Wittig reaction stereoselective

The path to the complex heptacycle 87 (Scheme 17 a) commences with a stereoselective Wittig reaction between 2-deoxy-D-ribose (110) and (ethoxycarbonylethylidene)triphenylphosphorane (see Scheme 21). This reaction takes advantage of a ring-chain tauto-... 

The carbonyl group of l-oxo-2-alkenylphosphonates (57) undergoes stereoselective Wittig reaction with acylidenephosphoranes... 

The condensation of the cinnamaldehyde derivative (18) with ethoxycarbonyl-pentyltriphenylphosphorane in DMF gave a mixture of tram-trans- and trans-cis-products.22 A general method to prepare l-substituted-m-9-alkenes (19), using a stereoselective Wittig reaction, has been described.23 2,2-Dimethyl-3-butenal (20)... 

Schemes 2.3a-c outline the retro-synthetic production of BTX B (1). The final approach to PbTx-2 involved separate assembly of the ABCDEFG and UK ring systems 4 and 5, their coupling, and final elaboration to the end. The didehydrooxocane ring in BTX B (ring H) was thus designated as the final ring to be constructed. Retro-synthetic cleavage of the indicated C-0 bond in 1 and removal of the terminal electrophilic groupings reveal hydroxy dithioketal 3 as a plausible precursor. Tricyclic aldehyde 4 and heptacyclic phosphonium salt 5 can thus be defined as potential precursors to 3. The reliable and usually stereoselective Wittig reaction would be employed to accomplish the union of compounds 4 and 5 (Scheme 2.3a). Tricyclic aldehyde 4 was traced retro-synthetically to D-mannose 10.

Z)-condenses with aldehydes in a (Z)-stereoselective Wittig reaction to afford diethyl acetals of (Z)-a,jS-unsaturated aldehydes in 57-86% yield. Hydrolysis of the acetals with p-TsOH as catalyst in acetone-water or with moist silica gel (2 days at 23°) affords the corresponding (Z)-unsaturated aldehydes in 47-98% yield. The product usually contains 4-14% of the (E)-isomer. These results contrast with those of Wittig reactions of formylmethylenetriphenylphosphorane and l,3-dioxolan-2-ylmethylenetriphenyl-phosphorane (5, 269), which afford (E)-unsaturated aldehydes. 

A commoner way to make heterocycles by pericyclic reactions is to use 1,3-dipolar cycloadditions. These often occur without catalysis and so are compatible with many other reactions. The starting material 182 for this asymmetric synthesis of swainsonine was derived from a natural sugar (chiral pool strategy, chapter 23). An exceptionally stereoselective Wittig reaction gave the Z-alkene 183 (chapter 15) and the alcohol was converted into the azide 184 with diphenylphos-phoryl azide.24... 

Stereoselective Wittig reaction. A stereoselective synthesis has been reported of the sex pheromone of the Egyptian cotton leafworm, (Z)-9,(E)-11-tetradecadien-l-yl acetate (3), by the Wittig reaction of (E)-2-pentenal (1) with (2) in dry dimethyl sulfoxide with dimsylsodium as base followed by deprotection and acetylation. The 9Z,11E- and 9E, 11 E-isomers were obtained in a... 

Stereoselective Wittig reaction. The reagent has been used in a novel synthesis of 25-hydroxycholesterol (5). Thus the /-steroid aldehyde 2 reacts with 1 to form 3. Two features of the reaction are noteworthy. The 20-methyl group does not epimerize and the A -double bond of the major product has the E-con-... 

In the case of acetylenic carotenoids, mentioned in Section B, appropriate choice of the aldehyde and an acetylenic phosphonium salt as synthons results in a stereoselective Wittig reaction to give the (9Z)-isomer [12,14,15] (Scheme 5). 

A sequence for the regiospecific production of the allylic alcohols (37) from terminal isopropylidene groups (Scheme 17), involving phenylsulphenyl chloride addition, was reported last year (3,140). More recently, extension of this work (also Scheme 17)" has provided a route to the internal alcohol isomer (38), and a new alternative scheme" leads either to (37) or to the cw-terminal isomer (39) this latter depends on a stereoselective Wittig reaction modification. 

A Stereoselective Wittig Reaction Giving Mostly cis Product... 

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