Investigations of benzothienopyrans and related compounds

Abstract

The synthesis of isomeric benzothienopyran-4-ones has been investigated. 2-Methoxybenzo[b}thiophene has been used to prepare 2,2-dimethyl-2,3-dihydrobenzothieno[2,3-b] pyran-4-one via acylation with 3,3-dimethylacryloyl chloride.
2-Acetyl-3-hydroxybenzo[bjthiophene, readily available in a single step from thiosalicylic acid, has been used to obtain a range of 2,2-disubstituted 2,3-dihydrobenzothieno[3,2-bjpyran-4-ones by direct condensation with an aliphatic or alicyclic
ketone under pyrrolidine catalysis. A greatly improved route was developed which relied upon formation of the dianion of 2-acetyl-3-hydroxybenzo[b]thiophene. The dianion was intercepted with a wide range of ketones to give the corresponding
/3-hydroxy ketones. Cyclisation to the benzothieno[3,2-b]pyranones results on treatment with acid.
Reaction of the dianion with tetraphenylcyclopentadienone gave a fulvene derivative that was resistant to cyclisation. With 2,5-diethyl-3,4-diphenylcyclopentadienone, stereospecific 1,6-addition occurred. The structure of the product was
elucidated using X-ray crystallography.
Reduction of the benzothienopyranones follows the expected course, as does their reaction with Grignard reagents. However, with PhMgBr, 2,2-diphenyl-3-methyl-2,3-dihydrobenzothieno[3,2-b]pyran-4-one underwent a novel reaction sequence to give a 3-phenylbenzo[b]thiophene derivative, the structure of which was established by X-ray crystallography. The benzothienopyranones are readily cleaved by alkoxide under mild conditions. Reactions of the resulting 2-alkenoyl-3-
hydroxybenzo[b] thiophenes with hydrazine derivatives afford novel pyrazolidines. The preparation of 2,2-disubstituted benzothieno[2,3-b]- and [3,2-b]- pyrans has been investigated. The [2,3-b] isomers exist preferentially as their dienone
valence tautomers. In the latter series, the pyran form predominates and the compounds exhibit photochromism providing that an aryl substituent is present at C-4.
Attempts to effect acid catalysed dehydration of the benzothieno[3,2-b]pyran-4-ols failed. However, treatment with N,N-dimethylcarbamyl chloride effects a novel acylative ring cleavage to give 2-(alka- 1 ,3-dien- I -yl)-3(dimethylaminocarbamyloxy) benzo[b]thiophenes. Electrocyclisation of these carbamates has been accomplished and developed as a new approach to dibenzothiophenes and fused-ring derivatives.
The synthesis of chroman-4-ones possessing C-2 spiro-annulated 9-(9Hfluorene), 1 '-indane, 3'-(2,3-dihydrobenzofuran), and 3'-(2,3-dihydrobenzo-[b]thiophene) units has been accomplished and their oxidative ring expansion by organoiodine(Ill) reagents e.g. PhI(OH)OTs has been investigated. The indane derivative afforded 1 2H-benzo[a]xanthen- 12-one and its 5,6-dihydro derivative together with 5,6-dihydro-7H-benzo[c]xanthen-7-one, whilst the benzofuran affords a ring cleaved product together with the tetracyclic rotenoid core, rotoxen-12(611)-one.
Conjugate reduction of these systems provided both the cis and trans tetrahydrobenzoxanthones, and the trans-B/C fused rotenoid, respectively. Treatment of the spiro benzo[b]thiophene derivative with PhI(OH)OTs in methanol initiated a novel Pummerer-type reaction and rearrangement products were not observed.