Wittig reaction £-selective

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). 

Aryl and alkyl trimethylsilyl ethers can often be cleaved by refluxing in aqueous methanol, an advantage for acid- or base-sensitive substrates. The ethers are stable to Grignard and Wittig reactions and to reduction with lithium aluminum hydride at —15°. Aryl -butyldimethylsilyl ethers and other sterically more demanding silyl ethers require acid- or fluoride ion-catalyzed hydrolysis for removal. Increased steric bulk also improves their stability to a much harsher set of conditions. An excellent review of the selective deprotection of alkyl silyl ethers and aryl silyl ethers has been published. ... 

Barron, L. D., and Vrbancich, J. Natural Vibrational Raman Optical Activity. 123,151-182 (1984) Bestmann, H. J., Vostrowsky, O. Selected Topics of the Wittig Reaction in the Synthesis of Natural Products. 109, 85-163 (1983). 

In our work with aminolysis of vinylepoxides (see Section 9.2.1.1), the substrates were routinely synthesized by SAE followed by Swern/Wittig reactions (Table 9.3, Entries 1-4) [48, 49]. This procedure is well suited for terminal olefins, but dis-ubstituted olefins can seldom be obtained with useful (E Z) selectivities. Nakata recently synthesized some advanced intermediates towards natural products in this manner (Entries 5, 6) [50, 51]. 

Olefin cross metathesis starts to compete with traditional C=C bondforming reactions such as the Wittig reaction and its modifications, as illustrated by the increasing use of electron-deficient conjugated alkenes for the ( )-selective construction of enals and enoates. 

It should be noted here that the lithium salt of hexamethyldisilazane li-HMDS 492 (and Na-HMDS-(486) and K-HMDS in Sections 5.1.2 and 5.1.3), which is readily obtained on treatment of a solution of HMDS 2 in hexane or THF with butyUithium at -78 °C, is not only a very useful and selective strong base, e.g. for Wittig reactions, but can also add to carbonyl groups to yield the silylated Schiff bases or nitriles (cf. Sections 4.7 and 5.1.3) or to nitriles to afford N-silylated ami-dines. Alkylation of the Li-HMDS 492, e.g. with allyl bromide, affords, furthermore, N,N-bis(trimethylsilylated) primary amines such as 43 [64]. The combina-... 

Quebrachitol was converted into iL-c/j/roinositol (105). Exhaustive O-isopropylidenation of 105 with 2,2-dimethoxypropane, selective removal of the 3,4-0-protective group, and preferential 3-0-benzylation gave compound 106. Oxidation of 106 with dimethyl sulfoxide-oxalyl chloride provided the inosose 107. Wittig reaction of 107 with methyl(triphenyl)phos-phonium bromide and butyllithium, and subsequent hydroboration and oxidation furnished compound 108. A series of reactions, namely, protection of the primary hydroxyl group, 0-debenzylation, formation of A-methyl dithiocarbonate, deoxygenation with tributyltin hydride, and removal of the protective groups, converted 108 into 7. 

The stereoselectivity of the Wittig reaction is believed to be the result of steric effects that develop as the ylide and carbonyl compound approach one another. The three phenyl substituents on phosphorus impose large steric demands that govern the formation of the diastereomeric adducts.240 Reactions of unstabilized phosphoranes are believed to proceed through an early TS, and steric factors usually make these reactions selective for the d.v-alkcnc.241 Ultimately, however, the precise stereoselectivity is dependent on a number of variables, including reactant structure, the base used for ylide formation, the presence of other ions, solvent, and temperature.242... 

Tellurium sources, 22-24 Thermodynamics in cyclo-oligomerization, 185-186 butadiene insertion, 187-188 reductive elimination, 193, 194 selectivity control, 212 polysilane isomerisation, 158-160 see also Stability Thermolysis, 135, 136, 158 THF (tetrahydrofuran), 97, 150, 153 Thio-Wittig reaction, 37 Tin, 121... 

Syntheses of (l )-frans-isomers were reported by Crombie [24] and Elliott [25] starting from (1 /t Wran.v-chrysanthemic acid by means of the Wittig reaction. Their method were convenient to obtain (Z)-isomer (Scheme 10, step a) but not appropriate for the synthesis of ( )-isomer because of the (Z)-selective nature of the Wittig reaction in the case of nonstabilized ylides. It was very difficult to separate the pure ( )-isomer out of the (E)- and (Z)-mixture. This problem was overcome by use of the Takai s method (Scheme 10, step b) [26]. The ( )-selectivity of the double bond was fairly high (E Z = 89 11) (Scheme 10). 

Boron-Wittig reaction (12, 12-13). The direct reaction of the anion of an alkyldimesitylborane at -78° with an aromatic aldehyde followed by oxidation results in an (E)-alkene in low yield. The intermediate adduct can be isolated in about 80% yield as the silyl ether of a iyn-l,2-diol by addition of CISi(CH,), to the reaction, and this product on desilylation (HF, CHjCN) affords (E)-alkenes with high selectivity. Somewhat lower (E)-selectivity obtains in a one-pot reaction. In contrast, addition of trifluoroacetic anhydride (slight excess) to the reaction at -78° to -110° results in a (Z)-alkene with almost comparable selectivity (Z/E 9 1). 

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