Silyl dienol ethers alkylation

The regioselectivity of alkylations of silyl dienol ethers has been studied87,88. These reactions favor y-alkylation products. In contrast, alkylations of the corresponding lithium enolates mainly occur in the a-position. Substituents on the silyl diene unit, as well as the substituents at the silicon, strongly influence the regioselectivity of the reaction87 91,... 

It is important to note that, under the same conditions, isophorone or other /3-alkyl cyclic enones predominantly afford the exocyclic conjugated silyl dienol ether (Scheme 3). 

The position of alkylation of homoannular silyl dienol ethers is apparently dramatically influenced by the nature of the alkylating agent and catalyst. Reetz and cowoikers reported that 1-trimethylsiloxy-... 

Lewis acid catalyzed alkylations of cross-conjugated silyl dienol ethers provide routes to a -alkylated ketones. A short synthesis of the sesquiterpene ( )-ar-turmerone has been accomplished using the cross-conjugated TMS dienol ether of mesityl oxide (equation 9). ... 

The total synthesis of ( )-trichostatin A has been carried out by two routes [67] one uses the y-alkylation of a silyl dienol ether with an acetal which gives the ether in a Mukaiyama reaction. A Wittig olefination followed by DDQ oxidation gives a ketoester which reacts with hydroxylamine to give trichostatin A in 22% overall yield (Scheme 20). 

Synthesis and y-alkylation of pyrrole-based silyl dienol ether Zanardi et al. [88]. 

Dienols with 2 equiv. of 1 gave the corresponding bimetallic alkoxide organozir-conocene complexes however, protolysis allowed recovery of the alcohol functionality (Scheme 8-29) [107]. Alcohols can also be easily converted to ethers. Alkyl, aryl, silyl [85,112,183, 210] and THP [17, 153, 211, 219] ethers are stable under hydrozirconation conditions side products were observed only with the trimethylsUyl group [220, 221]. 

LDA, LiCl) afforded the dienolate intermediate, which was alkylated with iodoethane. Four additional steps including a reductive cleavage of the chiral auxiliary, the protection of the resulting primary alcohol, the oxidative cleavage of the double bond under Johnson-Lemieux conditions, and the formation of the silyl enol ether led to the desired fragment 187. 

This was followed shortly by a stereo- and enantiocontrolled synthesis of (—)-chimonanthine (154) and calycanthine (150) as well as a second route to meso-chimonanthine (152). The central step in this synthesis features the use of a double Heck cyclization to create vicinal quaternary carbon centers in high yields and with complete stereocontrol 124). The synthesis commenced with a double alkylation of the lithium dienolate of dimethyl succinate 194 and tartrate-derived diiodide 195 to give a diastereomeric mixture of the saturated diesters. Subsequent oxidation of the diesters, followed in succession by aminolysis, A-benzylation, removal of the benzyl ethers, and silylation, provided the cyclization substrate 197, which on Heck cyclization yielded a single product, 198, a pentacyclic bisoxindole, subsequently shown to have the meso relationship of the two oxindole groups. Further manipulations of 198 led eventually to the diazide derivative 199, which can be processed to we.so-chimonanthine (152), following the procedure established in the preceding synthesis (Scheme 14). 

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