New functionalized cycloheptatrienes via tropone σ-adducts

Abstract

We report that, on treatment with phenyl-lithium, followed by triethyloxonium tetrafluoroborate in diethyl ether at -78 "C, (i) 2-chloro-and 2-bromo-tropone give the corresponding 1 -ethoxy-2-halogeno-7phenylcycloheptatriene ; (ii) 2-fluorotropone gives 2-ethoxy-3,7-diphenylcycloheptatriene ; and (iii) tropone affords 1 -ethoxy-2-phenylcycloheptatriene. Conceivably, in case (i) the tropone undergoes phenyl-lithium attack at C-7, followed by 0-alkylation of the enolate, while in case (iii) the intermediate enolate suffers base-catalysed rearrangement and, finally, in case (iii) nucleophilic replacement of fluorine by phenyl-lithium from the tropone precedes phenyl-lithium attack at C-7 of the resulting 2-phenyltropone. This gives a better synthesis, than those available so far, of 2-chloro-7-aryltropones ; in addition, a 1 -a I koxy -2c hloro-7p henylcyclo heptatriene undergoes selenium dioxide oxidation to 2c hloro -7p henyltropone.Cycloheptatrienes, owing to a variety of possible ring transformations, are potentially useful intermediates in organic synthesis. This requires, however, that suitably functionalized cycloheptatrienes are made easily available. Besides other laboratories,' our own has contributed to the progress in this field in recent years. ' We now report on the reactions of cycloheptatrienes carrying halogen or/and alkoxy substituents. These were prepared by adding an aryl-lithium reagent to a tropone and trapping the resulting enolate with an electrophile.The results in the Table show that 2-chloro-(l), 2-chloro-[3,5,7-'H3]-(2), and 2-bromo-tropone (3), when the electrophile is either the Meerwein reagent or trimethylchlorosilane, give the corresponding 1 -alkoxy-( 7)-( 9) or 1-trimethylsiloxy-2-halogeno-7-phenycycloheptatriene (1 I ) . The only difference, in the two cases, is that while the chloro compounds (7) and (8) proved to be quite stable, the bromo compound (9) was labile. 2-Phenyltropone (4) shows similar behaviour, giving a 1 -alkoxy-2,7-diphenylcycloheptatriene (10).2-Fluorotropone (5) shows different behaviour, giving the same product (10) as obtained from the reaction of 2-phenylt ropone.

4‐π‐Photocyclization: Scope and Synthetic Applications

“…[20] Thus, irradiation of BF 3 •9 in acetonitrile for 30 minutes resulted in 80 % conversion, and the photocycloadduct 12 was isolated in 59 % yield (Scheme 6). [21] Similar results were also observed through adding one equivalent of BF 3 •Et 2 O (or excess sulfuric acid) to a solution of tropone in acetonitrile, followed by irradiation under the above conditions. The authors showed that absorption peak at ca.…”

“…Similarly, Cavazza, Zandomeneghi and Pietra reported that complexation of tropone to Lewis acids also allowed selective 4‐π‐photocyclization rather than dimerization . Thus, irradiation of BF 3 ·9 in acetonitrile for 30 minutes resulted in 80 % conversion, and the photocycloadduct 12 was isolated in 59 % yield (Scheme ) . Similar results were also observed through adding one equivalent of BF 3 · Et 2 O (or excess sulfuric acid) to a solution of tropone in acetonitrile, followed by irradiation under the above conditions.…”

Acetylene and Ethylene: Universal C2 Molecular Units in Cycloaddition Reactions

“…The resulting mixture was easily separated, and further isolated compounds were used in the synthesis of natural troponoid 4-isopropyltropone (nezukone). 66 Low yields for the reaction of cyclopentenones are confirmed by other examples. 2-Meth­oxybicyclo[3.2.0]hepta-2,6-dien-4-one was isolated in 30% yield as a result of acetylene addition to cyclopent-2-ene-1,4-dione monoacetal.…”

Changes in the photochemistry of tropone or 2-methoxytropone on complexation with acids

“…The quantum yields for the (1) to (3) (Scheme 1) and (4) to ( 5 ) (Scheme 2) processes are of the same order of magnitude as for the (1) to (2) process (Scheme l), which was determined to be 0.05. Thus, the quantum yield for the (4) to ( 5 ) valence isomerization has a dramatically higher yield than the photodimerizations of (4) (Scheme 2 ) as shown in parallel experiments in which by using a merry-go-round apparatus, tropone was recovered unchanged from irradiation in non-acidic CH3CN solutions, while 30% of the tropone.BF3 adduct was converted into ( 5 ) .…”

“…On irradiation with UV light in either organic solvents or acidic or neutral water, 2-methoxytropone (1) undergoes valence isomerization to give 1-methoxybicyclo[ 3. ( 5 ) and a norbornenone derivative in neat FS03H at -75 "C, which were only spectroscopically observed in situ as protonated species. 4 We show now that on complexation with acids in CH3CN the photochemistry of tropone shifts from dimerizations to valence photoisomerization giving a single product (Scheme 2), while with 2-methoxytropone there is a change in the course of the valence photoisomerization from product (2) to Thus, irradiation with an immersion 125 W Pyrex-filtered medium-pressure Hg lamp of a solution of 0.58 mmol of tropone-boron trifluoride monohydrate adducts in CH3CN (130 mi) for 30 min up to 80% conversion, followed by evaporation and silica-gel TLC with 3 : 2 Et20/petroleum ether, gave bicyclo[3.2.O]hepta-3,6-dien-2-one6 (5) (0 -27 mmol, 59%, RF 0.5).…”