Abstract
The photochemical reaction between 3β-acetoxyandrost-5-en-17-one 4 or (–)-methone 5 and furan was preceded by isomerization of the ketones to the corresponding γδ-unsaturated aldehydes which subsequently added to furan furnishing 6-substituted 2,7-dioxabicyclo-[3.2.0]hept-3-enes, 8 and 9. Under the same conditions 1,2:5,6-di- O-isopropylidene α- d-ribo-hexofuranos-3-ulose 12 reacted with furan without any isomerization and afforded all four stereoisomeric bicyclic photoadducts 14–17. The ratio of the products resulting from the approach of the furan molecule to 12 from the less hindered and from the more hindered sides was 3:1. This pointed at the decisive role of steric factors in controlling the course of photochemical cycloaddition.
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