Contributions to benzothiopyran chemistry

Abstract

A series of 2,3-dihydro-4H-[1]benzothiopyran-4-ones has been prepared by the cyclocondensation of a thiophenol with cz,13-unsaturated carboxylic acids in methanesuiphonic acid and selectively oxidised to their 1-oxides and 1,1-dioxides. Reduction and dehydration of the resulting benzothiopyran-4-ols gave the 2H-[1]benzothiopyrans and their 1-oxides and 1,1-dioxides, although the 1- oxides were more conveniently obtained by oxidation of the 2H- [1]benzothiopyrans.
The influence of the oxidation state of the sulphur heteroatom on the reactivity of the thiopyran ring has been studied. Based on hydroboration studies and the behavior towards triazolinediones, 2H-[1]benzothiopyrans are more reactive than their oxidised counterparts.
Novel 2,3-dihydro-4H-[ 1 ]benzothiopyran-3 ,4-diones have been synthesised and their chemistry investigated. Although in some instances the two carbonyl functions exhibit comparable reactivity, reaction with phosphorus ylides occurs regiospecifically at the 3-position. The use of excess ylide promoted an a-ketol rearrangement and gave a stabilised ylide which has been characterised by Xray crystallography.
2,3-Dihydro-4H-[1]benzothiopyran-3-ones, the synthesis of which has been greatly improved, have been converted into their 4-dimethylaminomethylene derivatives and these together with the 3,4-diones and the 2,3-dihydro-3-(hydroxymethylene)-4H-[l]benzothiopyran-4-ones have been used as biselectrophiles to prepare a range of 3,4-heteroannulated [l]benzothiopyrans.
The new thiopyrano[2,3-c][ljbenzothiopyran ring system has been obtained by a one-pot thionation-hetero Diels-Alder reaction sequence from 2,3-dihydro-4-(dimethylaminomethylene)-4H-[1]benzothiopyran-3-one and a43-unsaturated
aldehydes. The linear thiopyrano[3,2-b][l]benzothiopyran system has been similarly prepared from the 2-dimethylaminomethylene analogue. 6H, 1 2H-[ 1 ]Benzothiopyrano[3,4-b][ 1]benzopyran- 12-ones have been
synthesised by 2-fluorobenzoylation of 3-pyrrolidino-2H-[ 1]benzothiopyTan and intramolecular cyclisation of the resulting diketones. Selective reduction of the a43-unsaturated carbonyl function has been achieved, providing a facile access to 5-thiorotenoids. Attemps to produce these thiorotenoids by the thermal condensation of ethoxycarbonyl derivatives of benzothiopyran-3-ones with 4- methoxyphenol failed, but gave instead the novel bis[lbenzothiopyrano[3,2- b:4,3-e]pyran ring system.