Form cleaning

These water-sensitive derivatives can be used to form cleanly the t-butyl ethers of serine and threonine. They are cleaved with aqueous acid or base. ... 

This silyl hydrazone formation-oxidation sequence was originally developed as a practical alternative to the synthesis and oxidation of unsubstituted hydrazones by Myers and Furrow [31]. The formation of hydrazones directly from hydrazine and ketones is invariably complicated by azine formation. In contrast, silyl hydrazones can be formed cleanly from /V,/V -bis(7< rt-butyldimethylsilyl)hydrazine and aldehydes and ketones with nearly complete exclusion of azine formation. The resulting silylhydrazones undergo many of the reactions of conventional hydrazones (Wolff-Kishner reduction, oxidation to diazo intermediate, formation of geminal and vinyl iodides) with equal or greater efficiency. It is also noteworthy that application of hydrazine in this setting may also have led to cleavage of the acetate substituents. 

Substitution of the VCP is tolerated both on and adjacent to the cyclopropane ring. Diester-substituted and heteroatom (O, NTs) tethers are well tolerated. Reactions were conducted with 2-10 mol% catalyst at up to 0.20 M, as illustrated. Most importantly, reactions with the naphthalene catalyst were found to be more rapid than those with other catalysts. For example substrate 54 is readily converted in >99% yield to cycloadduct 55 in only 15 min at room temperature (entry 1). Complex 93 efficiently catalyzes the reactions of both alkynes and alkenes with VCPs, offering greater generahty than thus far observed with non-rhodium catalysts. This catalyst is particularly advantageous in the cases of substrates 100 and 102, for which the desired product is not formed cleanly with Wilkinson s catalyst due to product isomerization. 

Tubes up to 10-12 mm in diameter can be cut in this way. When properly notched, a tube breaks easily, forming clean edges without burrs. To cut wider tubes, tie a thick thread wetted with ethonol around the notched spot. Ignite the thread, and the glass at this spot will crack smoothly. 

The crude product is purified by washing with a small quantity (2 X 50 ml.) of ice water and vacuum-dried for 2 hours. The yield is 150 g. (78%). The product forms clean yellow crystals (m.p. 29 to 30°). For further purifi-... 

The method is less successful with benzylic and allylic halides, and problems of selectivity with these substrates have been overcome in the past via an initial substitution with lithium butyltelluride the organolithium is then formed cleanly from the telluride 135 or 136 (which need not be isolated) by tellurium-lithium exchange.112... 

Transition-metal-catalysed epoxidations work only on allylic alcohols, so there is one limitation to the method, but otherwise there are few restrictions on what can be epoxidized enantioselectively. When this reaction was discovered in 1981 it was by far the best asymmetric reaction known. Because of its importance, a lot of work went into discovering exactly how the reaction worked, and the scheme below shows what is believed to be the active complex, formed from two titanium atoms bridged by two tartrate ligands (shown in gold). Each titanium atom retains two of its isopropoxide ligands, and is coordinated to one of the carbonyl groups of the tartrate ligand. The reaction works best if the titanium and tartrate are left to stir for a while so that these dimers can form cleanly. 

Another useful reaction of these Diels-Alder adducts is shown in equation (54). Dihydrothiazine oxide (123) from ( . )-2,4-hexadiene can be opened with a Grignard reagent to allylic sulfoxide (124) which undergoes a stereoselective 2,3-sigmatropic rearrangement via the envelope-like transition state conformation shown, having a quasi-equatorial methyl group to afford sulfenate ester (125). Desulfurization of (125) provides E)-threo smino alcohol derivative (126) in excellent overall yield. If ( ,Z)-2,4-hexadiene is used, the E)-erythro epimer of (126) is formed cleanly. 

Reaction of M2Xi0 (X = C1, Br) with NaL (L /V,/V -disubstituted dithiocarbamate) is complicated by side reactions and solvent dependency. In CH2C12, the two product salts [ML4][MX6] and [ML4]X have similar solubilities. A selection of both forms was isolated, as were the monothiocarbamato salts [M(OSCN Mc2)4][MCl6] which formed cleanly at —78°C. Variable-temperature 111 NMR was employed in a detailed study of the stereoisomerism and metal-centered rearrangements of the eight-coordinate [ML4]+ ions.397 The mmmm dodecahedral arrangement is present in [Nb(S2CNEt2)4]Br.356... 

UV detector. A small-diameter tube is placed in the side arm of the tee and runs to the inlet to the MS. The tubing is usually either PEEK (polyether-etherketone) or fused silica. A smaller-inner-diameter tube penetrates into the source and acts as a restrictor, limiting the flow through this side arm. The fluid pressure drops below the two-phase region somewhere in the transfer line. The small droplets of modifier formed clean the transfer line and rapidly delivers the solute to the MS source. 

Liquid dosage forms Creams/ointments/ suppositories Solid dosage forms Cleaning... 

However, the scope of the first approach [Eq. (ct)] is limited by the availability of suitable thexylmonoalkylboranes, which are formed cleanly only from alkenes with intermediate steric requirements. The second method [Eq. (cu)] suffers from low yields of the xylalkenylboranes owing to competing dihydroboration of thexylborane ... 

The pyrolysis of 1-methylsilacyclobutane appears to be similarly complicated as the pyrolysis of silacyclobutane itself. In the most detailed investigation available to date, the results were rationalized assuming that 1-methylsilene is formed cleanly in the initial pyrolysis step and is capable of subsequent isomerization to dimethylsilylene, a step to be discussed in more detail below75, 205. It is of interest to note that under these high-... 

The Weinreb group has recently developed stereoselective methodology for synthesis of unsaturated vicinal diamine derivatives from dihydrothiazine imines.49 For example, when cycloadduct 54 prepared from (E, )-2,4-hexadiene was treated with phenylmagnesium bromide followed by trimethyl phosphite, E-threo vicinal diamine 57 was formed cleanly in good yield [Ea. <27)1. 

In the case of vinyldiazomethanes with a single electron-withdrawing group, such as 9, the unsaturated tropane system 91 is not formed cleanly under the traditional reaction conditions of rhodium(II) ace-tate/dichloromethane (Scheme 33). A major side product is the alkylation product 92, derived from reaction at the vinyl terminus of the vinylcarbenoid. The side reaction, however, is readily eliminated by using rhodium(II) hexanoate/hexane, and under these conditions the tropane system 91 is formed in 75% isolated yield. 

A phenolic oxygen participates in facile oxypalladation [16], Different chemos-electivity is observed depending on the catalytic species. The 2H-l-benzopyran 66 was formed cleanly from 2-allylphenol (65) by endo cyclization with Pd2(dba)3 or... 

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