition and intramolecular 5-exo-tet elimination of a bromide anion. Nitrate, acetoxy and chloride anions are compared as leaving groups in the formation of the in situ generated o-QM.

Several novel 1,1-disubstituted-8-hydroxynaphthalen-2(1H)-ones have been efficiently synthesized via a two-step sequence from 2-hydroxy-1-naphthaldehyde oxime. The methodology involves oxidative ring closure and alkoxylation to 3a-alkoxynaphtho[1,8-de][1,2]oxazin-4(3aH)-ones, followed by thermal ring-opening. Both thermal and microwave irradiation conditions were used. A novel one-pot reaction of oxime to 8-isopropoxynaphthalene-1,7-diol using microwave irradiation is also reported.

N,N-Dimethylformamide was found to act as solvent and catalyst in the dehydration of aldoximes to nitriles. The reaction ­required heating at 135 °C and yields of nitriles were moderate to good. (Benzylideneaminooxy)formaldehyde was detected as an ­intermediate in one of the reactions.

The synthesis of (E)-3-[1-(toluene-4-sulfonyl)-1H-pyrrol-3-yl]prop-2-ene derivatives from 1-(toluene-4-sulfonyl)-1H-pyrrole-3-carbaldehyde by the Horner-Wadsworth-Emmons (HWE) reaction, is described. However, the HWE reaction with diethyl methoxymethylphosphonate and diethyl (2-methoxyethoxy)methylphosphonate gave a different product, namely, diethyl {1-methoxy(or methoxyethoxy)-2-[1-(toluene-4-sulfonyl)-1H-pyrrol-3-yl]vinyl}phosphonates, respectively. (E)-3-[1-(Toluene-4-sulfonyl)-1H-pyrrol-3-yl]prop-2-enes were easily hydrolysed to (E)-3-(1H-pyrrol-3-yl)prop-2-enes, two of which are natural products.

2-Amino-4-pyrrolidinothieno[2,3-d]pyrimidine-6-carboxylic acid (4) (ATPC) is an unnatural amino acid with promise in applications as a building block for the synthesis of peptidomimetics. ATPC was obtained from both 3a and 3b thienopyrimidines by hydrolysis and hydrogenolysis, respectively. The synthesis of eleven ATPC-amino acids and two ATPC-peptides is described. ATPC is incorporated as N-terminal moiety in solution or solid-phase peptide synthesis using Boc or Fmoc methodology and without protection of the ATPC amino group.