• K Rabindran

      Articles written in Proceedings – Section A

    • A new synthesis of thiophenes and thiapyrans - Part VII. Bromothionaphthenes and 5-nitrothionaphthene

      K Rabindran A V Sunthankar B D Tilak

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      5-Bromothionaphthene and 7-bromothionaphthene were prepared by the cyclization ofp-bromophenylω-dimethoxyethyl sulphide ando-bromophenylω-dimethoxyethyl sulphide respectively.

      The new synthesis of thionaphthene may also be extended to negatively substituted thionaphthenes as shown by the synthesis of 5-nitrothionaphthene (VI) by the ring-closure ofp-nitrophenylω-dimethoxyethyl sulphide (V).o-Nitrophenylω-dimethoxyethyl sulphide could not be cyclized by the usual method. 5-Aminothionaphthene andp-aminophenylω-dimethoxyethyl sulphide were obtained by the reduction of (VI) and (V) respectively.

    • Thiophenes and thiapyrans - Part VIII. A new synthesis of dibenzothiophene and its derivatives; its scope and limitations

      K Rabindran B D Tilak

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      A new synthesis of dibenzothiophene (I) which has been extended to its substituted derivatives has been developed. It consists in the condensation of thiophenol with 2-bromocyclohexanone to give 2-phenylmercaptocyclohexanone (II), cyclization of (II) with phosphorus pentoxide to 1:2:3:4-tetrahydrodibenzothiophene and selenium dehydrogenation of the latter compound to give (I) (overall yield 52%).

      Starting with suitably substituted thiophenols, 2-methyl-, 3-methyl and 4-methyldibenzothiophenes and 4-methoxydibenzothiophene have been synthesized. The synthesis of 2-methoxydibenzothiophene could not be completed due to the low yield of 2-methoxy-6:7:8:9-tetrahydrodibenzothiophene. Attempts to cyclize 2-(4′-nitrophenylmercapto)cyclohexanone by means of phosphorus pentoxide by the usual method were unsuccessful.

      The scope and limitations of the new synthesis are discussed.

    • Sulphur isosters of carcinogenic hydrocarbons - Part II. 6∶12-Dimethylbenzo (1∶2-b, 5∶4-b′) dithionaphthene and thionaphtheno (4∶5∶4′∶5′) thionaphthene

      V V Ghaisas K Rabindran B D Tilak

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      With the view to study the significance of the 9∶10-phenanthrene double bond (“phenanthrene bridge”) in 9∶10-dimethyl-1∶2∶7∶8-dibenzanthracene (II), 6∶12-dimethylebenzo (1∶2b, 5∶4-b′)dithionaphthene (V) was prepared where both the phenanthrene bridges in (II) are removed by isosteric replacement with thiophene rings and themeso-positions are blocked by methyl groups. Compound (V) should prove noncarcinogenic.

      2∶7-Dimethoxyethylmercaptonaphthalene (IX) on cyclization gave thionaphtheno(4∶5∶4′:5′-)thionaphthene (VIII). The constitution (VIII) follows from the favoured α-cyclization in the naphthalene series and the similarity of its absorption spectrum with that of 3∶4-benzphenanthrene.

    • Thiophenes and thiapyrans - Part IX. Studies in the dehydrogenation of 1∶2∶3∶4-tetrahydrodibenzothiophene

      K Rabindran B D Tilak

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      The dehydrogenation of 1∶2∶3∶4-tetrahydrodibenzothiophene (I) was studied by using various dehydrogenating agents in order to prepare dibenzothiophene (II) in optimum yield under mild conditions. Dehydrogenation by means of selenium or palladised carbon gave good yields of (II), but the reaction was carried out at high temperatures (>275°). By employing N-bromosuccinimide, the dehydrogenation could be carried out in a facile manner at lower temperatures (76°). These results are of value in the synthesis of derivatives of (II). Thus, whereas dehydrogenation of the chlorotetrahydrobenzothiafluorene (III) by treatment with selenium gave the benzothiafluorene (IV) by simultaneous dehydrogenation and dechlorination, dehydrogenation with N-bromosuccinimide gave the chlorine-containing benzothiafluorene (V).

    • Thiophenes and thiapyrans - Part X. 1∶2-benzo-9-thiafluorene and 3∶4-benzo-9-thiafluorene

      K Rabindran B D Tilak

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      The hitherto unknown angular benzo-9-thiafluorenes, 1∶2-benzo-9-thiafluorene (II) and 3∶4-benzo-9-thiafluorene (III) have been synthesized. The former was prepared starting from α-thionaphthol and 2-bromocyclohexanone. Condensation of these compounds gave 2-(α-naphthylmercapto) cyclohexanone (IV) which on cyclization and dehydrogenation gave (II). The constitution of (II) was proved by its unambiguous synthesis starting from 8-chloro-1-thionaphthol and 2-bromocyclohexanone. Starting from β-thionaphthol, (III) was prepared by similar steps. The constitution of (III) followed from its non-identity with 2∶3-benzo-9-thiafluorene.

    • Thiophenes and thiapyrans - Part XI. A new route to polycyclic thiophenes

      K Rabindran B D Tilak

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      Polycyclic thiophenes have been prepared starting from aryl thiols and 2-bromo-1-tetralone (1). Condensation of thiophenol with (I) gave 2-(phenylmercapto)-1-tetralone, which on cyclization gave 1:2-dihydro-3:4-benzo-9-thiafluorene. Dehydrogenation of the latter gave 3:4-benzo-9-thiafluorene (overall yield 72%). Starting fromβ-thionaphthol andα-thionaphthol, 3:4:5:6-benzo-9-thiafluorene (V) and 1:2:5:6-dibenzo-9-thiafluorene (VI) were synthesized similarly (overall yield, 78% and 76% respectively).

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