Regioselective photocyclization of (e)‐2‐(2,3,6‐trichlorostyryl)‐benzothiazole and synthesis of 3,4‐dichloro‐ and 4‐chlorobenzothiazolo[3,2‐α]quinolinium chlorides: A synthetic and theoretical study

A high degree of regioselectivity is observed in the photochemically induced cyclization of (E)‐2‐(2,3,6‐trichlorostyryl)benzothiazole ( 1a ). According to the proposed mechanism, this compound was expected to afford two products, 3,4‐dichloro‐ and 1,4‐dichlorobenzothiazolo[3,2‐α]quinolinium chlorides ( 4a and 4a' , respectively). However, this reaction produced 3,4‐dichlorobenzazolo[3,2‐α]quinolinium chloride ( 4a ) as the sole product. On the other hand, irradiation of (E)‐2‐(2,3,5‐trichlorostyryl)benzothiazole ( 1b ) failed to produce the expected 1,3‐dichlorobenzothiazolo[3,2‐α]quinolinium chloride ( 4b ). Furthermore, (E)‐2,3‐difluorostrylbenzothiazole ( 1c ) also failed to give the corresponding 1‐fluorobenzothiazolo[3,2‐α]quinolinium fluoride ( 4c ). Interestingly, the irradiation of 2,6‐dichlorostyrylbenzothialole ( 1d ) produced 4‐chlorobenzothiazolo[3,2‐α]quinolinium chloride ( 4d ) in excellent yield. This paper presents the results of these investigations and a mechanistic rationale for the outcome of this reaction based on steric arguments and theoretical studies using a combination of molecular mechanics (MM+) and semiempirical quantum mechanical calculations (PM3/RHF/CI). Two‐dimensional high field nmr methods were employed to make complete assignments of the proton and carbon spectra of all new compounds.