Photochemistry of Bicyclo[2.2.2]oct-7-ene-2,5-diones and the Corresponding 5-Hydroxyimino and 5-Methylene Derivatives

Abstract

Synthesis and photochemistry of several title compounds 1-3 containing multiple chromophoric systems are described. The Diels-Alder reactions of 2,6,6-trimethylcyclohexa-2,4-dienone (5) with acetylenes 6a-d provided the adducts 7a-d, which upon hydrolysis furnished the desired bicyclo[2.2.2]octenediones 1a-d. Oximes 2a-d were prepared from diones 1a-d by treatment with hydroxylamine hydrochloride in pyridine. 5-Methylenebicyclo[2.2.2]oct-7-en-2-ones 3a-d were obtained via chemoselective Wittig reaction of the corresponding diones 1a-d. Bicyclo[2.2.2]octenediones 1a-c underwent chemoselective oxa-di-pi-methane rearrangement under sensitized conditions and suffered formal ketene extrusion upon direct irradiation. Direct irradiation of 1d afforded 11d via formal ketene extrusion but under sensitization it remained unchanged. Oximes 2a-d suffered ketene extrusion upon direct irradiation and E/Z isomerization under sensitized conditions. On the other hand, 5-methylenebicyclo[2.2.2]oct-7-en-2-ones 3a-d generally underwent 1,3-acyl shift. The plausible courses of all these photochemical processes are discussed.

Diels−Alder Reactions of 4-Triflyloxy-2,6,6-trimethyl-2,4-cyclohexadienone. An Expedient Methodology for the Synthesis of Bicyclo[2.2.2]oct-5-en-2-ones and Bicyclo[2.2.2]octa-5,7-dien-2-ones

“…General procedure for the synthesis 23 and spectral analysis of 1,3,3-trimethylbicyclo[2.2.2]oct-7-ene-2,5-diones ( 7a , 7b ,21c 7c , 7d 20 ) were reported earlier.…”

“…While the first route was found to be inefficient, we were able to prepare bicyclo[2.2.2]octenediones ( 7 ), a very useful class of compounds with no easy access, and transformed the diones 7 into the desired compounds 2 using [Pd(PPh 3 ) 2 Cl 2 −Bu 3 N−HCO 2 H]. The overall result of this four-step route could have been accomplished in one step by the Diels−Alder reaction of an acetylene with 2,6,6-trimethyl-2,4-cyclohexadienone ( 12 ) except for the problems associated with synthesis and handling of 12 . 10b,,16a,, Alternatively, we were able to identify and explore the possibility of using 4-triflyloxy-2,6,6-trimethylcyclohexa-2,4-dienone ( 13 ) instead of 4 and thereby reducing the number of synthetic steps to two .…”

Formation of a Three‐Membered Ring

“…Acetophenone is also a convenient sensitizer for the ODPM rearrangement. Bicyclo[2.2.2]oct-7-ene-2,5-diones 53a-c undergo a highly chemoselective ODPM rearrangement under acetophenone-sensitized irradiation (350 nm) to yield 54a,c and 55b, in excellent yields (Table 4.2) [33]. However, phenols 56a,b, which presum- ably arise via a-cleavage and dimethylketene elimination, were obtained under different conditions, such as irradiation in acetone.…”

“…However, phenols 56a,b, which presum- ably arise via a-cleavage and dimethylketene elimination, were obtained under different conditions, such as irradiation in acetone. On the other hand, a compound such as 53d is unreactive, as a result of the localization of excitation energy in the triplet on the stilbene moiety, which is too low in energy to allow the rearrangement to occur [33]. Bicyclo[2.2.2]oct-5,7-dien-2-ones 57a-c, which are endowed with the structural features required for both the ODPM and DPM rearrangements, undergo acetophenone-sensitized DPM rearrangement to afford the compounds 58a-c in good yields (Scheme 4.24) [34].…”

Efficient Synthesis and Subsequent Transformations of Phenylsulfanylbicyclo[2.2.2]octenones and Phenylselenylbicyclo[2.2.2]octenones

“…These reactions at first sight look to have a di-π-methane or oxa-di-π-methane rearrangement-type reaction pathways. Though the fate of electronically excited triplets cannot be compared with that of the radicals in the ground states, we recently reported, during our study of photochemical reactions on bicyclo[2.2.2]octadienones, the competition between di-π-methane and oxa-di-π-methane bridgings. , …”