Lewis Acid-Controlled Regioselectivity in Reactions of Styrenyl Systems with Benzoquinone Monoimides:  New Regioselective Syntheses of Substituted 2-Aryl-2,3-dihydrobenzofurans, 2-Aryl-2,3-dihydroindoles, and 2-Arylindoles

Abstract

Reactions of 4-(N-phenylsulfonyl)-2-alkoxy-1,4-benzoquinone monoimines 2-4 with electron-rich propenylbenzenes promoted by BF 3 yield 7-alkoxy-2-aryl-3-methyl-5-[(N-phenylsulfonyl)amino]-2,3-dihydrobenzofurans 5-7 nearly exclusively, whereas promotion of the reactions by Ti 4+ gives mixtures of the dihydrobenzofurans and their N-(phenylsulfonyl)-6-alkoxy-2-aryl-5-hydroxy-3-methyl-2,3-dihydroindole isomers 8-10, depending upon substituents present on the propenylbenzene. However, reactions promoted with excess Ti 4+ , as mixtures of TiCl 4 :Ti(OiPr) 4 , give the dihydroindoles as nearly the exclusive products. Evidence for a mechanism involving initial 5 + 2 cycloaddition of the Lewis acid-bound quinone monoimide with the propenylbenzene is found in reactions of styrenes 1f/g with monoimide 3 in which 7-aryl-3-hydroxy-6-methylbicyclo[3.2.1]oct-3-ene-2,8-diones 33 (5 + 2 adducts) are isolated. These reactions have been applied to stereoselective syntheses of pterocarpans bearing N-phenylsulfonyl groups, azapterocarpans and diazapterocarpans. In addition, DDQ oxidation of derivatives of several of the 2-aryl-2,3-dihydroindoles afford the corresponding 2-arylindoles in good yield. Finally, the experimental details of a general synthetic approach to 7-alkoxy-benzofuranoid neolignans, including (()-licarin B and eupomatenoids-1 and -12 are reported.

Ring contraction of N-acetyl-2-aryl-1,2,3,4-tetrahydro-4-quinolones with [hydroxy(tosyloxy)iodo]benzene to trans methyl N-acetyl-2-aryl-2,3-dihydroindol-3-carboxylates

“…A side product, methyl p-toluenesulfonate (6), was also isolated (Scheme 2) in small quantity. The structures of 5a-e and 6 11 were established by physical and spectroscopic techniques (IR, 1 H and 13 Ar Scheme 1 12 The coupling constant between C 2 -H and C 3 -H of cis 2,3-dihydroindoles where nitrogen is protected with nitrosyl or arylsulfonyl is 8-9 Hz, whereas that of trans isomer is < 4 Hz. The less coupling constant (1.8 Hz) in 5a-e is perhaps due to p-p interaction between N-acetyl and C 2 -aryl groups, thus significantly altering the C 2 -H/C 3 -H dihedral angle.…”

“…The less coupling constant (1.8 Hz) in 5a-e is perhaps due to p-p interaction between N-acetyl and C 2 -aryl groups, thus significantly altering the C 2 -H/C 3 -H dihedral angle. 13 The probable mechanism involves the ketalisation of 4 with TMOF in presence of either HClO 4 or H 2 SO 4 to afford intermediate enol ether 7. The electrophilic attack of HTIB on the double bond of enol ether 7 furnished the iodine (III) complex, 8.…”

Recent Progress in the Synthesis of 2,3-Dihydrobenzofurans

“…40 In particular, the addition of β-methylstyrenes bearing strong electron-donating groups to solutions of 2-alkoxy-4-(N-phenylsulfonyl)benzoquinone monoamines and BF 3 ·Et 2 O in dichloromethane at low temperature afforded trans-2,3-dihydrobenzofurans in good yields and high stereoselectivities (Scheme 11). However, it should be noted that the formation of 2,3-dihydrobenzofurans was not exclusive and, in some cases, small amounts of dihydroindoles were found.…”

Synthesis of Pterocarpans

“…Recently, microwave heating has been used to improve yields of this cycloaddition reaction catalyzed with Zn(II) [97]. Table 3) [26,[98][99][100][101][102]. However, attempted reactions of 2H-thiachromenes with quinones failed to provide 5-thiapterocarpans [26].…”