**10. Conclusion**

**9. Other transformations**

24 Alkenes

A wealth of further transformations have been found to be possible in combination with Wittig/HWE reactions. Thus, cyclopropanation of alkenes using sulfur-ylide reagent **166** can be run in tandem with a Wittig reaction with a conjugated phosphorane such as **21**. This combination of reactions can be performed with the preparation of the aldehyde as the Wittig sub-

Generally, non-stabilized phosphoranes are basic. This basicity has been used by Knüppel et al. in the transformation of α,α-dibromoenone **168** with excess methylenetriphenylphosphorane, where the phosphorane induces a Corey-Fuchs-reaction-type dehydrobromination/debromination to generate a terminal alkyne, which together with the concomitantly run Wittig-olefination delivers **169**, an intermediate to the trisnorsesquiterpene (−)-clavukerin A (**171**) (**Scheme 43**)

**Scheme 43.** Wittig-olefination—Corey-Fuchs-reaction-type dehydrobromination/debromination.

in one pot (**Scheme 42**) [177].

strate by oxidation of the corresponding alcohol **165** with MnO2

**Scheme 42.** One-pot oxidation—Wittig-olefination—cyclopropanation.

**Scheme 41.** Tandem Wittig-olefination—aza-Wittig-rearrangement.

Due to the fact that phosphoranes and phosphonates are stable under more diverse conditions than was initially realized, it has become possible to perform reaction cascades and one-pot reactions with Wittig- and HWE reactions as an integral part of the reaction sequence. Frequently, Wittig olefination reactions are carried out with *in situ* prepared phosphonium salts and phosphoranes [17, 22–27]. One-pot oxidation—Wittig olefination reactions are also quite common [40, 43–110], especially when the carbonyl component is labile [89, 97]. Often, the oxidant of choice is MnO2 [40, 43–46, 79–97], although a number of reactions are known where transformations were carried with air oxygen using metals and metal oxides as catalysts [72–78]. As many Wittig- and HWE reactions tolerate metal catalysts, this allows the running of Wittig/HWE reactions in combination with metal catalyzed cross coupling reactions and olefinations such as Heck [114–123], Suzuki [111–113], Sonogashira [119–121], and metathesis reactions [179–181]. The alkenes gained in the olefination reactions can be submitted to cycloaddition reactions, including Diels Alder reactions [69]. Furthermore, the alkenes lend themselves to 1,2-addition reactions [71, 140] in one-pot procedures. In cases where enones or enaldehydes are produced in the olefination reaction, a 1,4-addition becomes a possibility; this includes the Michael addition [149–151]. Also, the combination of ring opening of cyclic hemiacetals or acetals, olefination reaction and a 1,4-addition leading to ring closure is quite common [154–156]. The outcome of one-pot sequences of olefination reaction—electrocyclic rearrangement can be predicted less easily. Nevertheless, one-pot Wittig olefination—Claisen- [173], Wittig olefination—Cope- [169], and Wittig olefination—aza Wittig [176] rearrangement reactions have been published. Lastly, Wittig olefination and HWE reactions have been combined with functional group transformations, including the hydrolysis of an ester function [152] and the reduction of a carbonyl group [148].

The prospects of multi-step, one-pot reactions and reaction cascades incorporating Wittig reagents can be seen in the rich chemistry of ketenylidenetriphenylphosphorane (**178**) (**Scheme 45**) [182–185], which has been reviewed earlier [182, 186, 187]. Lastly, catalytic Wittig reactions can be seen as a subset of tandem reactions involving phosphoranes. Further research in specifically this area will help make the Wittig olefination more atom-economical and environmentally sustainable, so that this reliable alkene forming reaction will remain a competitive olefination strategy of choice.

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**Scheme 45.** Cascade reactions with ketenylidenetriphenylphosphorane (**178**).
