Theoretical Problem 20 covers several organic synthesis transformations and their mechanisms, includ โ Organic Chemistry Chemistry Question
Theoretical Problem 20
Theoretical Problem 20 covers several organic synthesis transformations and their mechanisms, including the Robinson annulation, the acid-catalyzed biomimetic cyclization building the steroid (gonane) skeleton, and the enzymatic stereoselective reduction of a prochiral alpha-halo ketone followed by basic cyclization to form enantiopure epoxides.
Suggest a mechanism for the following transformation (Robinson-anellation).
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Rationalise the build-up of the gonane skeleton.
[VISUAL]
A prochiral ฮฑ-chloro ketone was reduced to halohydrins enzimatically (R is an alkyl group). One alcohol dehydrogenase enzyme (Rhodococcus ruber) produces the R isomer, and another (Lactobacillus brevis) gives the S isomer. Under basic conditions enantiopure epoxides were produced from the halohydrins.
Give the stereostructures of the halohydrin intermediates and those of the epoxides. Detail the mechanism of the ring closure reaction.
[VISUAL]
Model Answer
The mechanism for the ring closure:
[VISUAL]
The reaction does not alter the configuration around the chiral center; thus the products are:
[VISUAL]