Wittig-type condensation

Subsequent synthesis of Vitamin D metaboUtes kivolved oxidative degradation of the vitamin D molecule to obtain the C- and D-ring portion with the kitact side chain. Recombkiation of this molecule with an appropriate stmcture containing the A-ring was then carried out by a Wittig-type condensation. 

Another approach to higher carbon sugars is based on the Wittig-type condensation between two properly activated sugar sub-units. The... 

The dodecapentanedioic acid moiety was prepared by a double Wittig type condensation of octatrienedial, followed by amidation of one of the carboxy groups with 2-amino-3-hydroxy-cyclopenten-2-one. Final condensation with the peptolide afforded enopeptin B (63) in 8% yield (Scheme 37). Other similar fragment strategies also gave poor yields. 

A new method for Pd-catalyzed aza-Wittig-type condensation of isoxazol-5(4fd)-ones with aldehydes has been reported. For instance, decarboxylation/ring-opening of isoxazolone 33 furnished variously substituted 2-azabuta-l,3-dienes 34 (14CEJ1490). 

A second example of step-growth polycondensations with formation of the ole-finic double-bond are Wittig- and Wittig-Horner-type condensations. The Wittig-type polycondensations involve AA/BB-type reactions of aromatic bisal-dehydes with bisphosphonium ylides [99,100] with formation of PPV derivatives (75) and lead to products of only moderate molecular weight (DP 10-20). 

Recently, Meier et al. synthesized the most extended OP Vs (81) known to date [104]. Four different synthetic pathways were used for the generation of these OP Vs the final step involves the formation of one or two olefinic double bonds via a Wittig-Horner-type, a Siegrist-type or a McMurry-type condensation. 

The Orthogonal Approach (Combined Heck- and Wittig-Horner-Type Condensations) to PPM Oligomers... 

A tandem enzymatic aldol-intramolecular Homer-Wadsworth-Emmons reaction has been used in the synthesis of a cyclitol.310 The key steps are illustrated in Scheme 8.33. The phosphonate aldehyde was condensed with dihydroxyacetone phosphate (DHAP) in water with FDP aldolase to give the aldol adduct, which cyclizes with an intramolecular Horner-Wadsworth-Emmons reaction to give the cyclo-pentene product. The one-pot reaction takes place in aqueous solution at slightly acidic (pH 6.1-6.8) conditions. The aqueous Wittig-type reaction has also been investigated in DNA-templated synthesis.311... 

Reaction 39 shows the condensation of acylstannanes with primary amines. The only isomer produced was tentatively ascribed the Z-configuration291. Acylstannanes undergo Wittig-type processes with organophosphorus compounds to yield vinyl stannanes, as illustrated in reactions 40 and 41. In the latter reaction products are preferably of Z-configuration292. 

The synthesis of the mixed furan-thiophene-containing macrocycle dioxa-dithia[24]porphyrin-(2.2.2.2) 4.166 was reported by Strand, et al. in 1977. This was accomplished using Wittig-type chemistry, and involved the condensation reaction between 2,5-diformylfuran 4.163 and the thiophene-based phosphonium salt... 

In the late 1960s, as part of an effort devoted to the preparation of high-order annulene polyoxides, J. A. Elix attempted a Wittig-type self-condensation of 5-formyl-2-furfurylphosphonium chloride 1.19 While the products that resulted were predominantly polymeric, small quantities of macrocyclic products were isolated. These latter consisted of trimers, tetramers, and pentamers (discussed in Chapters 2, 4, and 6, respectively), as well as a hexamer. Unfortunately, this hex-americ [36]annulene hexoxide product represented by structure 7.80, was only obtained in 0.05% yield (Scheme 7.7.1). Thus, Elix was unable to elucidate the exact configurational arrangement of the double bonds in this product. Still, as discussed below, this issue could be addressed at least in part, by spectroscopic means. 

The UV-vis spectrum of 7.80 closely resembles those of the smaller pentaoxa[30]pentaphyrin-(2.2.2.2.2) isomers that were also isolated from the original Wittig-type self-condensation (see Chapter 6). It was found, however, that the hex-aoxa[36]hexaphyrin-(2.2.2.2.2.2) 7.80 displays absorbances in its UV-vis spectrum that are substantially red-shifted compared to those of pentaoxa[30]pentaphyrin-(2.2.2.2.2), or its smaller tetrameric and trimeric congeners (see Chapters 4 and 2, respectively). This was rationalized in terms of the greater n-conjugation present in 7.80. 

The Knoevenagel and Wittig type reactions have been used for the creation of the 6-bond. The condensation of octadecanal with methyl p-(chlorophenyl)sulfinyl acetate in the presence of piperidine afforded the corresponding acetate after acetylation of the hydroxy group which, after subsequent molibdenum-catalyzed elimination, gives the dienic ester as a 4 1 E,E E,Z mixture. After final amidation, the amide trichonine (13) [40] is obtained (Scheme 2). A similar strategy has been used for the synthesis of piperovatine (21) with total E,E-stereoselectivity [41]. 

Vilsmeier reaction to the chloroaldehyde (106), followed by reductive meth-ylation, yielded the aldehyde 107 (Scheme 14) with the precedented stereochemical outcome. Attempted halogenation of the derived neopentyl-type alcohol was unsuccessful under a variety of conditions, so an alternative scheme involving two successive Wittig-type reactions was developed. The aldehyde 107 was converted to the unsaturated aldehyde 108 by the method of Nagata and Hayase. Reduction with triethylsilane and tris (triphenylphosphine) rhodium chloride was followed by condensation with isopropylidene phosphorane to give the desired product 109. 

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