Wittig reagents asymmetric

In a second synthesis of LTB4, Corey s group made use of the excellent method of asymmetric epoxidation developed by Sharpless and co-workers to obtain an optically active epoxy alcohol (Scheme 3.11). In this sequence, the Wittig reagent 27 seems to condense much more cleanly with (S)-21a than did 24 (Scheme 3.10). The key and very elegant step in the synthesis was the hydrolysis and rearrangement of the intermediate 28, which undergoes a very... 

The C2-symmetric cyclohexane 102 that carries two thiourea groups induces the asymmetric coupling of a triphenylphosphoranylacetic ester with aldimines to give stabilized Wittig reagents containing a chirality center. ... 

Lithiated phosphine oxides, the so-called Horner-Wittig reagents, are important intermediates in organic syntheses, for example in stereoselective syntheses of alkenes and asymmetric syntheses of chiral compounds [61, 62], Despite the importance of these reagents very little is known about their structures. The first lithiated phosphine oxide containing Li-C bonds has been reported recently. The self-assembled... 

A multiple one-pot iterative asymmetric hydroformylation-Wittig olefination was realized with a single catalyst loading (Scheme 5.137) [26]. Noteworthy, after each run, depressurization and injection of the allyl-substituted Wittig reagent was necessary in order to avoid the undesired hydroformylation of the Wittig ylide. In this manner, the dimer and the trimer of 4-hydroxyvalerate were prepared. 

Different types of the reagents (see Fig. 8-4) have been applied in asymmetric Wittig-type reactions. Because no new sp3 stereocenter is formed in a Witting-type reaction, a substrate containing a stereogenic or pro-stereogenic unit apart from the carbonyl group is usually required to induce an asymmetric process. 

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... 

Intermolecular aza-Wittig reaction has been described for the one-step synthesis of pyrazolofl, 5-a Ipyrimidinc and imidazo[l,2-fc]pyrazole derivatives.37 The asymmetric synthesis of ft-quaternary azacycles has been accomplished by aza-Wittig reaction mediated by chiral phosphorus(ni) reagents.38... 

Single-carbon elongation can also be achieved with the use of organometallic reagents (MeMgl, McjZnATiCl [22], sulfonium ylides [23], or by Wittig olefination, followed by asymmetric epoxidation [24],... 

Rehwinkel, H., Skupsch, J., and Vorbrtiggen, H., E- or Z-selective Homer-Wittig reactions of suhsti-tuted bicyclo[3.3.0]octane-3-ones with chiral phosphonoacetates. Tetrahedron Lett., 29, 1775, 1988. Tiillis, J.S., Vares, L., Kann, N., Norrby, P.-O., and Rein, T., Reagent control of geometric selectivity and enantiotopic group preference in asymmetric Horner-Wadsworth-Emmons reactions with meso-dialdehydes, J. Org. Chem., 63, 8284, 1998. 

Then came a nasty surprise. The ketone 82 is rather crowded and would not undergo a Wittig reaction with Ph3P=CH2, but Johnson turned this to advantage by using his asymmetric reagent 87 to resolve the compound and do the methylenation in one step. Optically active 88 can be isolated in 42% yield (out of a maximum of 50%) and gives natural (-)-panasinene 81. 

The sensitivity of the five-centered transition state of the [2,3] Wittig rearrangement to perturbation by steric or stereoelectronic factors presents an enhanced opportunity for substrate-, auxiliary- and/or reagent-induced asymmetric induction. For selected [2.3] Wittig substrates. 

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