1.2- Diols alkene protection

The alkenes 149 and 150 are easily cleaved by ozonolysis either directly or after protection of the hydroxy gronp. Depending on the workup conditions of the ozonolysis, either diols 151 or 0-protected aldehydes 152 can be obtained. The C2 symmetric ketone 153 dr 75 25) is available from another addition of the dilithium reagent 148 (R = Ph)... 

Some additional examples, where the stereochemical outcome of the cycloaddition to chiral alkenes has been explained in terms of the Honk—Jager model, should also be mentioned. The diastereomer ratio found in the reaction of y-oxy-a,p-unsamrated sulfones (166), with Morita-Baylis-Hillman adducts [i.e., ot-(a -hydro-xyalkyl)-acrylates (167)] (Scheme 6.27), with dispiroketal-protected 3-butene-l,2-diol (168), and with a,p-unsamrated carbonyl sugar and sugar nitroolefin (169) derivatives, all agree well with this model. 

To complete the synthesis, the ketone of 2 was homologated to the alkene 12. Selective oxidative cleavage of the two vinyl groups followed by reduction provided the diol, the less encumbered alcohol of which was protected to deliver the fully-differentiated ester 13. Oxidation followed by ester cleavage then gave 3. 

Among the other reagents listed, activated Mn02 is specific for the oxidation of allylic and ben/ylie alcohols.22 KM11O4 is used for the oxidation of alkenes to diols.23 Ceriv(lV) ammonium nitrate (CAN) accomplishes oxidative cleavage of the PMB protecting group. 

A detailed investigation showed that PhB(OH)2 does not interfere with the chiral ligand, leaving the enantioselective step of alkene oxidation intact. Its main role-apart from protecting the diol products against potential over-oxidation - is to remove the diol via an electrophilic cleavage, which is in contrast to the conventional hydrolytic cleavage of the AD protocols.126... 

The aminocyclitol moiety was synthesized in a stereocontrolled manner from cis-2-butene-l,4-diol (Scheme 40)112 by conversion into epoxide 321 via Sharpless asymmetric epoxidation in 88% yield.111 Oxidation of 321 with IBX, followed by a Wittig reaction with methyl-triphenylphosphonium bromide and KHMDS, produced alkene 322. Dihydroxylation of the double bond of 322 with OSO4 gave the diol 323, which underwent protection of the primary hydroxyl group as the TBDMS ether to furnish 324. The secondary alcohol of 324 was oxidized with Dess-Martin periodinane to... 

A new procedure for GSL synthesis via olefin cross metathesis (164) is highly versatile in terms of the hydrophobic agly-cone. A protected 5 carbon amino alkene diol is the central building block to which the protected carbohydrate donor, long chain fatty acid, or, by olefin cross metathesis, the long alkenyl chain of the base can be coupled, in a variety of sequences. This atypical synthetic flexibility should allow a stmctural approach to dissecting the role of the lipid moiety in GSL receptor function and intracellular trafficking. 

We invoke 7c-stacking of the alkene with a phenyl moiety on the silicon protecting group (since this high degree of selectivity was only observed for TBDPS and not with TBS), in the precursor to explain this remarkable selectivity (Figure 1). Lewis acid induced reduction of the epoxide with sodium cyanoborohydride led regioselectively to the 1,3-diol (11) the hydride attacks the more substituted position via an S 2 mechanism. ... 

Diols are easily accessible from alkenes by the reaction with osmium tetroxide, and constitute another protection of the double bond since a variety of methods for the reformation of the alkenes from them is available. Among the important methods which proceed stereospecifically, the desulfurization of 2-thioxo-l,3-dioxolanes (e.g. 102, Corey-Winter reaction),the acid-catalyzed decomposition of... 

With a more complicated substrate 48, Suzuki et al. [34] removed the benzyl protecting group by use of calcium in ammonia, in which part of the diol product underwent partial saturation of the triple bond (Scheme 4.13). Hydrogenation of this mixture gave the fully saturated diol 49 in 87% yield. A thorough study was performed on selective reduction of benzyloxy alkyne 50 to give a mixture of hydroxy alkyne 51 and hydroxy trans-alkene 52 in 81-93% overall yields (Scheme 4.14) [12]. The study involved the use of different quantities of calcium and lithium in liquid ammonia. 

The O-benzylated aldehyde 517 was also coiu erted into the a,p-unsaturated ester 522 through Wadsworth-Emmons reactioiT-- with ethyl 2-(diethoxyphosphono)acetate5 ° in excellent yield (Scheme 88). The intermediate alkene 522 was subjected to Sharpless asymmetric dihydroxylalion " to afford the diol ester 523 in excellent yield and with a diastereoselectivity in excess of 95 5. Subsequent to alkali-catalyzed hydrolysis of 523, the carboxylic acid obtained was condensed with the p-lolucncsulfonalc salt of glycine benzyl ester or phenylalanine benzyl ester, by the action of 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC), to afford the benzyl-protected amide derivatives catalytic... 

More recently, the effects of functional groups on the alkene have been explored. Enol ethers are more nucleophilic than ordinary alkenes and the two enol ethers E- and Z-64, give good yields of different diastereoisomers of the half-protected diol 65. Though 64 is a trisubstituted alkene, it reacts rapidly with IpCiBH with good enantioselectivity.13... 

The solution to the chemoselectivity problem is minimal protection ap-mcthoxybcnzyl group for the hydroxyaldehyde and an acetal 158 for the diol unit in the product 157 of the enzyme-catalysed aldol reaction. The solution to the alkene geometry problem is going to be intramolecular trapping by the one remaining free OH group. 

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