Synthesis of 4,6-dimethyl-tetrahydro- and hexahydro-dibenzothiophene

Synthesis of 4,6‐Dimethyldibenzothiophene and 1,2,3,4‐Tetrahydro‐4,6‐dimethyldibenzothiophene via Tilak Annulation

“…After evaporation, 6a (2.4 g, 75%; cis/trans-mixture) was obtained. This procedure was repeated five times to obtain 12 g of product [8].…”

“…Analogously, one can react 4 with 2-halo-3-methylcyclohexanone to give 3-methyl-2-[(2-methylphenyl)sulfanyl]cyclohexanone (5a) and let this molecule undergo Tilak annulation to form 2a (Scheme 1), as shown by some of us in a preliminary communication [8]. Aoyama et al demonstrated that one could even combine the base-catalyzed coupling of the a-halo ketone with the arenethiol and the subsequent acid-catalyzed Tilak cyclization, when the base and acid are supported on separate supports, e.g., Na 2 CO 3 / SiO 2 and PPA/SiO 2 [9].…”

“…We investigated the use of cyclohex-2-en-1-one (9), because it could give 6a in only two steps (Scheme 2 [8]). The first step, the bromination of 9, and the final step, the hydrolysis of the AlÀO bond, proved to be easy.…”

“…The first step, the bromination of 9, and the final step, the hydrolysis of the AlÀO bond, proved to be easy. However, the conjugate addition of Me 3 Al to 2-bromocyclohex-2-en-1-one (10) [11] required safety precautions, and, therefore, only small batches (2 g) at a time could be prepared [8]. With the thus obtained 6a (cis/trans-mixture) and 4, the synthesis of 2a presented no problem (Scheme 1).…”

“…We prepared 7 by bromination of 3-methylcyclohex-2-en-1-one (11; Scheme 3), extraction with Et 2 O, and chromatographic purification on a silica gel column. Hydrogenation of 7 might, in principle, give 6a, which could be used for the synthesis of 2a [8]. However, when halogen atoms are present in a,bunsaturated aldehydes and ketones, the hydrogenolysis of the CÀX bond is faster than the hydrogenation of the C¼C bond.…”