Homoaldol reactions complexes

Z)-awh-4-Hydroxy-l-aIkenyl carbamates 363, when subjected to substrate-directed, vanadyl-catalysed epoxidation , lead to diastereomerically pure epoxides of type 364 (equation 99)247,252,269 qqjggg epoxides are highly reactive in the presence of Lewis or Brpnsted acids to form -hydroxylactol ethers 366 in some cases the intermediate lactol carbamates 365 could be isolated . However, most epoxides 364 survive purification by silica gel chromatography . The asymmetric homoaldol reaction, coupled with directed epoxidation, and solvolysis rapidly leads to high stereochemical complexity. Some examples are collected in equation 99. The furanosides 368 and 370, readily available from (/f)-0-benzyl lactaldehyde via the corresponding enol carbamates 367 and 369, respectively, have been employed in a short synthesis of the key intermediates of the Kinoshita rifamycin S synthesis . 1,5-Dienyl carbamates such as 371, obtained from 2-substituted enals, provide a facile access to branched carbohydrate analogues . 

Lithium-metal exchange in the lithium-)—)-sparteine complexes 399 or 402, respectively, by diethylaluminium chloride or triisopropoxytitanium chloride proceeds with inversion providing useful reagents for enantioselective homoaldol reactions... 

Thermal degradation led to the oxo compounds including [(N02)N0)][Ti0(F3CS03)4], Tilv triflate complexes efFiciently catalyze a variety of reactions including the conversion of acetophenones to 1,3,5-triarylbenzenes,658 the nucleophilic ring opening of epoxides,659 Diels—Alder reactions,660 selective Claisen and Dieckmann ester condensations,661 esterification reactions,662 Fries rearrangements,663 homoaldol reactions,664 sequential cationic and anionic polymerizations,641 and the stereoselective synthesis of m-arabinofuranosides.606... 

Hoppe has extended this work to d3 reagents 159 (homoenolates - see chapter 13 for achiral versions) by the addition of a double bond.29 Lithiation occurs at C-l by removal of one of the enantiotopic protons at C-3. Aldol reaction with acetone occurs at C-3 of the complex 160 as expected for a homoenolate (chapter 13) giving a single enantiomer of the homoaldol product 161. All these reactions use an excess of sparteine. 

The elements of the 4b transition group were also stabilized by triflates. Titanium is among the most moisture-sensitive elements. Air- and moisture-stable titanium(IV) triflates were prepared by its co-complexation with organic ligands. Reported examples referred to Cj-symmetric amine tris(phenolate) [36] or Binol ligands [37]. These compounds demonstrated efficiency as Lewis acid catalysts for formal a a-Diels-Alder (Equation (8.15)) and the homoaldol (the addition of silyloxycyclopropanes to aldehydes) (Equation (8.16)) reactions. 

The titanium- ate complexes of a-methoxy allylic phosphine oxides, generated in situ by reaction of the corresponding lithium anion and Ti(0-i-Pr)4, condense with aldehydes exclusively at the a-position to produce homoallylic alcohols in a diastereose-lective fashion. The overall result is the three-carbon homologation of the original aldehyde, and this protocol has been used in a synthesis of (-)-aplysin-20 from nerolidol. The titanium- ate complex produced by reaction of the chiral lithium anion of an ( )-crotyl carbamate with Ti(0- -Pr)4 affords -y-condensation products (homoaldols) on reaction with aldehydes. Allyl anions produced by the reductive metalation of allyl phenyl sulfides condense with a,p-unsaturated aldehydes in a 1,2-manner at the more substituted (a) allyl terminus in the presence of Ti(0-i-Pr)4. 1,2-Addition of dialky Izincs to a,p-unsaturated aldehydes can be achieved with useful levels of enantiocontrol when the reaction is conducted using a chiral titanium(IV) catalyst in the presence of Ti(0- -Pr)4 (eq 20). Higher ee values are observed when an a-substituent (e.g. bromine) is attached to the substrate aldehyde, but a -substituent cis-related to the carbonyl group has the opposite effect. 

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