Studies in Pyrimidine-Annelated Heterocycles1 by Tandem Cyclization: Regioselective Synthesis of [6,6]Pyranopyran by Intramolecular [1,6] Michael Addition

α-Carbonyl Rh-Carbenoid Initiated Cascade Assembly of Diazobarbiturates with Alkylidene Pyrazolones for Synthesis of Spirofuropyrimidines

Catalyzed by Rh2(esp)2 (10 mol%) and (±)-BINAP (20 mol%) in DCE at 80 °C, the cascade assembly between diazobarbiturates and alkylidene pyrazolones proceeded readily and produced spiro-furopyrimidines in 38-96% chemical yields. The chemical structure of the prepared spirofuro-pyrimidines was firmly confirmed by X-ray diffraction analysis.

Studies in Sigmatropic Rearrangement: Thermal Rearrangement of 3-(4′-aryloxybut-2-ynyloxy)thiochromen-4-ones

Reaction of 3-hydroxythiochromen-4-ones 1a,b with appropriate1-aryloxy-4-chlorobut-2-ynes 2a–f furnished 3-(4′-aryloxybut-2-ynyloxy)thiochromenones 3a–g in 80–90% yields. Substrates 3a–g on refluxing in chlorobenzene for 8–10 h gave 3-aryloxymethyl-2-methylfuro[3,2- b]-thiochromen-9-ones in 90–95% yields.

Radical-mediated cyclization reaction — Regioselective synthesis of pyrimidine- and coumarin-annulated [6,6]-fused oxygen heterocycles

The n-Bu3SnH-AIBN mediated aryl radical cyclization of various 8-[(2-bromophenoxy)methyl]-1,3-dimethyl-2,3,4,6-tetrahydro-1H-pyrano[3,2-d]pyrimidine-2,4-diones and 1-[(2-bromophenoxy)methyl]-3,5-dihydropyrano[2,3-c]coumarins was investigated with the formation of [6,6]-fused oxygen heterocycles as a single regioisomer.Key words: n-Bu3SnH-AIBN, aryl radical cyclization, 6-endo-trig, oxygen heterocycles, coumarin derivatives, pyrimidine derivatives.

Lewis acid catalyzed Claisen rearrangement: Regioselective synthesis of oxygen, nitrogen, and sulfur heterocycles

Regioselective synthesis of pentacyclic heterocycles containing oxygen, nitrogen, and sulfur has been achieved in good to excellent yields by the sequential Claisen rearrangement of but-2-ynyl ethers and sulfides containing quinolone moiety. The substrates ethers and sulfides were prepared from 1-aryloxy-4-chlorobut-2-ynes with N-alkyl-4-hydroxyquinoline-2(1H)-ones.Key words: regioselectivity, [3,3] sigmatropic rearrangement, nucleophilicity of sulfur, aluminium chloride, heterocycles.

Regioselective synthesis of biologically interesting pentacyclic polyheterocycles by sequential thio-Claisen and AlCl3 catalyzed oxy-Claisen rearrangement of 4-(4′-aryloxybut-2′-ynylthio)-1-phenyl-1,8-naphthyridin-2(1H)-one

A number of 4-aryloxymethyl-6-phenyl-2H-thiopyrano[3,2-c][1,8]naphthyridin-5(6H)-ones were regio selectively synthesized in 82%–95% yields by the thermal Claisen rearrangement of 4-(4′-aryloxybut-2′-ynylthio)-1-phenyl-[1,8]naphthyridin-2(1H)-ones. These products were then subjected to a second Claisen rearrangement in the presence of a Lewis acid catalyst, anhyd. AlCl3, to give hitherto unreported pentacyclic heterocycles in 75%–90% yields. The same final products were also obtained in low yield upon refluxing 4-aryloxymethyl-6-phenyl-2H-thio pyrano-[3,2-c][1,8]naphthyridin-5(6H)-ones in N,N-diethyl aniline for 12–14 h. This method was found to be more effective than thermal Claisen rearrangement.Key words: [3,3] sigmatropic rearrangement, regioselective synthesis, phase transfer catalysis, sequential Claisen rearrangement, Lewis acid catalyzed Claisen rearrangement, single crystal X-ray.

Synthesis of pyrimidine annulated furothiopyrans — An efficient sequential and tandem catalyzed Claisen rearrangement – intramolecular hydroaryloxylation

Regioselective synthesis of a hitherto unreported furothiopyran moiety fused at the C-5 and C-6 positions of a pyrimidine heterocycle was achieved by the application of sequential Claisen rearrangement in which a second aromatic Claisen rearrangement and intramolecular hydroaryloxylation were catalyzed by aluminum chloride. The second aromatic Claisen rearrangement step was also studied under thermal conditions to give mostly isomerized exocyclic compounds. The precursor endocyclic compounds were synthesized by thermal [3,3] sigmatropic rearrangement of the corresponding sulfide.Key words: aluminum chloride, sequential Claisen rearrangement, hydroaryloxylation, furothiopyran, pyrimidine.

Studies in sigmatropic rearrangement: synthesis of a [6,6]pyranothiopyran ring system by sequential claisen rearrangement and pyridine hydrotribromide mediated regioselective "6-Endo" cyclization

[reaction: see text] 4-(4'-Aryloxybut-2'-ynylthio)[1]benzopyran-2-ones are refluxed in chlorobenzene to afford 4-aryloxymethylthiopyrano[3,2-c][1]benzopyran-5(2H)-ones which are subsequently subjected to heating in o-dichlorobenzene in the presence of N,N-diethylaniline and then treated with pyridine hydrotribromide to give [6,6]pyranothiopyrans in almost quantitative yield.