Homer reaction

The phosphorus ylides of the Wittig reaction can be replaced by trimethylsilylmethyl-carbanions (Peterson reaction). These silylated carbanions add to carbonyl groups and can easily be eliminated with base to give olefins. The only by-products are volatile silanols. They are more easily removed than the phosphine oxides or phosphates of the more conventional Wittig or Homer reactions (D.J. Peterson, 1968). 

The vitamin D3 metabolite la,25-dihydroxycholecalciferol is a lifesaving drug in treatment of defective bone formation due to renal failure. Retrosynthetic analysis (E.G. Baggjolint, 1982) revealed the obvious precursors shown below, a (2-cyclohexylideneethyl)diphenylphosphine oxide (A) and an octahydro-4f/-inden-4-one (B), to be connected in a Wittig-Homer reaction (cf. section 1.5). 

The phosphonates obtained by the Arbuzov reaction are starting materials for the Wittig-Homer reaction (Wittig reaction), for example, appropriate phosphonates have been used for the synthesis of vitamin A and its derivatives/... 

In the other approach, again harmalane (150) was treated with methyl 2-(di-ethylphosphono)acrylate (174), resulting in iminophosphonate 175. By its sodium borohydride reduction and subsequent lactonization, the amidophospho-nate 176 has been obtained, Wittig-Homer reaction of 176 with acetaldehyde followed by selective reduction of the carbonyl group of the enamide function supplied ( )-deplancheine in good yield (116). 

The total syntheses of these pepper alkaloids are not those of pyrrolidines but rather syntheses of their acid parts. Thus dihydrowisanidine (137) has been prepared by a series of reactions, the key step of which is the formation of the carbon-carbon double bond by a Wittig-Homer reaction (217, 218). Schemes 41 and 42 summarize two syntheses of okolasine from sesamolmethyl ether (279) of course, routes to okolasine also yield the corresponding piperidine alkaloid wisanine. Molybdenum-catalyzed elimination of allylic acetate (149) yielded (E,E)-diene ester 150 en route to trichonine (220) worthy of note is the use of an aluminum amide in the preparation of amide 143 from ester 150 (Scheme 43). 

Phosphonates and related compounds, which can subsequently be used in the Wittig-Homer reaction (see Section 6.5), are readily alkylated in good yield (Table 6.8) [67-71], Mono-alkylation is observed with mildly basic conditions at 45°C [67] and dialkylation under stronger basic conditions at 60°C [70], Reaction of a,to-dihaloalkanes with phosphonocarboxylates leads to cycloalkylphosphonates [72], The methylenebisphosphonate reacts in a similar manner. 

It should also be noted that a new reagent for trifluoroethylidenation, trifluor-oethyldiphenylphosphine, can be used in Homer reactions under TBAF activation... 

Derivatives of the general formula (7) in Table 6 have been successfully used as probases and their properties in this context are being further explored. In common with the azobenzenes and ethenetetracarboxylate esters, the fluoren-9-ylidene derivatives usually display two reversible one-electron peaks in cyclic voltammetric experiments. Although disproportionation is possible (cf. Scheme 12) it is the dianions which are the effective bases. It was shown early on that the radical-anions of such derivatives are long-lived in relatively acidic conditions (e.g. in DMF solution the first reduction peak of Ph C -.QCN) remains reversible in the presence of a 570-fold molar excess of acetic acid, at 0.1 V s ). Even the dianions are relatively weak bases, useful mainly for ylid formation from phosphonium and sulphonium salts (pKj s 11-15) they are not sufficiently basic to effect the Wittig-Homer reaction which involves deprotonation of phosphonate esters... 

Wit tig, Wittig-Homer, and Knoevenagel reactions These reactions can be carried out with A1203 or KF supported on A1203 without solvent, but addition of water catalyzes both Wittig and Wittig-Homer reactions. Under these conditions trimethylsulfonium iodide undergoes reaction to form epoxides (equation I). 

Wittig-Homer reaction with 1,4- and 1,5-dialdehydes.1 Reaction of these aldehydes with 1 equiv. of a Wittig-Horner phosphonate in an aqueous medium with K2C03 as base is accompanied by an intramolecular aldolization to provide five- or six-membered cycloalkenols. 

Use of the Wittig and Homer reactions with quinucIidin-3-one (2) and other j9-oxoquinuclidine derivatives afforded a variety of 3-substituted quinuclidines.127, 140 155-157 Examples are the syntheses of 3-methylene- (114), 3-ethylidene- (115),157 3-alkoxycarbonyl-methylene- (116),128 and 3-formyIquinuclidines (76)140 (Scheme 2). 

Widman-Stoermer synthesis, 3, 43 Wild-fire toxin, 7, 249 Willardiine, synthesis, 3, 146 Willgerodt reaction thiophene synthesis by, 4, 883 Williamson reaction oxetane synthesis by, 7, 390-391 Withasomnine occurrence, 5, 302 Wittig reaction crown ethers and, 7, 759 Wittig-Homer reactions crown ethers and, 7, 759 Wolff rearrangement oxirenes in, 7, 120, 126... 

Keywords inclusion complex, stereoselective Wittig-Homer reaction, carbeth-oxymethylene cyclohexane... 

Wittig-Homer reactions.3 The reaction of the phosphonate 2 with the aldehyde 1 under literature conditions gives the fraras-stilbene 3 in only about 10% yield. Addition of 15-crown-5 raises the yield to 45%. This step was used in a synthesis of pallescensin E (4), a furanosesquiterpene in a marine sponge. 

Moison, H., Texier-Boullet, F. and Foucaud, A. Knoevenagel, Wittig and Wittig-Homer reactions in the presence of magnesium oxide or zinc oxide. Tetrahedron, 1987, 43, 537-542. 

In addition to the Wittig- und Homer-Wadsworth-Emmons reactions, we know a third alkene-forming reaction between carbonyl and phosphororganic compounds, i.e. the Wittig-Homer reaction. In Section 11.2, you will learn that in the course of this reaction a. syn-elim-ination of Ph2P(=0)0 takes places, i.e. another / elimination of I let1/I let2. 

The Wittig-Homer reaction performed in a single operation, as described, is flawed by the lack of stereocontrol. That s because the phosphorylated lithium alkoxide D is formed without diastereoselectivity. So the alkene E is also formed without diastereoselectivity—via a syn anti-mixtme of the phosphorylated potassium alkoxides F and a trans. cis-mixture of the oxaphosphetane G, all of which are essentially 1 1 mixtures of diastereomers. It is generally not easy to separate a cis alkene from its trans isomer. 

Single alkene diastereomers are accessible through a Wittig-Homer reaction only if it is performed in two steps (Figure 11.10). A 1 1 mixture of the phosphorylated lithium alkoxides syn- and anti-D is still formed but if the mixture is protonated at this point, the resulting phosphorylated alcohol diastereomers C can usually be separated without difficulty. The suitable diastereomer will be deprotonated with potassium-ferf-butoxide in the second step and then be converted into the stereouniform trans- or cis-alkene E via stereospecific oxaphosphetane formation and fragmentation. 

---