Electrophilic Cyclisation of Bis(4-methoxybenzylthio)acetylene − Competition Between Ar2-6 and Ar1-5 Routes, Yielding 1H-2-Benzothiopyrans or Spiro Derivatives of Cyclohexadienone

Abstract

Treatment of bis(4‐methoxybenzylthio)acetylene (1) with iodine monochloride yields different products in the presence or absence of nucleophiles such as water or alcohols. Normally, the electrophilic cyclisation of bis(benzylthio)acetylenes produces 1H‐2‐benzothiopyrans 2 by intramolecular ortho attack on the aromatic ring by a vinyl cation formed in situ (Ar2‐6 cyclisation). In the case of 1, however, the high electron density in the ipso position of the aromatic ring favours ipso attack (Ar1‐5 route). The fate of the ipso σ complex is determined by the presence or absence of nucleophiles in the reaction medium. When nucleophiles are excluded, the σ complex is stabilised by 1,2‐migration and formation of 1H‐2‐benzothiopyran 2a. In the presence of water, the σ complex yields spirocyclohexadienone dihydrothiophenes 3a and 3d. In the presence of 3‐methylbenzyl alcohol, the methoxy substituent of the 2‐benzothiopyran ring is exchanged by the 3‐methylbenzyloxy group in product 2d. These findings are consistent with the formation of 2a, 2c, 2d and 2f by ipso − and not ortho − attack on the 4‐methoxyphenyl ring. Similar results were obtained both with ICl and from a proton‐induced cyclisation. In one‐pot syntheses, 3a and 3d were transformed into 2‐benzothiopyrylium salts 4a and 4b by tritylium tetrafluoroborate, and 3a and 3b were rearranged into the 6‐hydroxy‐substituted 2‐benzothiopyrans 2b and 2g by proton catalysis. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Iodocyclization of N-Aryl-3-phenylpropiolamides by I2/CAN: A Convenient Route for the Selective Synthesis of Quinolin-2-ones

“…4 In this report, interestingly the aryl ring bearing active para-substituents (OMe), N-4-methoxyphenyl perfluoroalkyl propargyl imines showed selective ipso-cyclization to an azaspiro compound in excellent yield [Scheme 1 (A)]. Indeed, the same results were reported by the groups of Fanghanel, 5 Larock, 6 and Li 7 for the iodocyclization of N-(para-substituted aryl)propiolamides to ipso-cyclized azaspiro compounds using iodine and iodine monochloride as electrophiles. Thus, it was generalized that the ipso-cyclization can be achieved from arylalkynes bearing active para-substituents, such as methoxy and N,N-dimethylamino groups.…”

“…J = 7.55, 1.7 Hz, 2 H), 6.95-6.90 (m, 2 H), 4.33 (t, J = 5.85, 2 H), 8.04 (t, J = 6.23 Hz, 2 H), 2.24-2.14 (d, J = 6.04 Hz, 2 H) 13. C NMR (75 5. MHz): d = 158.52, 156.07, 141.36, 136.17, 130.22, 128.59, 128.40, 128.26, 126.73, 124.77, 121.82, 121.24, 101.09, 44.47, 27.66, 20.77.…”

Construction of Alkenyl‐Functionalized Spirocarbocyclic Scaffolds from Alkyne‐Containing Phenol‐Based Biaryls via Sequential Iodine‐Induced Cyclization/Dearomatization and Pd‐Catalyzed Coupling of N‐Tosylhydrazones

“…[ 12 ] To design a convenient sequential reaction, we considered to use iodine as the initiator to promote a cyclization reaction of alkyne‐containing phenol‐based biaryls, which could afford iodinated spirocyclopentyl cyclohexadienone product (Scheme 1f). Therefore, as a continuation of our interest in developing transition‐metal‐catalyzed cascade reactions, [11,14] herein, we wish to report a highly efficient sequential process for the construction of iodinated spirocyclopentyl cyclohexadienone derivatives from alkyne‐containing phenol‐based biaryls in the presence of iodine, [ 13 ] which subsequently coupled with N ‐tosylhydrazones [15] catalyzed by palladium to afford the alkenyl‐functionalized spirocarbocyclic scaffolds (Scheme 1f).…”

Dearomatization through Halofunctionalization Reactions

“…In 2003, Fanghänel and co‐workers reported an example of dearomatization through halogenation when they studied electrophilic cyclization of bis( p ‐methoxybenzylthio)acetylene 1 (Scheme ) . In a basic aqueous solution, iodination of the alkyne moiety by ICl and subsequent ipso electrophilic attack of the 4‐methoxybenzylthio moiety at the iodonium intermediate afforded spiro cyclohexadienone 2 in 72 % yield.…”