Issue in Honor of Prof. S. Swaminathan ARKIVOC 2005 (xi) 146-160
1a and 4a. Thus, we performed the reaction of excess a-diazo ketone 1a with acenaphthenequinone in the presence of Rh2(OAc)4 to furnish bis-dioxabicyclo[2.2.1]heptanone ring system 10a as a minor isomer along with mono-dioxabicyclo[2.2.1]heptanone ring systems 10b,c as a diastereomeric mixture. And the crude nmr spectrum showed the formation of products in the ratio of 1:1:1.5. Similarly, diazo ketone 4a with acenaphthenequinone afforded bis-epoxy-bridged cycloadduct 11a as a minor isomer along with mono-epoxy-bridged cycloadducts 11b,c as a diastereomeric mixture and the crude nmr spectrum showed the formation of products in the ratio of 1:1:1.5. The bis-cycloadducts 10a and 11a were obtained only in poor (20 and 25%) yields even in the presence of excess diazo ketones. A reason for this may be due to steric hindrance arising from the proximity of an already installed dioxabicyclo[2.2.1]heptanone ring system.
Table 2. Reaction of carbonyl ylides 3 or 6 with anthraquinone and acenaphthenequinone
Entry X Y Z Product Yield (%)a 1 -(CH2)4-CH3 7 60 2 CH3 CH3 CH3 8 70 3 CH3 -(CH2)2-9 75b 4 -(CH2)4-CH3 10a 20c 5 CH3 CH3 CH3 11a 25c
a
Yields (unoptimized) refer to isolated and chromatographically pure compounds. b Ref.16. c Only the yield of bis-cycloadduct is provided.
After studying the tandem reaction of diazo ketones with 1,2- and 1,4-diketo functionalities placed on a rigid ring system, we extended the reaction of cyclic carbonyl ylides 3 and 6 with substrates having 1,4-diketo-functionalities on a flexible ring system. Thus, an excess of diazo ketone 1a was reacted with 1,4-cyclohexanedione in the presence of a catalytic amount of Rh2(OAc)4 under an argon atmosphere. The crude reaction mixture was investigated by 1H NMR spectroscopy, which indicated formation of the bis-dioxabicyclo[2.2.1]heptanone ring system as a mixture of diastereomers in the ratio of 1:3. These diastereomers were separated by column chromatography to afford products 13a and 13b in 45 and 17% yield, respectively (Scheme 3, Table 3). The IR spectrum of compound 13a showed a band at 1763 cm-1 for the presence of a keto-functionality in a strained ring. The 1H NMR spectrum of compound 13a exhibited two singlets for both the bridgehead protons (Ha) at 4.23 and 4.21 ppm. Characteristically, 13C NMR spectrum of the product 13a showed a single resonance at 87.2, 214.1 ppm for the bridgehead17 (C-Ha) and the carbonyl (C=O) carbons, respectively. Further, the single crystal X-ray crystallographic analysis18 of compound 13a (Figure 1) clearly revealed that the stereochemistry of the interesting bis-cycloadduct 13a has the trans-geometry on cyclohexane ring system. Based on the interrelated spectroscopic analyses, the minor isomer was tentatively assigned as biscycloadduct 13b with cis-geometry.
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