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Stereoselective isomerisations of 4-(2′,5′-dimethoxyphenyl)-2,5-dimethyl-1,3-dioxolanes and their 2′-chloro-5′-methoxyphenyl analogues. Temperature-dependent diastereoselective formation of isochromanes

Giles, R.G.F., Rickards, R.W. and Senanayake, B.S. (1997) Stereoselective isomerisations of 4-(2′,5′-dimethoxyphenyl)-2,5-dimethyl-1,3-dioxolanes and their 2′-chloro-5′-methoxyphenyl analogues. Temperature-dependent diastereoselective formation of isochromanes. Journal of the Chemical Society, Perkin Transactions 1 (22). pp. 3361-3370.

Link to Published Version: http://dx.doi.org/10.1039/A704543C
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Abstract

Stereoselective isomerisations of rel-(2R,4S,5R)-4-(2′,5′-dimethoxyphenyl)-2,5-dimethyl-1,3-dioxolane 1 and the 2∶1 epimeric mixture of rel-(2S,4R,5R)- and rel-(2R,4R,5R)-4-(2′,5′-dimethoxyphenyl)-2,5-dimethyl-1,3-dioxolanes 2 and 3 with titanium tetrachloride at –78 °C afford rel-(1R,3R,4S)- and rel-(1S,3R,4R)-4-hydroxy-5,8-dimethoxy-1,3-dimethylisochromanes 25 and 30, respectively. The yields are only moderate owing to the competing influence of the 2′-methoxy group in the starting dioxolanes, and are improved significantly when this group is replaced by a 2′-chloro substituent. rel-(2R,4S,5R)-4-(2′-Chloro-5′-methoxyphenyl)-2,5-dimethyl-1,3-dioxolane 13 under similar conditions is isomerised smoothly to rel-(1R,3R,4S)-5-chloro-4-hydroxy-8-methoxy-1,3-dimethylisochromane 34 as the sole reaction product. In contrast the C-2 epimers rel-(2R,4R,5R)- and rel-(2S,4R,5R)-4-(2′-chloro-5′-methoxyphenyl)-2,5-dimethyl-1,3-dioxolanes 14 and 15 each favour the formation of rel-(1R,3R,4R)-5-chloro-4-hydroxy-8-methoxy-1,3-dimethylisochromane 36 at –78 °C, while at –95 °C the 1S product 35 predominates. Dioxolane 14 isomerises more rapidly than its C-2 epimer 15, and both these reactions are under kinetic not thermodynamic control.

Publication Type: Journal Article
Murdoch Affiliation: School of Mathematical and Physical Sciences
Publisher: Royal Society of Chemistry
Copyright: © 1997 Royal Society of Chemistry
URI: http://researchrepository.murdoch.edu.au/id/eprint/35541
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