Asymmetric epoxidation-ring expansion

This chapter begins by classifying the combinations of oxidation/reduction processes with subsequent cationic transformations, though to date the details of only two examples have been published. The first example comprises an asymmetric epoxidation/ring expansion domino process of aryl-substituted cyclopropyl-idenes (e. g., 7-1) to provide chiral cyclobutanones 7-3 via 7-2, which was first described by Fukumoto and coworkers (Scheme 7.1) [2]. 

Scheme 7.1. Domino asymmetric epoxidation/ring expansion reaction.

With compound 64 available, vanadyl acetylacetonate catalyzed epoxidation [44] accompanied by simultaneous cyclization, afforded the corresponding tetrahydrofuran and its diastereomer in a 4 1 ratio (Scheme 12). Ring expansion of the corresponding mesylate 65 with silver (I) carbonate afforded compound 66 in a 42% yield for the two steps [45]. Extension of the side chain in six steps, followed by an asymmetric epoxidation, gave product 67 stereoselectively. The cyclization of 67 with titanium tetraisopropoxide in a manner consistent with model studies [27d], afforded bicyclic ether 68 in 65% yield. Transformation to the epoxide under standard conditions afforded fragment 69 ready to be coupled with the D-ring side chain. 

The catalytic asymmetric synthesis of (2S,3S)-3-hydroxy-2-phenylpiperidine was developed by J. Lee et al. using an intramolecular epoxide opening 5-exo-tet) followed by ring expansion. The acyclic c/s-epoxide substrate was prepared in good yield and in greater than 94% ee by the Jacobsen epoxidation from the corresponding (Z)-alkene. ... 

Oxetans.—Stable oxetens are rare another example (56), this time stabilized by the pstfluoroalkyl effect , has been reported. The Paterno-Buchi reaction cannot be used for the synthesis of 2,2-dialkyl-oxetans from aliphatic ketones. A convenient, one-step conversion of such ketones into oxetans involves use of the sulphoximide (57) and proceeds via ring-expansion of an epoxide intermediate. Unusually high asymmetric induction has been observed in the Paterno-Buchi reaction of (-)-menthyl phenylglyoxylate with tetramethylethylene. The visi-... 

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