Wittig synthesis, olefinated

The synthesis in Scheme 13.21 starts with a lactone that is available in enantiomer-ically pure form. It was first subjected to an enolate alkylation that was stereocontrolled by the convex shape of the cis ring junction (Step A). A stereospecific Pd-mediated allylic substitution followed by LiAlH4 reduction generated the first key intermediate (Step B). This compound was oxidized with NaI04, converted to the methyl ester, and subjected to a base-catalyzed conjugation. After oxidation of the primary alcohol to an aldehyde, a Wittig-Horner olefination completed the side chain. 

Reviews have featured epoxidation, cyclopropanation, aziridination, olefination, and rearrangement reactions of asymmetric ylides 66 non-phosphorus stabilized carbanions in alkene synthesis 67 phosphorus ylides and related compounds 68 the Wittig reaction 69,70 and [2,3]-Wittig rearrangement of a-phosphonylated sulfonium and ammonium ylides.71 Reactions of carbanions with electrophilic reagents, including alkylation and Wittig-Homer olefination reactions, have been discussed with reference to Hammett per correlations.72... 

In many cases, the second side chain of the cyclopentane ring of prostaglandins possesses a double bond with ( )-geometry at C-13. The introduction of this side chain is usually achieved by olefination with phosphonate anions according to the Homer technique. However, because of the vicinal oxygen function at C-15 it is also recommended to introduce the side chain via ( )-stereoselective Wittig synthesis... 

The same synthetic approach starting from imidazolemethanol and via Wittig-Schweizer olefination, which provided an improved synthesis of oroidin, was used... 

The Corey-Winter olefin synthesis has been performed on p-lactam derivatives.18 This olefination was employed in cases where traditional Wittig-type olefination of the corresponding aldehyde led to disappointing results. In one example, treatment of 40 with carbonyldiimidazole and exposure of the resulting thionocarbonate to P(OMe)3 afforded olefin 41 in 81% yield. 

The most important coupling reaction for the synthesis of carotenoids is the Wittig reaction or Wittig-carbonyl olefination or Wittig condensation in which a phosphorus ylide 3 reacts with a carbonyl compound 4 to give an olefin 5 and triphenylphosphine oxide (6) Scheme 1). 

Wittig phosphonate olefin synthesis. The phosphonate synthesis of a, 3-unsaturated nitriles and esters can be carried out by addition of both substrates simultaneously to a two-phase system of methylene chloride and 50% aqueous... 

A concise synthesis of substituted stUbenes from propargylic phosphonium salts by a cobalt-catalyzed Diels-Alder/Wittig olefination reaction has been described (Figure 1.4) [37]. It was shown that the cobalt(I)Diels-Alder reaction of propargylic phosphonium salts and alkyne-fimctionalized phosphonium salts with 1,3-dienes led to dihydroaromatic phosphonium salt intermediates that were directly used in a one-pot Wittig-type olefination reaction with aldehydes. Subsequent oxidation led to styrene- and stUbene-type products with the formation of three new carbon-carbon bonds. The reaction gives predominantly the Econfigured products. 

Secondly, Wittig-type olefination involving a-halomethyltri-phenylphosphoranes can afford the desired alkenyl halides in moderate to good yields with good stereoselectivities favoring the Z-isomer (eq 4). However, phosphonium salts suffer from tedious preparation and the triphenylphosphine oxide by-product can be hard to remove from the final product. The >Z-selectivity can also be poor, as observed in the Cassiol total synthesis (eq 5). ... 

Interestingly, a phosphine oxide was used as a reagent in a Wittig-Homer olefination aiming at the synthesis of the key intermediate of vitamin D3... 

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