Reactions of 2-methoxy-1,4-benzoquinones 2 and 3 with (E)-propenylbenzenes 1 promoted at −78
°C by Ti(IV)−TADDOLates prepared from diol-(+)-4 afford (1R,6R,7R,8R)-8-aryl-3-methoxy-7-methylbicyclo[4.2.0]oct-3-en-2,5-diones 5/8 or (1R,5R,6R,7R)-7-aryl-3-hydroxy-6-methylbicyclo[3.2.1]oct-3-en-2,8-diones 7/10 in good yield and high ee. (2S,3S)-2-Aryl-6-methoxy-3-methyl-2,3-dihydrobenzofuran-5-ols 6/9 are also found, but in slightly lower ee. Cyclobutanes 5/8 cleanly and
efficiently rearrange to the dihydrobenzofurans 6/9 without loss of enantiomeric purity upon
treatment with the Ti−TADDOLates at higher temperatures. Reactions of (Z)-propenylbenzene
17 and of indene with 2 and 3 give products in moderate enantiomeric purity. Products obtained
from reactions of 1-anisylcycloalkenes with 2 differ significantly in yield and enantiomeric purity.
In the latter reactions, the ee's of the cyclobutane products are consistently much higher than
those of the dihydrobenzofuran products. More significantly, products of different absolute configuration result from different cycloalkenes. With 1-anisylcyclopentene or 1-anisylcyclohexene, all of
the products are of similar configuration and are obtained in comparable yields and ee's. However,
1-anisylcycloheptene affords products that are diastereomeric with those of the 1-anisylcyclopentene,
and in lower ee's. A mechanistic model is proposed. Application of these reactions to the
enantioselective synthesis of the pterocarpan class of isoflavonoid natural products is also reported.
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.
BF3-promoted reactions of
2-alkoxy-1-(N-benzoyl)-4-(N-benzenesulfonyl)-1,4-benzoquinone
bisimines
(2/3) with β-methylstyrenes yield
trans-5-(N-benzoylamino)-6-alkoxy-2-aryl-3-methyl-2,3-dihydroindoles (4/5) or
3-(N-benzoylamino)-6-aryl-7-methyl-4,8-dioxobicyclo[3.2.1]oct-2-enes
(6) regio- and
stereoselectively. The former are produced in reactions with
styrenes bearing alkoxy substituents,
whereas with neutral or electron deficient styrenes, the latter are
found. As applications of this
new methodology to the synthesis of biologically interesting molecules,
reactions of 2H-chromenes
and 7-methoxy-N-toluenesulfonyl-1,2-dihydroquinoline with
the bisimines have been developed as
routes to substituted aza- and diazapterocarpans. Syntheses of
2-arylindoles via oxidation of the
dihydroindoles 4 are also reported.
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