Wittig transform

Another example is a chiral olefinic alcohol, which is disconnected at the double bond by a refro-Wittig transform. In the resulting 4-hydroxypentanal we recognize again glutamic acid, if methods are available to convert regio- and stereoselectively... 

In the first step a Wittig transformation is performed. The a-position of the phosphonium salt 12 is C,H-acidic, and therefore the strong base NaHMDS abstracts a proton forming the labile ylide 38. This ylide is a carbon nucleophile, which attacks the ketone moiety of Segment B (7) forming the oxaphosphetane intermediates cw-39 and trans-39. The mode of action is thought to proceed via a [2+2]-cycloaddition. 

The Corey process is also useful for the synthesis of PGs of the 1 and 3 series. Catalytic hydrogenation of (34) (see Fig. 5) with 5% Pd/C at — 15-20°C results in selective reduction of the 5,6-double bond. Subsequent transformations analogous to those in Figure 5 lead to PGE (9) and PGF (10). The key step for synthesis of the PG series is the Wittig reaction of (29) with the appropriate unsaturated CO-chain yUde (170). 

The retron for the Claisen rearrangement transform (see above) is easily established by the application of a Wittig disconnection at each of the equivalent terminal double bonds of 57... 

Antithetic conversion of a TGT by molecular rearrangement into a symmetrical precursor with the possibility for disconnection into two identical molecules. This case can be illustrated by the application of the Wittig rearrangement transform which converts 139 to 140 or the pinacol rearrangement transform which changes spiro ketone 141 into diol 142. 

Nonclassical Wittig reaction, olefination by phosphoranes interaction with carboxylic acid derivatives, in transformation of heterocycles 99JCS(P1)3049. 

The aza-Wittig rearrangement in synthesis and transformations of N-hetero-cycles 97S497. 

Another agent of this general type is nalmefene (47) Despite their useful characteristics, opiates display tolerance, addiction, abuse, and some toxic side effects Antagonists combat some of these effects, most notably respiratory depression and addiction Nalmefene reputedly has significant oral activity as a narcotic antagonist The synthesis of nalmefine concludes by Wittig olefination of naltrexone (46) to nalmefene (47) This molecular transformation resulted in a significant increase in oral potency as well (141... 

The adjacent iodine and lactone groupings in 16 constitute the structural prerequisite, or retron, for the iodolactonization transform.15 It was anticipated that the action of iodine on unsaturated carboxylic acid 17 would induce iodolactonization16 to give iodo-lactone 16. The cis C20-C21 double bond in 17 provides a convenient opportunity for molecular simplification. In the synthetic direction, a Wittig reaction17 between the nonstabilized phosphorous ylide derived from 19 and aldehyde 18 could result in the formation of cis alkene 17. Enantiomerically pure (/ )-citronellic acid (20) and (+)-/ -hydroxyisobutyric acid (11) are readily available sources of chirality that could be converted in a straightforward manner into optically active building blocks 18 and 19, respectively. 

The lithium etiolate of acetaldehyde DMH has recently been utilized in the opening reaction of the ot-epoxide obtained by DM DO oxidation ofenol ether 142, to provide hemiacetal 143 after mild oxidative acid hydrolysis. The protected carbonyl functionality was subsequently used for the introduction of the trans enyne chain through a Wittig olefmation reaction to provide alcohol 144, which was then transformed into (+)-laurenyne (Scheme 8.37) [71]. 

A particularly interesting reaction is the C-formylation[2] of alkylidenephosphoranes by A-formylimidazole to give a-formyl derivatives (C6H5)3P=C(R)CHO which can be further transformed into a,/ -unsaturated aldehydes by Wittig reaction, or hydrolyzed to produce aldehydes RCH2-CHO. 

The versatility of the described domino SN2/Wittig reaction is underlined by its application in the total synthesis of (Z)-pterulinic acid (2-236) [130], a natural product isolated from fermentations of the Pterula sp 82168 species (Scheme 2.55). Reaction of 2-234 and 2-231 led to 2-235 in 68% yield, which was then transformed into 2-236 [129],... 

A combination of a Claisen rearrangement, a Wittig rearrangement and a Wittig reaction was described by Mali and coworkers for the synthesis of 6-prenylcou-marins. In these transformations, 2-prenyloxybenzaldehydes was employed as substrate [105]. 

The final example of a domino process under high pressure, to be discussed in this chapter, is a combination of a Horner-Wittig-Emmons (HWE) reaction with a Michael addition developed by Reiser and coworkers [5]. Hence, reaction of a mixture of an aldehyde such as 10-18, a phosphonate 10-19 and a nucleophile 10-20 in the presence of triethylamine at 8 kbar led to 10-21. By this method, (3-amino esters, 3-thio esters and 3-thio nitriles can be prepared in high yield (Scheme 10.4). Many of these transformations do not occur under standard conditions, thereby underlining the importance of high pressure in organic chemistry. 

A similar procedure was applied to the synthesis of quinazolidine 189 from precursor 188 in the total synthesis of the natural product known as ( )-quinolizidine 2071 190, an alkaloid isolated from the skin of the Madagascar mantelline frog Mantella baroni, that shows an exceptional axial stereochemistry for the ethyl group at C-l. Quinolizidine 189 was transformed into 190 by oxidation and two consecutive Wittig methylenations (Scheme 34) <1999CC2281>. 

An important transformation in organic synthesis is the Wittig olefination. Dai and coworkers have described highly regioselective Wittig olefinations of cyclohexa-... 

The concept of using diazaquinones for diene system protection was accomplished in the case of some vitamin D derivatives, as shown in Scheme 37 (86JOC4819). Acetyl vitamin D2152 was protected with phtha-lazinedione 83a, providing a mixture of both possible stereoisomeric adducts from which 153 was isolated. Then usual ozonolysis procedure afforded aldehyde 154, which after the Wittig coupling and hydrogenation provided 155. The most difficult part of the sequence proved to be the deprotection of this compound, but this transformation was accomplished... 

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