Aza-Nazarov cyclization cascades

Benzamides with tethered acetal groups undergo reactions in CF₃SO₃H to give ring-fused isoindolinones by a cyclization cascade. The reaction initially forms an N-acyliminium ion which then gives the isoindolinone by the aza-Nazarov reaction. An unusual variant also cyclizes at the allylic position.     

Reaction of arylidene derivatives of Meldrum's acid with 3-amino-1,2,4-triazole

The condensation of arylidene derivatives of Meldrum's acid with 3-amino-1,2,4-triazole in nitrobenzene leads to 4,5,6,7-tetrahydro-1,2,4-triazolo [1,5-a]pyrimidin-5-ones. In DMF the reaction proceeds with the formation of arylsubstituted N-(2H-1,2,4-triazol-3-yl)-3-(2H-1,2,4-triazol-3-ylamino)propionamides. Ukrainian Research Institute for the Pharmacotherapy of Endocrine Diseases, Kharkov 310002. Kharkov State University, Kharkov 310077, Ukraine     

Carboxyketenes from 4-hydroxy-1,3-oxazin-6-ones and Meldrum's acid derivatives

New 4-hydroxy-1,3-oxazin-6-ones 8 and 16 were prepared from chlorocarbonyl(phenyl)ketene and amides. The flash vacuum thermolysis (FVT) reactions of these compounds and the 4-methoxy derivative 17 were investigated by Ar matrix isolation IR spectroscopy and online mass spectrometry including MS/MS analysis. Carboxy(phenyl)ketene 10 is formed as the major product by thermal fragmentation of 4-hydroxy-1,3-oxazin-6-one 8. This takes place via the unstable 6-hydroxy tautomer 9. Another tautomer, the 5H-isomer 12, leads to the formation of benzoyl isocyanate 13 as a minor product together with phenylketene 14. Carboxy(phenyl)ketene 10 remains detectable at high FVT temperatures but undergoes thermal decarboxylation to phenylketene 14. The same carboxy(phenyl)ketene 10 is also produced in significant amounts by FVT of 5-phenyl-Meldrum's acid 18 via the unstable enol tautomer 19. A small amount of the unsubstituted carboxyketene 20 is observable on FVT of Meldrum's acid 1 itself.     

Radical cyclization of 4-aryl-1-iodobutene derivatives to form dihydronaphthalene scaffold

Synthesis of dihydronaphthalene derivatives was carried out by allyltributylstannane-mediated vinyl radical cyclization as the key step from 4-aryl-1-iodobutene derivatives, which were derived from Baylis-Hillman adducts via vinyl radical cyclization.     

Asymmetric cyclization of unsaturated aldehydes catalyzed by a chiral lewis acid

A highly enantioselective cyclization of prochiral unsaturated aldehydes has been accomplished with a chiral zinc reagent derived from dimethylzinc and ($\text{R}$)-(+)-1,1′-bi-2-naphthol.     

Meldrum's acid and its analogs

2-Substituted or 2,2-disubstituted 4,6-dioxo-1,3-dioxanes, which are analogs of Meldrum's acid and were obtained by reaction of malonic acid with acetic anhydride and the appropriate carbonyl compound in the presence of sulfuric acid, exist in solution preferably in a single form according to the PMR and ¹³C NMR spectral data. Fluctuation of the CH₂ group is not observed. The mass spectra are characteristic and are distinguished by splitting out of the carbonyl compound from the molecular ion or successive splitting out of CO₂ and ketene molecules. The Meldrum acid analogs do not affect plant growth but have a depressive effect on the central nervous system of animals and low toxicity.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1482–1486, November, 1975.     

Microwave-mediated intramolecular Diels-Alder cyclization of biodihydroxylated benzoic acid derivatives

The synthetic utility of biodihydroxylated benzoic acid derivatives for the construction of bridge bicyclo scaffolds was investigated. Biodihydroxylation of benzoic acid using Ralstonia eutropha B9 gave (1S,2R)-1,2-dihydroxycyclohexa-3,5-diene-1-carboxylic acid (DCD) in high optical purity (>95% ee). Protection of the intermediate and subsequent functional group transformation gave the required cyclization precursors in moderate to excellent overall yields. Subsequent intramolecular Diels-Alder cyclization of biodihydroxylated benzoic acid derivatives was carried out using either thermal or microwave conditions. Enantiomerically pure products with five chiral centers were obtained in 4-6 steps from achiral starting material.     

Electrophilic intramolecular cyclization of functional derivatives of unsaturated compounds: V. Cyclization of anilides of styrylacetic acids in polyphosphoric acid

Anilides of styrylacetic acids at heating in polyphosphoric acid undergo an intramolecular cyclization giving 1,5-diarylpyrrolidin-2-ones, 5-aryl-1,3,4,5-tetrahydro-2H-benzazepin-2-ones, and 5-(3-fluorophenyl) dihydrofuran-2(3H)-one, where the ratio of the products depends on the character of the aromatic substituents in the amide and styrene fragments of the substrate.     

Polyphosphorous acid catalyzed cyclization in the synthesis of cryptolepine derivatives

11-Oxo-10,11-dihydroxy-5H-indolo[3,2,b]quinoline 7-carboxylic acid was obtained specifically by polyphosphorous acid catalyzed cyclization with optimal reaction conditions.Biological assays showed that it potentially inhibits the proteasomal chymotrypsin-like activity in vitro and suppresses breast cancer cell growth.     

Novel cyclization cascades to functionalized indanes and tetrahydronaphthalenes

Cyclization cascades involving C-C bond formations followed by lactonization reactions provide fast access to structurally complex tricyclic indane and tetrahydronaphthalene derivatives.     

SmI2-mediated dialdehyde cyclization cascades

Dialdehydes undergo sequenced SmI₂-mediated cyclization cascades generating four contiguous stereocenters with high diastereocontrol.     

Wavelength-dependent photochemistry of Diazo Meldrum's acid and its spirocyclic isomer,Diazirino Meldrum's acid: Wolff rearrangement versus isomerization

Photoreaction of diazo Meldrum's acid (1) shows a unique wavelength selectivity. At 254 nm it results in efficient (phi(254) = 0.34) Wolff rearrangement, while irradiation with 355 nm light leads to a completely different process, isomerization into corresponding cyclic alpha,alpha"-dicarbonyl diazirine 2 (phi(350) = 0.024). UV photolysis of diazirine 2 is accompanied by two competing processes: loss of nitrogen followed by the Wolff rearrangement and isomerization into diazo compound 1. Thermal decomposition of 1 leads to clean Wolff rearrangement, while heating of 2 causes quantitative conversion into diazo isomer 1.     

Radical cyclization approach to spirocyclohexadienones

Cyclization of an aryl radical at the ipso position of a p-O-aryl-substituted acetamide or benzamide generates oxindoles or quinolones bearing spirocyclohexadienone rings. This versatile reaction is applied to formal syntheses of the vasopressin inhibitor SR121463A and aza-galanthamine.