Studies directed towards the total synthesis of morphine alkaloids

Application of metallated enamines to the synthesis of morphine related congeners is reported. A formal total synthesis of (±)-morphine has been completed.     

Flow synthesis of tricyclic spiropiperidines as building blocks for the histrionicotoxin family of alkaloids

A domino cyclization reaction of the bis-unsaturated ketone 3 with hydroxylamine proceeds with good yield and high stereoselectivity, in a flow reactor system. The tricyclic spiropiperidine products 5 and 2 obtained are valuable building blocks for the synthesis of analogues of the histrionicotoxin alkaloids.     

Synthesis of azaspirocyclic compounds via organoiron complexes. Potential synthetic routes to histrionicotoxin and cephalotaxus alkaloids.

Reaction of cyclohexadienylium-Fe(CO)₃ complexes (7) and (8) with sodiomalono-nitrile gives the carbo-spirocyclic derivatives (9) and (10), whilst reaction with benzylamine and removal of iron leads to the azaspirocyclic enones (15) and (16), respectively.     

Spiroleucettadine: synthetic studies and investigations towards structural revision

Synthetic efforts towards spiroleucettadine are described, including the enantioselective synthesis of the presumed biosynthetic precursor. High level density functional theory calculations were used to predict the ¹³C NMR shifts of possible alternative structures and, along with a re-evaluation of the available NMR data, allow the proposal of revised structures for this spirocyclic alkaloid.     

Synthetic studies directed towards asmarines; construction of the tetrahydrodiazepinopurine moiety by ring closing metathesis

Asmarines are tetrahydro[1,4]diazepino[1,2,3-g,h]purine derivatives isolated from marine sponges (Raspailia sp). They possess profound cytotoxic activity towards cancer cell lines, and are thus attractive synthetic targets. The tetrahydrodiazepinopurine ring skeleton has been prepared employing the RCM reaction on Boc-protected 6-allylamino-7-(propen-1-yl)purine as the key step for the construction of the seven-membered ring. 7-(Propen-1-yl)purines were formed by a novel rearrangement of 7-allylpurines under basic conditions. Boc-protected N⁶,7-diallylpurine also participated in RCM to give the eight-membered ring analog of the diazepinopurine.     

An altemative synthetic approach towards erythrinan and homoerythrinan alkaloids by tandem semipinacol/intramolecular Schmidt reaction

An alternative construction of A-B-D ring-system of erythrinan and homoerythrinan alkaloids by TiCl_4-mediated tandem semipinacol/intramolecular Schmidt reaction ofα-siloxyepoxyazide was addressed and the unusual epoxidation stereoselectivity was observed in preparation of the substrate once again.     

An alternative synthetic approach towards erythrinan and homoerythrinan alkaloids by tandem semipinacol/intramolecular Schmidt reaction

An alternative construction of A-B-D ring-system of erythrinan and homoerythrinan alkaloids by TiCl4-mediated tandem semipinacol/intramolecular Schmidt reaction of α-siloxyepoxyazide was addressed and the unusual epoxidation stereoselectivity was observed in preparation of the substrate once again.     

Biomimetic macrocycle-forming Diels-Alder reaction of an iminium dienophile: synthetic studies directed toward gymnodimine

The macrocyclic core of gymnodimine has been constructed via an intramolecular Diels-Alder reaction of an alpha,beta-unsaturated iminium dienophile in water. The cycloaddition furnished a single exo-product, along with two endo-products. Through X-ray analysis of a suitable derivative, the stereochemistry of the exo-product was established, thereby demonstrating that its stereochemistry matches that of gymnodimine. In contrast, macrocyclization of an analogous alpha,beta-unsaturated ketone dienophile gave only undesired endo-products. Interestingly, the imine dienophile shows remarkable stability in water. [reaction: see text]     

On the synthetic approach to the protopine alkaloids

The synthetic approach to the protopine alkaloids is described. Efficient construction of the alkaloid models 16a and 16b was accomplished by the general synthetic method consisting of photo-oxygenative ring enlargement of tetrahydrobenzindenoazepines 13a and 13b by singlet oxygen and further elaboration of the resultant 10-membered amido-ketone products 14a and 14b.     

Novel steroid mimics directed towards the estradiol skeleton

A series of non-symmetrical tri- and tetra-substituted ureas have been prepared to mimic the rigid tetracyclic core of estradiol. Tetra-substituted ureas were prepared by a five-step protocol, involving activation and displacement of a carbonyldiimidazole adduct in 48-67% overall yield. This method was unsuccessful for tri-substituted ureas, which were prepared in 42-74% yields by an alternative four-step method, which included a one-pot 4-nitrophenyl carbamate formation/displacement sequence.     

A short synthetic route to the calystegine alkaloids

An efficient strategy is described for the synthesis of enantiopure calystegine alkaloids. The key step employs a zinc-mediated fragmentation of benzyl-protected methyl 6-iodo-glycosides followed by in situ formation of the benzyl imine and Barbier-type allylation with zinc, magnesium, or indium metal. Stereochemistry in the pivotal allylation is controlled by the choice of the metal. The functionalized 1,8-nonadienes, thus formed, are converted into cycloheptenes by ring-closing olefin metathesis. Regioselective hydroboration and oxidation give the corresponding cycloheptanones, which are deprotected to afford the desired calystegines. Hereby, calystegine B(2), B(3), and B(4) are prepared from D-glucose, D-galactose, and D-mannose, respectively. This route constitutes the shortest synthesis of calystegine B(2) and gives rise to the first total syntheses of calystegine B(3) and B(4).     

A synthetic approach to the Stemona alkaloids.

This paper describes our work developing a strategy for the construction of the typical core structure of the Stemona alkaloids. The approach is to control the relative stereochemistry of the groups on the core 1-azabicyclo[5.3.0]decane ring system by a [3,3] sigmatropic rearrangement of an acylimmonium ion followed by selective reduction. After optimization, this reaction sequence afforded the desired diastereomer in 62% yield. Further efforts were directed toward elaboration of the characteristic butyrolactone substituent.     

Studies directed towards the total synthesis of anthracycline antibiotics

At present anthracycline antibiotics have proven to be the most exciting agents in cancer chemotherapy. Both adriamycin and daunomycin are proven to be effective against a variety of human tumour cells, despite their cardiotoxicity. However, some synthetic analogues, such as 4-demethoxydaunomycin, are shown to be better therapeutic ratios compared to adriamycin or daunomycin. Various approaches have been successfully made for the total synthesis of (±) 4-demethoxydaunomycinone, the aglycone of (±) 4=demethoxydaunomycin, starting from benzoquinone, napthalene or anthraquinone precursors. Finally an elegant approach for the stereoconvergent synthesis of (+) 4-demethoxydaunomycinone has been worked out. New methods involving both Diels-Alder and Friedal-Crafts acylation, have been developed for the total synthesis of daunomycinone and 11-deoxydaunomycinone. The synthesis of L-daunosamine, the amino sugar unit present in the antitumor anthracyclines has been successfully elaborated starting either from D-glucose or D-glucosamine.