A simple route for the synthesis of 4-chlorochromenes and chroman-4-ones

A one pot synthesis of a number of 4-chlorochromenes and chroman-4-ones was achieved from γ-chloropropargyl aryl ethers proceeding through Claisen rearrangement, depending upon the solvent of choice

Thermal behaviour of aryl γ-haloprcpargyl ethers

A systematic study of the behaviour of aryl γ-halopropargyl ethers under thermal condition was undertaken. Aryl γ-bromopropargyl ethers 2̲ underwent unique transformation in N,N-diethylanillne (215°C, 6 h) giving rise to a mixture of products 3̲,4̲ and 5̲, whereas, under similar conditions aryl γ-chloropropargyl ethers ulbar|8, afforded 4-chlorochromenes, 9. A remarkable substituent and solvent effect has been observed in the thermolysis of these aryl γ-bromo and γ-chloropropargyl ethers, rendering this transformation as a method for the synthesis of a number of substit uted 4-bromochromenes 3̲, 4-chlorochromenes 9̲ and chroman-4-ones 7̲. In contrast, solution thermolysis of aryl γ-iodopropargyl ether 11̲ afforded aryl propargyl ether 1̲ as the major product

Photochemical behaviour of 3.4-epoxyprecocene-I and related epoxychromans

A systematic study of photochemical behaviour of 3,4-epoxyprecocene-I and related epoxychromans was undertaken. Upon irradiation in acetone or cyclohexane, 3,4-epoxyprecocene-I 1̲ was found to undergo photoisomerisation to the corresponding 3-chromanone 2̲. In contrast, the photochemical behaviour of analogous 3,4-dihydro-3,4-epoxy-2H-1-benzopyrans 3 was found to be dependent upon the nature of solvent. Irradiation of 3 in cyclohexene led to photodecarbonylation leading to the formation of 2,3-dihydrobenzofurans 4̲, whereas irradiation in acetone resulted in the formation of chramanones 5̲

Radical cyclisation : synthesis of 4H-furo(3,2-c)-1-benzopyrans

3,4-dihydro-3-bromo-4-(prop-2-ynyloxy)-2H-1-benzopyrans (3) undergo radical cyclisation when treated with nBu<SUB>3</SUB>SnH and AIBN to give good yield of 2,3,3a,9b-tetrahydro-3-methylene-4H-furo (3,2-c)benzopyrans (4)

Photochemical studies on 3,4-epoxy-3,4-dihydro-2H-1-benzopyrans

Photochemical behaviour of 3,4-epoxy-3,4-dihydro-2H-1-benzopyrans, intermediates in the synthesis of pterocarpans, has been studied in various solvents. While ring contraction leading to 2,3-dihydrobenzofurans through a photodecarbonylation process was observed in cyclohexane, photoisomerisation leading to 3-chromanones was observed in acetone. This study has also been extended to antijuvenile hormone precocene(I) oxide

Synthesis of pterocarpans by means of a "disfavored" 5-endo-trig radical cyclization reaction

A successful synthesis of pterocarpans 1, based on a "disfavored" 5-endo-trig radical cyclization reaction, has been accomplished. The radical precursor 4-(2'-bromoaryloxy)-2H-chromene 8 was synthesized in six steps, starting from aryl propynyl ether 2. On treatment with tributyltin hydride in refluxing benzene, aryl enol ether 8 underwent radical cyclization to furnish the pterocarpan 1. A deuterium label study was performed to prove the occurrence of 5-endo-trig radical cyclization. This methodology was extended further to synthesize a class of hitherto unknown pterocarpans 13. A novel and stereoselective route towards the synthesis of aryl ethers of bromohydrins by means of the Mitsunobu reaction is also described