Synthesis of 1,7‐difunctionalized cycloheptatrienes. Complications from rapid [1,7] sigmatropic shifts of unhindered alkylthio groups

Abstract

Abstract. 4,5-Benzotropone and 1,2-ethanedithiol gave the expected dithioketal 8 with boron trifluoride etherate in methanol. In contrast, tropone, under similar conditions, gave the bicyclic 1,7-disubstituted cycloheptatrienes 2, 3 and 4 on reaction with, respectively, 1,2-ethanedithiol, 1,3-propanedithiol, and o-benzenedithiol. Related, monocyclic 1,7-difunctionalized cycloheptatrienes 10-13 and 15ad were obtained from sodium thiolates and methoxy-or I-(alky1thio)tropenylium salts in dichloromethane-ethanol at -50°C. 1,7-Bis(aIkylthio)-substituted cycloheptatrienes with unhindered alkylthio groups showed a rapid interconversion of the alkylthio groups at the C( 1) and C (7) carbons (15a s 16).Unsuccesful attempts to obtain tropone dithioketals, which were desired as reaction intermediates, led us to search for and discover simple entries to cycloheptatrienes 1,7-difunctionalized with alkoxy or alkylthio groups. The reactions involve either tropone itself with thiols or tropenylium salts with thiolates. The results are interesting because access to regiospecifically difunctionalized cycloheptatrienes has, so far, proved difficult. In fact, addition of bases to monosubstituted tropenylium ions is known to give a mixture of I ,7-, 2,7-, and 3,7-disubstituted isomeric cycloheptatrienes*. The only exception is the addition of methoxide to methoxytropenyhum ion at low temperature to give tropone dimethylaceta12. In Scheme 1 are reported the new products obtained in medium to low yields, from tropone 1 and dithiols under typical conditions for dithioacetalization of ketones3. The balance of materials consists of intractable red gums. Structures 2 4 , which are consistent with elemental, mass, and UV data, are fully supported by 'H NMR spectra. In fact, for both 2 and 3 we observed a multiplet for H(7) and

One hundred years of benzotropone chemistry

“…d. Ketalization of 4,5-benzotropone (11): Cavazza’s group reported their unsuccessful attempts to obtain tropone dithioketals such as 155 [ 132 ]. The treatment of 4,5-benzotropone ( 11 ) with 1,2-ethanedithiol and BF 3 .…”

Cyclopropacycloheptathiophenones and thiols: unexpected rearrangement with dithiols leading to benzo- and cyclo-octa-thiophenes. Spectroscopic and mechanistic studies

“…spectrum contains an SEt group signal, and a vinyl proton resonance but no cyclopropyl signal. The formation of compound (8) can be explained by means of a [1,4] addition of thiol to the protonated cyclopropacycloheptathiophenone. After proton abstraction, the Michael adduct tautomerizes into the ketone (8) (see Scheme 3).…”

“…These results have led us to propose a possible mechanism for the rearrangement of compound ( 6) (see Scheme 6). After protonation the cyclopropacyclohepta[c]thiophensne (6) undergoes a [ 1,4] addition with ethanedithiol, followed by prototropy. The Michael adduct (i) tautomerizes to a protonated ketone (ii) which undergoes a non-classical rearrangement, with nucleophilic sulphur attack at C-8 followed by C(7)-C(8) bond cleavage resulting from the electrophilic character of the oxonium.…”

“…7H-Benzocyclohepten-7-one (1) and ethane-1,Zdithiol give the expected dithioacetal (2), but cyclohepta-2,4,6-trienone (3) leads to the bicyclic system (4). 4 In a previous p~blication,~ we reported the synthesis of the mono-and di-cyclopropane compounds (6) and (7): the increased values of vco in both compounds (6) and (7) (1 655 and 1 683 cm-' respectively) as opposed to the 'anomalous' value, 1606 cm-', for (5) led us to investigate their behaviour with respect to thiols.…”

“…

Cyclopropa [3,4]cyclohepta[1,2-c]thiophen-5-one reacts in an acidic medium with thiols to give addition or rearrangement compounds: [l ,4] addition occurs with ethanethiol, but with 2mercaptoethanol isomerization of the cyclopropyl ring to a vinyl group occurs, and the thioacetal was not obtained. The most surprising results are observed with ethane-l,2-dithiol and propane-l,3dithiol, where a benzo [&]thiophene is isolated.

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