Peterson alkenation oxidation

Unlike the Peterson alkenation, which is in principle similar, the phosphine oxide anion addition can be controlled to produce predominantly the erthyro isomer (206). The threo isomer can be obtained by selective reduction of the a-ketophosphine oxide (210), allowing highly stereoselective alkene fonna-tion. Since a two-step sequence is employed, this reaction does not require a stabilizing functionality to be conjugated to the phosphine oxide in order to produce the alkene. In fact, unlike the phosphonate HWE reagents, the reaction of a ketophosphine oxide (211) with a carbonyl derivative does not occur to produce the unsaturated carbonyl (213 Scheme 30). ° The addition step is presumably too rapidly reversible and the elimination of phosphine oxide too slow. 

In analogy to the Peterson alkenation, the intermediate hydroxyphosphine oxidn (269) cm be prepared by addition to epoxide derivatives (268 Scheme 36). Overall yields are high for this process, and this sequence can be applied to the synthesis of phosphonate intermediates as well. Warren has studied hydroxy-directed epoxidation. Provided the allylic phosphine oxide is trisubstituted, as is (270) in equation (6S), these oxidations proceed with good selectivity. Ring opening can then be undertaken to generate the hyth-oxyphosphine oxide. 

Homo-Peterson Alkenation. Styrene oxide reacts with lithio-phenylthiobis(trimethylsilyl)methane to afford cyclopropane-containing products (eq 6)7 This reaction is limited, due to the complexity of its mechanism the alkenation reagent must serve to generate both alkenic and carbenic species. For this reason, only styrene oxide and trimethylsilyloxirane undergo this transformation. 

The heteroalkenes were prepared by Peterson-type alkenation of the corresponding aldehydes with bis(trimethylsilyl)phenylthiomethyllithium [PhS(Me3Si)2CLi] followed by oxidation to sulfones. 

The rest of the synthesis is straightforward but you should notice the catalytic Ru(III) oxidation used both on 65 and 67, the periodate cleavage to replace ozonolysis in the formation of 68 and the Peterson reaction (chapter 15) used to make the alkene 69. This method was used for the synthesis of quantities of both enantiomers of grandisol 34. 

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