Perkin communications. A new method for the preparation of 1,5-dicarbonyl compounds from bicyclic ketals

Abstract

Dicarbonyl compounds have been prepared from bicyclic ketals in high yield under mild conditions by using aluminium chloride-sodium iodide in methylene dichloride.Although Lewis acid induced rearrangements of bicyclic ketals are an attractive method for the synthesis of natural products,' they have been little investigated. In our studies of the 6,8dioxabicyclo[3.2. lloctane skeletal system which is easily prepared from methyl vinyl k e t ~n e , ~ we have found a new synthetic route from bicyclic ketals to 1,5-dicarbonyl compounds, with potential as intermediates for 6-membered heterocycles and cyclopentane derivative^.^ Mild reagent systems such as boron trifluoride-diethyl ethersodium iodide," chlorotrimethylsilane-sodium iodide and aluminium chloride-sodium iodide were used to cleave the bicyclic ketal with interesting results.

Introduction of Bicyclic Ketal Chemistry: Synthesis and Transformation Reaction of 6,8-Dioxabicyclo[3.2.1]octane Skeletal System

“…Two products, a cyclohexenone (66, 11%) and a substituted pyridine 67 (48%), were formed in one step from this reaction (Scheme 24). 95 Based on isolation of the diketone intermediate 58, 90 we suggest that the formation of cyclohexenone 66 occurs through the intramolecular aldol condensation via an enol intermediate 58a and the pyridine compound 67 arises from insertion of the MeCN to an enol 58b followed by aromatization. The role of the diketone 58 as intermediate was confirmed by treating it with AlI 3 in MeCN to give the expected products, 66 and 67, in a separate reaction.…”

“…Two products, a cyclohexenone (66, 11%) and a substituted pyridine 67 (48%), were formed in one step from this reaction (Scheme 24). 95 Based on isolation of the diketone intermediate 58, 90 An aromatic substituted derivative 72 was tolerated under these reaction conditions to give the expected products 73 and 74 in 12% and 37% yield, respectively (Scheme 25). Butyronitrile was used instead of MeCN to broaden the range of nitrogen source for the pyridine derivatives and resulted in the formation of pyridine 75 in 42% yield.…”

“…A new synthetic route from bicyclic ketals to 1,5-dicarbonyl compounds, which are potential intermediates for 6membered heterocycles and cyclopentane derivatives, has been discovered. 90 A solution of 5,7,7-trimethyl-6,8-dioxabicyclo[3.2.1]octane (11) in CH 2 Cl 2 reacted over 3 hours at ambient temperature with AlCl 3 (2 equiv)-NaI (2 equiv) to give, after basic work-up followed by short-path column chromatography, the 1,5-diketone (58, 71%, Scheme 22). This is a novel rearrangement reaction, which could involve a 1,2-hydride shift via an epoxide intermediate (path a).…”