Synthetic aspects of organosulphur chemistry
The thesis is concerned with approaches to substituted 2-phenyl- 1,3-oxathiolans and their cycloreversion to olefins. 2-(α-Methoxybenzylthio)acetophenone (l) was prepared by in situ alkylation of the thiolate generated by aminolysis of 0-ethyl-S-phenacyldithiocarbonate with α-chlorobenzyl methyl ether. Reduction of (l) with lithium aluminium hydride gave 2-(α-methoxybenzylthio)—1— phenylethanol which cyclised in the presence of p-toluenesulphonic acid to give 2,5-diphenyl-l,3-oxathiolan. α-(α'-Methoxybenzylthio)acetone (2), was prepared by a similar route to that used for (l). Compounds (1) and (2) and various other a-thiosubstituted ketones were investigated as potential starting materials for the synthesis of substituted β—(α-methoxybenzylthio)alcohols but the transformations attempted were unsuccessful. A reasonably flexible synthesis of substituted oxathiolans and hence the corresponding olefins was developed starting from α—(benzylthio)ketones. The olefins prepared were 2-methyl—3—phenyl—2— butene, 1,2-dimethylcyclohexene and the Z-(3) and E-(4) 3,4—dimethylhex— 3—enes. Alkylation of the a-(benzylthio)ketones proceeded regio— specifically α- to the thio and keto groups. Subsequent reaction with organometallic reagents gave β-benzylthioalcohols. Generally alkyllithiums gave the best yields and higher stereoselectivities in these additions. The benzylthio group was cleaved with sodium/ ammonia to give β-mercaptoalcohols which were condensed with benzaldehyde to give 2-phenyl-l,3-oxathiolans. Treatment of the oxathiolans with lithium diisopropylamide resulted in cycloreversion to olefins in high yields (75-100%). Stereochemical integrity was maintained throughout the reactions used to convert the β-benzylthioalcohols into olefins and consequently the stereoselectivity was determined at the β-benzylthioalcohol forming step. Thus the ratio found for (3) to (4) synthesised from a 3-benzylthioalcohol prepared by reaction of methyllithium with 3-benzylthio-3- methyl-4—hexanone was 3:7, whereas when the β-benzylthioalcohol was prepared from 3-benzylthio-3-methyl-2-pentanone by an ethyllithium reaction the ratio of (3) to (4) subsequently obtained was 6:4.