Wittig-type methodology

Synthesis of Higher-Carbon Sugars by the Wittig-Type Methodology... 

Preparation of an HCS by a Wittig-type methodology and subsequent standard transformations into a linear derivative should give the di-olefin ready for the RCM cyclization. 

We initiated our synthesis from known C12 alcohol 93, readily obtained by a Wittig-type methodology as shown in Figure 10.31 [41], Our strategy required hydrolysis of the methyl glycoside followed by the reduction of the resulting hemiacetal to triol 94a further standard transformations should provide diol 94b. 

In the preparation of 155, a Wittig-type reaction of 159 to 160 served to incorporate carbons needed to build the tetracyclic system of artemisinin. As seen in Scheme 12, we took a portion of diene 158 and bypassed the introduction of any other carbons. After sequential deprotection of 163 via 164 and 165, closure to a new, more compact tetracyclic peroxide 156 was accomplished with our existing methodology. 

Synthesis of o ,jS-Unsaturated Aldehydes. Vinyltrimethylsilane undergoes [3 + 2] cycloaddition reactions with the aldehyde-derived nitrone (7) to provide the corresponding (trimethylsi-lyl)isoxazolidine adduct (8) in 84% yield (eq 5). Treatment of (8) with aqueous HE furnishes the a,/S-unsaturated aldehyde (9) in 95% yield. This methodology is general for the homologation of aldehydes and serves as an alternative to the traditional Wittig-type aUcenations. 

These types of compounds can also be formed via a Wittig reaction [19]. Triphenylphosphine can be quatemized with a suitable alkyl iodide, and the resultant perfluoroalkylated phosphonium salt will react with aldehydes to give fluorinated alkenes which are easily hydrogenated (Scheme 3.4). This methodology has recently been expanded to the formation of perfluoroalkylated pyridines [20],... 

Because Michael-type reactions are among the most useful base-mediated cyclizations, the first examples presented in this chapter center around this chemistry. Thus, utilizing Wittig methodology, Vyplel et al. [37] prepared the hydroxyolefin (shown in Scheme 7.93) from the starting protected glucosamine. Treatment of this intermediate species with DBU effected cyclization of the 5-hydroxy group to the unsaturated ester in a Michael fashion. Notably, the cyclization provided the a anomer in 43% yield, with an additional 7% accounted for in the isolation of the (3 anomer. 

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