Alkene Inversion via Epoxides

The strict geometrical requirements for elimination can be put to further use, as illustrated by elegant procedures for the geometrical isomerization of alkenes. Trimethylsilyl potassium (10) and phenyldimethylsilyl lithium (11) both effect smooth conversion of oxiranes into alkenes, nucleophilic ring opening being followed by bond rotation and spontaneous syn fi-elimination  

To a solution of potassium methoxide (0.2 mmol) in HMPA (10 ml) (CAUTION—CANCER SUSPECT AGENT), heated to 65 °C, was added ( )-oct-4-ene oxide (1.2 mmol), then hexamethyldisilane (1.8 mmol) in HMPA (5 ml), and the yellow mixture was stirred at 65 °C for 3h. The cooled mixture was poured onto saturated brine, and extracted with pentane (2 X 25 ml). The combined organic extracts were dried and concentrated, to give oct-4-ene (1,15 mmol, 96%, 99 1 (Z) ( ) by g.l.c.). 

To ( )-stilbene oxide (25 mmol) in THF (35 ml) was added freshly prepared dimethylphenylsilyl lithium (25.35 mmol, 1.3 m in THF) dropwise at ambient temperature. The solution was stirred at ambient temperature for 4h, and then poured into saturated ammonium chloride solution (15 ml), diluted with ether, and the separated organic layer was dried and concentrated. The crude product (97 3 (Z) ( ), g.l.c.) was purified by chromatography on silica gel to give (Z)-stilbene (18.75 mmol, 75%). 

Lactams (azetidin-2-ones) can be employed usefully (12) in this alkene-forming procedure  

---

Compound 191 was transformed into the exo-alkene 193 via the respective spiro epoxide the enone 192 (11%) was obtained as a side product (Scheme 24).97 Compound 193 was deprotected, and the triol obtained was selectively mesylated at the allylic position to give, after acetylation, compound 194 (68%). Treatment of 194 with sodium acetate resulted in the inversion of configuration at C-l to give the tetra-N,O-acetyl derivative 195. Oxidation of 195 with osmium tetraoxide in aqueous acetone, followed by acetylation, afforded 196 (87%) and 197 (13%), whose acid hydrolysis provided the free bases 5 and 37, respectively. 

Inversion of Olefin Stereochemistry The preparation of alkenes via inversion of the double bond geometry is an important synthetic transformation. For example, interconversion of the (Z)-alkene to the (ff-isomer depicted below involves treatment of the (Z)-epoxide with the nucleophilic LiPPh2 followed by phosphorus alkylation to furnish the betaine,which undergoes 5yn-elimination to produce the (Ef-alkene. The alkene inversion works for di-, tri-, and tetra-substituted olefins. 

The topic of outer sphere reductive oxy-de-metallation is also prominent in work on the difunctionalization of alkenes [26]. The reaction of a dinuclear Pt(III) complex with olefin and water gave a hydroxy-alkyl complex next, the dinuclear metal unit serves as an electron sink and leaving group in an oxy-de-metallation at carbon, presumably via double inversion and an epoxide intermediate (Scheme 7) [26]. The process currently remains a stoichiometric oxidation. 

The proposed catalytic cycle starts by coordination of rhodium to both the oxygen and alkene component of the vinyl epoxide, forming complex 69, followed by oxidative insertion of rhodium into the allylic carbon-oxygen bond with retention of stereochemistry (Scheme 10.28). The initially formed Rh species 70 undergoes isomerization via the Jt-allyl Rh " complex 71 to the less strained species 72. Subsequent intermolecular nucleophilic Sn2 displacement with inversion affords the observed product 73 with concomitant release of Rh . 

---